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1Z00L OGY | LIBRARY |

Zoology Series

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THE

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HISTORY
MUSEUM

VOLUME 61 NUMBER1 29JUNE 1995 ~

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© The Natural History Museum, 1995

Zoology Series
ISSN 0968 — 0470 Vol. 61, No. 1, pp. 1-90

The Natural History Museum
Cromwell Road
London SW7 5BD Issued 29 June 1995

Typeset by Ann Buchan (Typesetters), Middlesex
Printed in Great Britain at The Alden Press, Oxford

Bull. nat. Hist. Mus. Lond. (Zool.) 61(1): 1-10 Issued 29 June 1995

A revised familial classification for certain
cirrhitoid genera (Teleostei, Percoidei
Cirrhitoidea), with comments on the group/s THE MATURAL

RY MUSEU

monophyly and taxonomic ranking HISTORY MU
-7 JUL 1995
PRESEN cD

ZOOLOGY LIBRA

P.H. GREENWOOD‘

Honorary Research Associate, J.L.B. Smith Institute of Ichthyology, Private Bag 1015, Graha

6140, South Africa and Visiting Research Fellow, The Natural History Museum, Cromwell Road,

London SW7 5BD
CONTENTS
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Urohyal morphology and the suprageneric classification of the cirrhitoid fishes, particularly the genus
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Synopsis. Previous suprageneric classifications of cirrhitoid fishes were based mainly on superficial characters.
Recent anatomical studies show that five morphologically distinct types of urohyal bone occur within the group, and
that on this basis certain taxa have been misplaced at both the generic and familial levels. The monophyly of the
cirrhitoid fishes, tacitly assumed by previous authors, is reviewed, and an hypothesis of their monophyly erected on
the basis of several synapomorphic features. It is also proposed, on the basis of those synapomorphies, that the

lineage be given subordinal rank within the Perciformes.

INTRODUCTION

During an investigation into the cranial and branchial muscles
associated with feeding in certain cirrhitoid fishes from South
African waters, it became apparent that five distinct morpho-
types of urohyal occur within the group. Two very distinct
types are found in genera currently classified as members of
the Cheilodactylidae; of these two forms, one also occurs in
genera referred to the family Latridae by Regan. Despite the
passage of over 80 years since Regan’s (1911) paper was
published, it remains the most recent suprageneric classifica-
tion of these fishes, which he arranged in five families, viz
Cirrhitidae, Cheilodactylidae, Aplodactylidae, Chironemidae
and Latridae, grouping them informally as the ‘cirrhitiform
percoids’.

Like its predecessor, namely Gill’s more detailed paper of
1862, Regan’s later analysis was based mainly on external

+ Dr Greenwood died 3 March 1995.

© The Natural History Museum, 1995

features. Apart from some comments by Regan on cranial
features, vertebral numbers and vertebral morphology, nei-
ther paper included any other anatomical information on
these fishes.

Although Regan (1911) expressed some doubts about the
familial status given by him to the five generic assemblages he
recognised (suggesting that subfamilial rank could be more
appropriate) he did not comment on Gill’s (1862) earlier
classfication which recognised four subfamilies within a single
family, the Cirrhitoidae; indeed, and inexplicably, Regan
(op. cit.) makes no reference at all to Gill’s paper.

Gill’s (op.cit) four subfamilies correspond, almost entirely,
to Regan’s families, except that Gill’s Latridinae included
two genera, Nemadactylus Richardson, 1839, and Cheilodac-
tylus Lacépéde, 1803, which Regan incorporated in his family
Cheilodactylidae; Gill’s list of included taxa in his concept of
the Latridinae also contained two genera not mentioned by
Regan, despite their being recognised at that time and still
being recognised today (viz Latridopsis Gill, 1862, and Acan-
tholatris Gill, 1862).

The present contribution, it is hoped, will play some part in
rewakening an interest in a phylogenetically based taxonomy
of Regan’s ‘cirrhitiform percoids’, and of their interrelation-
ships within, or perhaps outside, the Percoidei. Regan (1911)
gave no formal suprafamilial ranking to his ‘cirrhitiforms’,
but in some recent classifications (Nelson, 1994) the group
has been raised to a superfamily, the Cirrhitoidea. The use of
the terms ‘cirrhitoid’ and ‘cirrhitoids’ in this paper is thus to
be interpreted as a reference to the superfamily and not to the
suborder suggested for these fishes on p. 9.

METHODS AND MATERIALS

Species of the percoid families Serranidae and Centropomi-
dae served as outgroup representatives of basal members of
the Percoidei. The condition and composition of the sterno-
hyoideus muscles, and the morphology of the urohyal in these
taxa were taken to represent the plesiomorphic percoid
conditions.

Wherever possible, dry skeletal preparations and alizarin
transparencies were examined, as were radiographs and
dissections of preserved specimens.

STUDY MATERIAL. The symbols used in the following list are:
D. Sternohyoid muscle dissected and the urohyal removed or
examined in situ; X. Radiographed; S. Dry skeletal material
examined; an asterisk following a binomen indicates that this
name appears on the box housing the skeleton, and that its
specific identity could not be checked on the basis of the
characters preserved.

The familial grouping used in the list is that resulting from
the conclusions reached in this paper, and thus differs in some
respects from the classifications of Gill (1862) and Regan
(1911), and in some cases that of later authors (see p. 5
relating to the species ‘bergi’ and ‘gayi’ previously placed in
the genus Cheilodactylus and in the family Cheilodactylidae).

Institutional abbreviations are: BMNH, The Natural His-
tory Museum, London; DIFS, The Department of Ichthyol-
ogy and Fisheries Science, Rhodes University, South Africa;
RUSI, J.L.B. Smith Institute of Ichthyology, South Africa;
SAM, South African Museum, Cape Town.

STUDY MATERIAL.
(i) Outgroups.

Centropomidae; Glaucosomatidae; Ambassidae: The osteo-
logical and other material listed in Greenwood (1976).

Serranidae: as above, together with Epinephalus alexandri-
nus BMNH 1867.2.1: 69-72; (D).

(ii) Cirrhitoid fishes.
Aplodactylidae: Aplodactylus punctatus * (type species of
the genus): BMNH 1873.4.3: 157 (S).
A. lophodon: BMNH 1914.8.20: 214 (D).
Cheilodactylidae: | Cheilodactylus fasciatus (type species of
the genus): 3 specimens, DIFS unregistered (D).
C. fuscus: 1 specimen, DIFS unregistered (D).
C. pixi: 6 specimens, DIFS unregistered (D).
2 specimens, DIFS unregistered (S).
4 ‘paperfish’ larvae, 44-50 mm standard length,
RUSI 19842 (D).
1 ‘paperfish’ larva, 44 mm standard length, RUSI
19842 (S).
C. zonatus: BMNH 1907.12.23: 238 (S).

P.H. GREENWOOD

Chirodactylus brachydactylus: 3 specimens, DIFS

unregistered (D).

2 specimens, DIFS unregistered (S).

2 ‘paperfish’ larvae, 33 & 38 mm standard length.

RUSI, unregistered (D).

C. grandis: 1 ‘paperfish’ larva, 57 mm standard

length. RUSI 18404 (D).

Dactylophora nigricans*: BMNH 1869.2.24:8 (S).
Chironemidae: Chironemus —marmoratus*: | BMNH

1871.3.29: 28 (S).

Chironemus marmoratus: BMNH 1861.11.7: 6 (D).
Cirrhitidae: | Amblycirrhitus pinos: 2 alizarin preparations

ex BMNH 1976.7.14: 453-7 (S).

Amblycirrhitus pinos: BMNH 1984.7.16: 96 (D).

Cirrhitus maculatus*: BMNH unregistered (S).

Cirrhitichthys oxycephalus: RUSI 11658; BMNH

1929.9.20: 8 (D).

Cirrhitichthys oxycephalus: BMNH 1908.3.23: 77-79

(X).
Cyprinocirrhites polyactis: RUSI 12339 (D).
Gymnocirrhites arcatus (type species of the

genus): BMNH 1965.12.20: 10 (D).
Gymnocirrhites arcatus*: BMNH unregistered, col-
lected by Richardson (S).
Gymnocirrhites arcatus: BMNH 1865.3.2: 82-83 (X).
Oxycirrhites typus (type species of the genus):
BMNH 1929.6.12: 2 (D).
Paracirrhites forsteri: BMNH 1852.9.13: 119 (S)
Paracirrhites forsteri: BMNH 1969.7.16: 28-32; one
specimen(D).

Acantholatris bergi BMNH 1936.18.26: 439 (D)
& (X).
Acantholatris gayi: BMNH
1890.2.26: 49 (D) & (X).
Acantholatris monodactylus (type species of the
genus): BMNH 1960.1.8: 6-13; one (D), all (X).
Acantholatris monodactylus: BMNH unregistered,
ex Gough Isl.; 2 specimens, both (S).
Acantholatris monodactylus: RUSI 33484; 33485;
33624 (D).
Latridopsis ciliaris (type species of the genus):
BMNH 1872.7.1: 32 (S).
Latridopsis ciliaris: BMNH 1873.12.13: 55 (D) &
(X).
Latris lineata: BMNH 1855.9.19: 124 (D).
Latris lineata: SAM 22623 (D).
Mendosoma lineatum (type species of the genus):
BMNH 1960.1.8: 14-21; one (D), all (X).
Mendosoma lineatum: RUSI 33613; 33625 (D).
Nemadactylus macropterus: BMNH 1855.9.19: 314
(X).
Nemadactylus macropterus: BMNH 1872.7.1: 21
(D) & (X).

Latridae:

1879.5.14: 278;

Urohyal morphology and the suprageneric
classification of cirrhitoid fishes, particularly the
genus Acantholatris and species currently referred
to the genus Nemadactylus

Within the cirrhitoid fishes, five morphologically distinct
types of urohyal can be recognised (Figs 1 & 2). Since each

Howes.

itype can be correlated with one of the five family groups
recognised by Regan (1911), these will be referred to as the
cirrhitid, latrid, chironemid, aplodactylid and cheilodactylid
i types respectively.

| The cirrhitid type (Fig. 1A) appears to be a plesiomorphic
form, one fundamentally similar to that found in several basal
percoids (see Kusaka, 1974; also personal observations).

The latrid urohyal (Fig. 2) differs markedly from the
icirrhitid type, and also shows slightly more intrafamilial
|variation, particularly with regard to its posterior margin’s
|degree of indentation, the extent to which the ventral margin

REVISED CLASSIFICATION FOR CERTAIN CIRRHITOID GENERA

Fig. 1 The urohyal of:A. Paracirrhites forsteri; Cirrhitidae; left lateral view (BMNH 1852.9.13: 119). B. Chironemus marmoratus;
Chironemidae; left lateral and ventral views. (BMNH 1871.3.29:28). C. Aplodactylus punctatus; Aplodactylidae; left lateral and ventral
views. (BMNH 1873.4.3: 157). D. Dactylophora nigricans; Cheilodactylidae; left lateral view. (BMNH 1869.2.24:8). Relative to other
figures, this bone has been rotated through 90° to the left; arrow indicates dorsal prominence. Scale in millimetres. Drawn by Gordon

the bone’s upper margin is sharp or somewhat flattened. Like
the cirrhitid type, the latrid urohyal is also of a pleisomorphic
form, One occurring in such basal percoids as the Serranidae
and Centropomidae (Kusaka, 1974; Greenwood, 1976: 39,
fig. 21, and other personal observations).

Departure from the basal percoid form of urohyal is most
pronounced in the chironemid, aplodactylid and cheilodac-
tylid types of bone. In chironemids (Fig. 1B) the bone is
shallow, the ventral margin greatly flattened and expanded
bilaterally to form a broad shelf, while the dorsal margin is
also noticeably flattened and bilaterally produced into a shelf,
albeit one relatively narrower than that on the ventral aspect
of the bone; when compared with the urohyal in cirrhitids,

P.H. GREENWOOD

Fig. 2 The urohyal of four latrid species.A. Acantholatris monodactylus (BMNH unregistered; ex Gough Island); left lateral and ventral
views. B. Latris lineata (BMNH 1855.9.19:194); left lateral view. C. Mendosoma lineatum (BMNH 1960.1.8:14-21); left lateral and ventral
views. D. Latridopsis ciliaris (BMNH 1872.7.1:31); left lateral and ventral views. Scale in millimetres. Drawn by Gordon Howes.

the chironemid type is relatively foreshortened (cf Figs 1A &
1B).

A pronounced ventral shelf and overall foreshortening of
the bone is also characteristic of the aplodactylid urohyal
(Fig. 1C), but in this type the bone is relatively deeper than is
the chironemid urohyal, and the dorsal surface is produced
into only a narrow shelf.

When compared with all other types, the cheilodactylid
urohyal (Fig. 1D) is very distinctive. In lateral view it has
virtually the shape of an arrow-head with its tip directed
anteriorly, and with the two arms meeting at the somewhat
thickened apex from which a dorsally directed process arises.
The anterior edges of both arms are slightly broadened to
form a very narrow bilateral shelf that does not quite extend
to the posterior tip of either arm. Although, morphogeneti-
cally, the cheilodactylid type of urohyal could be derived

from a latrid type by a marked anterior extension and
deepening of the latter’s posterior indentation, coupled with
an increase in the angle subtended by the two arms so
formed, the two morphotypes are readily distinguishable.
Interestingly, the urohyal in the so-called ‘paperfish’ juvenile
stage (see p. 7) of a 44 mm standard length Cheilodactylus
pixi Smith, 1980, resembles that of the latrid type more
closely than does this bone in larger specimens; nevertheless,
the upper and lower arms of the urohyal in the ‘paperfish’
stage are more widely separated, the division between them
extends further anteriorly, and the anterior body of the bone
is less compressed and more barrel-like than that in any of the
adult latrid urohyals I have examined.

In his monograph on urohyal bones Kusaka (1974)
described and illustrates this bone in Goniistius zonatus
(Cuv., 1839), a taxon now, and previously (Gill, 1862),

referred to the family Cheilodactylidae (see Allen & Heem-
stra, 1976, for comments on the status of this genus; also p. 6
below). Kusaka, however, lists the species under the heading
Aplodactylidae. The bone depicted is certainly of the cheilo-
dactylid type and not of the aplodactylid type, and I presume
Kusaka’s placing the species in the Aplodactylidae is a /apsus.
This author (op. cit: ) also figured and described the urohyal
from a specimen supposedly of Cirrhitichthys aureus Temm.
& Schl., 1843 (Cirrhitidae). Unfortunately I have not been
able to examine a specimen of this species, but the bone
illustrated (and described as ‘shaped like a standing rat’) is
| unlike that in any cirrhitoid taxon I have examined, particu-
larly amongst members of the Cirrhitidae and even in a taxon
| such as Oxychirrhites typus Bleeker, 1857, whose elongate
and tubular snout is an unusual morphotype within both the
| Cirrhitidae and the cirrhitoids as a whole. If Kusaka’s figure
and description are accurate and the specimen was correctly
identified, then a sixth and highly distinctive form of urohyal,
| one far removed from that of other cirrhitids must be
recognised, and the higher taxonomic position of its possessor
or posessord be reconsidered (assuming, that is, the bone
| Kusaka examined was not teratological or damaged during
| preparation).

A typical cheilodactylid urohyal (Fig. 1D) occurs in all
members of the family (sensu Regan [1911] and subsequent
| authors) I have examined apart from Nemadactylus and
/ members of the genus Acantholatris, viz the type species A.
monodactylus (Carmichael, 1818), and the species A. gayi
| (Kner, 1869) and A. bergi (Norman, 1937).

Parenthetically it should be noted that A. gayi and A. bergi
were both placed in the genus Cheilodactylus, and the family
Cheilodactylidae, by Norman (1937). The former species was
| later transferred by Fowler (1945) to the genus Acantholatris,
with no explanation given for the change, but was retained in
| the family Cheilodactylidae. Neither author appears to have
| been aware, however, that Gill (1862) had included Acantho-
| latris in his subfamily Latridinae. Mann (1954: 266) followed
Fowler’s generic and familial placing of A. gayi, and listed the
| species bergi under Acantholatris in the index to that publica-
| tion. The reader is there referred to page 266 of the text. No
| mention is made of A. bergi on that page, but on page 85 (op.
| cit.) Acantholatris bergi Norman (the author’s name not
| enclosed in brackets) is listed amongst the ‘Invasores del
| Atlantico’. Mann (op cit.) is thus the first author to employ
this particular combination of names for the species. As
noted earlier (p. 1) Regan did not include Acantholatris in
any of his cirrhitiform families.

The familial classification of Acantholatris Gill,
1982 and Nemadactylus

The urohyal in all three Acantholatris species examined, and
in Nemadactylus macropterus, is virtually identical and differs
markedly from that in the cheilodactylids, cirrhitids, aplodac-
tylids and chironemids (see pp. 2-5 and cf Fig. 1 with Fig.
2A). Instead, it resembles the latrid type, both in detail and
in its gross morphology (cf Figs. 2A, B and C), especially in
its fan-like outline. This marked difference would suggest
that the latrid genera (as listed in Regan, 1911), together with
Acantholatris and Nemadactylus shared a recent common
ancestry distinct from that of the cheilodactylids. It also
| Suggests that the phyletic relationships of the two groups are
| obscured by uniting the cheilodactylids with the latrids in a

REVISED CLASSIFICATION FOR CERTAIN CIRRHITOID GENERA 5

single subfamily, as did Gill (1862).

Thus, in my view, based essentially on their urohyal
morphology and not negated by other characters (see, how-
ever, the pectoral fin character discussed below), Acanthola-
tris and Nemadactylus should be included in the family
Latridae, currently comprising species of the genera Latris,
Latridopsis and Mendosoma, the latter recently shown to be
monotypic by Gon & Heemstra (1987). In addition to the
urohyal characters, the genera listed above lack a suborbital
shelf, which in cheilodactylids is a prominent feature formed
from the posterior upper margin of the lachrymal bone and
the entire upper margins of the second and third suborbitals.
Also, unlike cheilodactylids, these genera have the basal
scaly sheath to the soft dorsal fin somewhat higher and thus
more prominent than that at the base of the spinous part of
the fin.

As in cheilodactylids, the latrids (here taken to include
Acantholatris and Nemadactylus) have 35 vertebrae (14
abdominal + 21 caudal elements including the urostylar
element; data from radiographs and dry skeletons listed on
p. 2). To judge from the dry skeletal and dissected material
available to me, parapophyses are present on all precaudal
centra in both families, and no ribs are sessile.

Possible lineages within the Latridae as now
expanded to include the genera Acantholatris and
Nemadactylus

Acantholatris and Nemadactylus differ noticeably from Latris,
Latridopsis and Mendosoma in having one of the lower,
unbranched pectoral rays (i.e. the fifth, sixth or seventh ray
from the bottom of the series) greatly elongated, its tip,
which extends beyond the fin’s margin, reaching to at least
the level of the anus and sometimes as far as the midpoint of
the anal fin.
There are also differences in the following features:

(i) In scale size, as shown by lateral-line scale counts. In
Latris, Latridopsis and Mendosoma these range from 112 to
120 in the two former taxa, and from 68-78 in Mendosoma
(data from Last et. al., 1983; Gon & Heemstra, 1987;
pers.obs.). In Nemadactylus macropterus the count is 59 or
60, and in other species 47-68 (pers obs.; Last et. al., 1983)
and in Acantholatris monodactylus, A. bergi and A. gayi the
range is from 50-60 (Norman, 1937; pers. obs.).

(ii) Anal fin length. In Latris, Latridopsis and Mendosoma,
the number of branched anal rays ranges from 17-35 (the
lowest counts occurring in Mendosoma, viz. 17-21, whereas
in Acantholatris species and Nemadactylus macropterus the
range is from 12-15, and other species of the genus, 16-19
(sources as above).

Pending a detailed generic revision of the various taxa
involved, especially the several Australian and New Zealand
species currently referred to the genus Nemadactylus it would
be premature to formally recognise the two groups as, for
instance, tribes or subfamilies of the Latridae, although
phylogenetically some split seems to have occurred within the
lineage.

The condition of the pectoral fin in the Latris-Latridopsis-
Mendosoma group of latrids provides something of a puzzle
since these taxa are the only cirrhitiforms not showing any
marked elongation of the uppermost unbranched ray in the
lower section of the pectoral fin, nor, as in most other

6

cirrhitoids, do any of these rays clearly extend beyond the fin
membrane, and none is markedly thickened. In having the
lowermost 5-9 rays unbranched, these fishes are, however,
typically cirrhitoid. This latter condition can be considered
one of the synapomorphies uniting cirrhitoid fishes.

A typically derived pectoral fin configuration occurs in
the Cirrhitidae, yet the family would appear to be the least
derived of all cirrhitoid taxa (see p. 8). In contrast, except
for the condition of the pectoral fin, members of the
Latris-Latridopsis-Mendosoma assemblage within the
Latridae share with Acantholatris and Nemadactylus, and
with the cheilodactylids, aplodactylids and chironemids,
the derived condition for all the osteological and myologi-
cal features discussed on page 5. That being so, it is
unlikely that the pectoral fin form in Latris, Latridopsis
and Mendosoma can be interpreted as a true retention of
the plesiomorph condition. If that was the case, then the
derived condition must have evolved more than once
within the cirrhitoids. A more parsimonious solution to the
problem therefore, would, be to interpret the pectoral fin
form in Latris, Latridopsis and Mendosoma as a secondary
reversal to a seemingly more plesiomorphic condition than
is found in any other cirrhitiforms, including the family
with the greatest number of plesiomorphic features, the
Cirrhitidae (see p. 8).

The geographical distribution of the two groups within
the Latridae has an interesting pattern. Of the taxa in the
long-finned assemblage, Nemadactylus (see p. 5) occurs
only in Australia, Tasmania and New Zealand, thus over-
lapping the entire range of Latridopsis a member of the
short-finned group and one restricted to that region; it
overlaps in part (New Zealand and Tasmania) that of the
widely distributed Mendosoma lineatum, also a member of
the short-finned group, and in part, that of Latris, another
member of the short-finned group (Australia; New
Zealand; Gough and Tristan da Cunha islands; Vema
Seamount; St Paul and Amsterdam islands). The other
long-finned taxon, Acantholatris, does not occur in Aus-
tralasia, but has a wide western distribution, including St
Paul, Amsterdam, and Gough Islands, Tristan da Cunha,
Vema Seamount Chile, Juan Fernandez and the western
coast of South America from Rio de Janeiro southwards.
This distribution thus widely overlaps that of the short-
finned, monotypic genus Mendosoma lineatum, viz St Paul,
Amsterdam and Gough islands, the coast of Chile and, as
noted above, New Zealand and Tasmania (the latter being
areas where Acantholatris does not occur); data from
Norman, 1937; Fowler, 1945; Mann, 1954; Smith 1984;
Last et. al., 1983; Gon & Heemstra, 1987; Paulin et. al.
1989; Andrew & Hecht, 1992; Andrew, pers.comm., 1993).

TAXONOMIC AND PHYLOGENETIC
CONCLUSIONS

Taxonomy

The material studied indicates, on osteological and myologi-
cal grounds (p. 5), that the species currently named Nema-
dactylus macropterus (the type species of Gill’s (1862) genus
Dactylopagrus; see Wheeler, 1986) should be classified in the
Latridae and not the Cheilodactylidae as it is at present.

P.H. GREENWOOD

The genus Acantholatris Gill (1862, type species Chaetodon
monodactylus Carmichael, 1818), was overlooked by Regan
(1911) in his synoptic review of cirrhitoid families, but is
currently placed in the family Cheilodactylidae (see p. 5).
However, on the basis of its urohyal morphology, and its
lacking a suborbital shelf (see p.5) the genus should be
classified in the Latridae. Regan (1911) differentiated the
Latridae from the Cheilodactylidae on the basis of the latrids
having feeble, unbranched pectoral rays that are not pro-
duced beyond the fin’s margin, and by their lacking a
suborbital shelf; in other feaures he noted that the two
families are similar. With the inclusion of Acantholatris and
Nemadactylus in the Latridae the nature of the pectoral fin no
longer serves as a differentiating feature (see p.5), the
principal diagnostic characters for the family now lying in the
form of the urohyal bone, the absence of a suborbital shelf,
and in the more prominent arrangement of the basal sheath-
ing scales of the soft dorsal fin (see p. 5).

Gill’s (1862) suprageneric classification included Nemadac-
tylus as a division — Nematodactyli — of his subfamily Latridi-
nae, in which subfamily but as another division to which he
gave the name Latrides he also included Latris, Latridopsis,
Mendosoma, Acantholatris, Chirodactylus, Cheilodactylus
and Goniistius. Regan (1911) on the other hand, but without
reference to Gill’s paper, treated the latter author’s four
subfamilies as families, and recognised a fifth, the Cheilodac-
tylidae, for two genera, viz Cheilodactylus and Nemadactylus;
no mention is made in Regan’s paper of the other taxa in
Gill’s Latridinae except for Latris and Mendosoma, which
Regan retained in his family Latridae.

The evidence presented here (pp. 2-5), especially that
based on urohyal morphology, would support Regan’s (1911)
classification with regard to the separation of Cheilodactylus
(and, although not mentioned by Regan, Chirodactylus and
Goniistius) from the other taxa included in Gill’s Latridinae,
and would support the inclusion of all three taxa in one
family, the Cheilodactylidae. The same evidence would also
support Gill’s inclusion of Latris, Latridopsis, Acantholatris,
Mendosoma and Nemadactylus in a single suprageneric
taxon. Since Regan’s (1911) familial ranking has been
accepted and used since that time, and until contraindicative
evidence is available to suggest otherwise, that ranking (i.e.
Latridae) is retained.

The anatomical and other features used in this paper
(pp. 2-5) would support the recognition of Gill’s (1862) and
Regan’s (1911) other suprageneric lineages, again, for the
reasons given above, as families and not subfamilies, viz. the
Cirrhitidae, Aplodactylidae (Gill’s Haplodactylinae) and Chi-
ronemidae.

At an intrafamilial taxonomic level, Allen & Heemstra
(1976) note that ‘The currently accepted classification of the
Cheilodactylidae ... is most unsatisfactory’ a sentiment I
would not only endorse, but would extend to other cirrhiti-
form families as well. In part this situation has resulted from
the use of mainly superficial characters, with little or no
attention paid to anatomical features, especially myological
and osteological ones. Thus on those grounds I cannot agree
with Allen & Heemstra’s (op.cit.) treating Acantholatris as a
subjective synonym of Cheilodactylus and its consequent
placement in the Cheilodactylidae (see above, p. 5). How-
ever, at least on the characters and specimens I have exam-
ined, I would endorse their synonymy of Whitley’s (1957)
genus Morwong (type species Cheilodactylus fuscus Castel-
nau, 1879) with Cheilodactylus.

REVISED CLASSIFICATION FOR CERTAIN CIRRHITOID GENERA i

The cirrhitoids as a monophyletic lineage

On the basis of several apparently synapomorphic character-
istics (see below) the cirrhitoids would seem to be a mono-
phyletic lineage, a conclusion implied by both Gill (1862) and
Regan (1911) who described the group as a ‘natural’ one but
gave no reasons for that conclusion. The derived characters
on which I would base an hypothesised monophyly of the
cirrhitoids are, taken in conjunction, a reduced number (15)
of principal caudal fin rays, the unbranched lowermost five to
nine rays in the pectoral fin (usually with their tips produced
beyond the fin membrane), the lower part of each cleithrum
greatly expanded anteroposteriorly and meeting its antimere
in a deep, carinate symphysis, an increased number of
vertebrae relative to other percoids (26-35, comprising 10-16
abdominal and 15-21 caudal elements), and the presence,
ventrally, in subadults of a peculiar, lipoid-filled sac (Fig. 3),
free from the overlying hypaxial muscles, and extending from
the urohyal, to which it is attached, to the anus, with the
lipoidal material apparently contained in hexagonal compart-
ments.

To the best of my knowledge, this lipoid sac has not previ-
ously been noted as a feature of subadult cirrhitoid fishes, nor
indeed of any other perciforms except the stichaeid Lumpenus
maculatus (see Falk-Petersen et al., 1984). I first observed it in
small specimens (the so-called ‘paperfish’ stage) of Cheilodacty-
lus pixi ca 43 to 44 mm standard length (Fig. 3), where its
presence results in the ‘pouter-pigeon’-like ventral profile of the
paperfish stage in this and other cirrhitoid species (see photo-
graphs in Whitley, 1957; Allen & Heemstra, 1976, and Nielsen,
1963). Subsequent dissections revealed a lipoid sac in members

of all but two of the cirrhitoid families I have dissected (see
p. 2). The exceptions are a chironemid, Chironemus marmora-
tus Gtinther, 1860 (160 mm standard length) and an aplodac-
tylid, Aplodactylus lophodon Ginther, 1859 (180 mm S.L.).
Since, however, the sac is a juvenile (i.e. sub-adult) feature in
the other taxa, and the exceptional specimens were, to judge
from their gonadial development, young adults, I suspect that it
would also be present in smaller specimens of these species. In
Cheilodactylus pixi, for example, the sac is well-developed in a
specimen of 44 mm standard length, but has disappeared in one
of 46 mm. Likewise, in Chirodactylus brachydactylus (Cuv.,
1830), it is present in a fish of 38 mm standard length, but absent
in one of 42 mm. The presence of a lipoid sac in specimens from
128 to 243 mm standard length of other species (referred in the
current literature to the Latridae and Cheilodactylidae) whose
maximum adult lengths are from 50 cm to one metre, suggests
that the size at which the lipoid sac disappears is positively
correlated with that at which members of a species become
adult. This supposition is borne out by the presence of the sac in
a juvenile Chirodactylus grandis (Gunther, 1860) of 57 mm
standard length, a species whose adults reach a length of one
metre, whereas it has disappeared, at a length of 42 mm, in
young Chirodactylus brachydactylus, whose adults reach a
length of 40 cm. Again, it is present in a specimen of Acanthola-
tris monodactylus 243 mm S.L.; adults of this species attain a
standard length of at least 65 cm. Thus, the sac’s apparent
absence in chironemids and aplodactylids could be artefactual,
and related to the size-range of the specimens I was able to
examine.

It is hoped to carry out a more detailed examination of the
lipoid sac when specimens suitably fixed for detailed histo-

Fig. 3 Chirodactylus pixi, 49 mm standard length (RUSI 19842) in right lateral view; partially dissected, and with the greater part of the
pectoral and the entire pelvic fin removed. The large anterior portion of the lipoid sac (LS) is clearly visible; part of its posterior portion is
also visible (x). Throughout its length, the wall of the sac, unlike the muscles above it, is heavily peppered with melanophores.

8

logical and histochemical study are available.

Intragroup relationships within the cirrhitoids,
and the ranking of the group

Within the cirrhitoids, the Cirrhitidae should be ranked as the
most plesiomorphic taxon, a view seemingly implicit in
Regan’s (1911) diagram of relationships. My reasons for
giving the family this ranking are based on urohyal form, the
low vertebral count (26-28) relative to that in other families,
the absence of parapophyses on the first three abdominal
vertebrae, sessile ribs associated with these vertebrae, the
presence of 3 predorsal bones (2 in the other cirrhitoids
radiographed or dissected) and the presence of a suborbital
shelf (which, however, is also developed in the Cheilodactyl-
idae [but see below]). Furthermore, the Cirrhitidae are the
only cirrhitoids with a basal percoid type of myotome
arrangement in the sternohyoideus muscle, a feature not
previously noted. That is, one in which the three pairs of
hypaxial myomeres forming the sternohyoideus muscle are all
arranged in a vertical series, with the first block covering the
anterior part of the urohyal (Fig. 4A). Members of all other
cirrhitoid families, in contrast, have the first (i.e. anterior)
hypaxial myotome of each side displaced ventrally so that it
now lies medially and horizontally (not, as in cirrhitids,
laterally and vertically) to form, with its antimere, a ventral
muscle, paired in most species but in some with the left and
right parts fused over most of their lengths to form a single
median muscle (Fig. 4B). Single or paired, this ventro-medial
component of the sternohyoid runs from the urohyal tip to
the prominent ventral projection at the symphysis of the left

P.H. GREENWOOD

and right cleithra, its origin thus being immediately before
that of the anterior infracarinalis muscle insertion.

In both the Cirrhitidae and in the other families, the
sternohyoideus myotomes, except the horizontal first myo-
tome in the latter group, are chevron-shaped with the apex
directed anteriorly. However, in the latrids, cheilodactylids,
chironemids and aplodactylids, the angle between the upper
and lower arms of the chevron is more acute, and the lower
arm is relatively longer than in cirrhitids.

Judging from the rather scant literature on the sternohyoid
muscle in teleosts (see Winterbottom, 1974), and from a
personal knowledge of the situation in percoids, the condition
of the muscle in the Cirrhitidae should be ranked as plesio-
morphic, that in the other cirrhitoid families as derived and
possibly a unifying synapomorphy for the Aplodactylidae,
Chironemidae, Latridae and Cheilodactylidae within the lin-
eage.

In Regan’s (1911) figure of cirrhitoid intrarelationships
referred to above, the Cirrhitidae occupy a basal (i.e. stem
position) and are linked, on the one hand to the Chironemi-
dae and Aplodactylidae, and on the other to the Cheilodac-
tylidae and Latridae (the generic composition of the families
being those given by Regan). No reasons were provided by
Regan for these supposed relationships, which presumably
were based mainly on superficial characters as well as a few
anatomical ones. With the anatomical information now avail-
able a different scheme of intragroup relationships at the
family level can be proposed (see Fig. 5, and Table 1).

In this scheme, apomorphic features (see Table 1) are
taken to be: (i) The derived form of urohyal, of which there
are three distinct types (see p. 3). (ii) The presence of a

Fig. 4 A. Cirrhitid type organization of sternohyoid
myotomes. Drawn from Paracirrhites forsteri;
semi-schematic. In this arrangement, the
sternohyoid muscles completely envelop the urohyal
bone. B. Basic organization of the sternohyoid
myotomes in all other families. Drawn from
Nemadactylus monopterus; semi-schematic. In taxa
of these families, varying an terior extents of the
urohyal are not covered by the sternohyoid muscle.
1-4: 1st to fourth myotomes of the sternohyoid
muscle; c: cleithrum; ce: cut edges of the sterno-
hyoid myotomes; uh: urohyal. Drawn by Anthea
Ribbink

REVISED CLASSIFICATION FOR CERTAIN CIRRHITOID GENERA

()
C nO”
ee <
~ ?
<s <° <
o co —
1 ais’ 2,18
= +—
| aes

1 or 2,6,8,10,12,14

Fig.5 Tentative scheme of phyletic relationships within the Cirrhitoidei. Asterisks indicate apomorphic characters (see also Table 1). Since
both the cirrhilid (1) and latrid (2) condition of the urohyal are rated as pleisomorphic, that bone in the common ancestor of all lineages is

taken to be 1 or 2.

derived myotomal arrangement of the sternohyoideus
muscles (p. 8). (iii) More than 28 vertebrae. (iv) Parapophy-
ses developed on the first three abdominal vertebrae, with
the first pleural rib associated with the second or third
vertebrae. (v) Predorsal bones reduced to 2. The three
different types of derived urohyal morphology (pp. 2-5) (ie in
chironemids, aplodactylids and cheilodactylids) are each
taken to be independently evolved apomorphies. Relation-
ships (Fig. ) suggested by these data are: (i) That the
citrhitids are the plesiomorph sister group to the other four
families combined. (ii) The chironemids are the sister taxon
of the aplodactylids, latrids and cheilodactylids combined,
and that for the moment this assemblage should be treated as
an unresolved trichotomy since no two lineages uniquely
share a recognisable synapomorphic feature. For example,
the urohyal in the latrids is of a basal percoid type, and
although that bone in the cheilodactylids and aplodactylids is
highly derived, each is unique to the families respectively.
The cheilodactylids it will be noted, retain the plesiomorphic
suborbital shelf, whereas it is lost in the latrids and aplodac-
tylids (and in the chironemid lineage as well). The value of
this feature as an indicator of relationship, however, is
problematical because it involves a loss (and not an acquisi-
tion) in the lineages concerned. Clearly, a greater number
and variety of characters must be sampled and their polarity
determined before this hypothesis of cirrhitoid intrarelation-
ships can be improved and the trichotomy resolved.

The same reservation would apply before any sister-group
hypothesis can be erected regarding the relationships of the
cirrhitoids within the Percomorpha. However, based on the

synapomorphic features discussed (above pp. 6-7) it seems
reasonable to hypothesize that the five families comprising
the lineage, given informal ranking as the ‘cirrhitiform per-
coids’ by Regan (1911), and suprafamilial status by recent
authors (Nelson, 1994), should be elevated to subordinal
status (Cirrhitoidei) within the Perciformes.

ACKNOWLEDGEMENTS. I am particularly indebted to Dr Colin Buxton
of Rhodes University’s Department of Ichthyology and Fisheries
Science who first aroused my interest in the anatomy of cirrhitoid
fishes, and who provided many specimens for dissection. Professor
Tom Hecht of that department is to be thanked for personally
obtaining a specimen of Cheilodactylus fuscus from Salamander Bay,
New South Wales, in which operation he was aided by Bill Talbot of
the Brackish Water Fish Culture Research Station there.

To Dr Phil Heemstra of the J.L.B. Smith Institute goes my
gratitude for the many discussions we have had about cirrhitoids, and
for his advice on the relevant literature, and to Tony Booth of DIFS
my appreciation for his painstaking preparation of several skeletons.
Once again it is a great pleasure to thank my old colleague Gordon
Howes for his elegant and accurate draftmanship, and also to thank
another former colleague, Oliver Crimmen of the Fish Section, The
Natural History Museum, London for his untiring help with matters
radiographic and bibliographical. Yet again, my special thanks go to
Huibre Tomlinson, for her patience and skills in producing the
typescripts, and to Robin Stobbs for producing certain of the
radiographs, and the photograph, used in this paper.

10

Table 1 Data matrix and characters.

P.H. GREENWOOD

Cirrhtidae

Amblycirrhitus pinos + 0 0 0 0
Cirrhitus maculatus + 0 0 0 0
Cirrhitichthys oxycephalus +f 0 0 0 0
Cyprinocirrhites polyactus + 0 0 0 0
Gymunocirrhites arcatus af 0 0 0 0
Oxycirrhites typus ats 0 0 0 0
Paracirrhites forsteri + 0 0 0 0
Chironemidae

Chironemus marmoratus 0 0 0 + 0
Latridae

Acantholatris bergi 0 + 0 0 0
A. gayi 0 2 0 0 0
A. monodactylus 0 se 0 0 0
Latridopsis ciliaris 0 + 0 0 0
Latris lineata 0 at 0 0 0
Mendosoma lineatum 0 +f 0 0 0
Nemadactylus macropterus 0 + 0 0 0
Aplodactylidae

Aplodactylus punctatus 0 0 + 0 0
A. lophodon 0 0 + 0 0
Cheilodactylidae

Cheilodactylus fasciatus 0 0 0 0 +
C. fuscus 0 0 0 0 +
C. pixi 0 0 0 0 +
C. zonatus 0 0 0 0 +
Chirodactylus brachydactylus 0 0 0 0 3f
C. grandis 0 0 0 0 a
Dactylophora nigricans 0 0 0 0 +

oo SSMS) (SYS SK) i=) oe ee ee oe Ores Coe

SSS SSS

15

See] &)
+++4+4+44+
SAA oe)
t+++4+4+4+4+
SSS) (Hea VS)
t+++4+4+4++
S(T VSN}
+++4+4+4++4
Wl) (=)

au
ns
=)
So
+
(>)
a
So
ns

t++tteest
SS) Sn) Sey)
t++++4+4+4
Soveyere eS)
t+++4+4+4+4
SS) Sao eye)
P+ett++tt+t
cocococo
t++++4+4++4

++
oo
++
(==)
+ +
oo
++
oo
++

t+ttt++
SS Sy SSS
t+tt++44+
SoS Sree
t+tt+tt+
SaqeeoeaS
t+tt+t+++
t+tttt+
(yi) (a>) (==)

Characters: 1. Cirrhitid-type urohyal (i.e. of a basal percoid type, but one differing from the latrid condition); 2. Latrid-type urohyal (i.e. of

basal percoid type; see 1); *3. Aplodactylid-type urohyal; *4. Chironemid-type urohyal; *5. Cheilodactylid-type urohyal; 6. Less than 28
vertebrae; *7. More than 28 vertebrae; 8. Parapophyses not developed on the first three (sometimes 4) abdominal vertebrae, but sessile
pleural ribs on one or two of these centra; *9. Parapophyses developed on the first three abdominal vertebrae. No sessile pleural ribs; 1st
pleural rib articulating with the parapophyses of the third abdominal vertebrae; 10. Three predorsal bones; *11. Two predorsal bones; 12.
Basal percoid-type of sternohyoid muscle; *13. Derived condition of the sternohyoid muscle; 14. Suborbital shelf present; *15. Suborbital

shelf absent. Asterisk indicates an apomorphic feature.

REFERENCES

Allen, G.R. & Heemstra, P.C. 1976. Cheilodactylus rubrolabiatus, a new
species of Morwong (Pisces:Cheilodactylidae) from Western Australia, with
a key to the Cheilodactylid fishes of Australia. Records of the Western
Australian Museum, 4(4): 311-325.

Andrew, T.G. & Hecht, T. 1992. Feeding biology of Acantholatris monodacty-
lus (Pisces:Cheilodactylidae) at Tristan da Cunha and Gough Island, South
Atlantic. South African Journal of Antarctic Research, 22: 41-49.

Bloch, M.E. & Schneider, J.G. 1801. Systema ichthyologia. 584 p. Berolini.

Falk-Falk-Petersen, S., Falk-Petersen, I-B, & Sargent, J.R. 1986. Structure and
function of an unusual lipid storage organ in the arctic fish Lumpenus
maculatus Fries. Sarsia, 71: 1-6.

Fowler, H.W. 1945. Fishes of Chile. Systematic catalog. Apartado de la Revista
de Historia Natural, Anos XLV-XLVII: 1-171.

Gill, T.H. 1862. Synopsis of the family of cirrhitoids. Proceedings of the
Academy of Natural Sciences of Philadelphia 14: 102-124.

Gon, O. & Heemstra, P.C. 1987. Mendosoma lineatum Guichenot 1848, first
record in the Atlantic Ocean, with a re-evaluation of the taxonomic status of
other species of the genus Mendosoma (Pisces, Latridae). Cybium 11 (2):
183-193.

Greenwood, P.H. 1976. A review of the family Centropomidae (Pisces,
Perciformes). Bulletin of the British Museum (Natural History) Zoology 29:
1-81.

Kusaka, T. 1974. The urohyal of fishes. xiv + 320 p. University of Tokyo Press,
Tokyo.

Last, P.R., Scott, E.O.G. & Talbot, F.H. 1983. Fishes of Tasmania, viii + 563

p. Tasmanian Fisheries Development Authority, Hobart.

Mann, G. 1954. Vida de los Peces en aguas Chilenas. 342 p. Ministero de
Agricultura and Universidad de Chile, Santiago.

Nelson, J.S. (1994). Fishes of the world; 3nd edition. xvii+600 p. J. Wiley &
Sons, New York.

Nielsen, J.G. 1963. On the development of Cheilodactylus variegatus Valenci-
ennes, 1833 (Cheilodactylidae) Copeia 1963(3): 528-533.

Norman, J.R. 1937. Coast fishes, part II. The Patagonian Region. Discovery
Reports. 16: 1-150.

Paulin, C., Stewart, A., Roberts, C. & McMillan, P. 1989. New Zealand fish, a
complete guide. National Museum of New Zealand, Miscellaneous Series.,
no. 19: xiv + 279.

Regan, C.T. 1911. On the cirrhitiform percoids. Annals and Magazine of
Natural History (8)7: 259-262.

Smith, M.M. 1984. Cheilodactylidae. In: Fishcher, W. & Bianchi, G. (eds).
FAO identification sheets for fishery purposes, western Indian Ocean. F.A.O.
Rome.

Wheeler, A.C. 1986. Catalogue of the natural history drawings commissioned
by Joseph Banks on the Endeavour voyage 1768-1771, held in the British
Museum (Natural History). Part 3: Zoology. Bulletin of the British Museum
(Natural History) Historical Series 13: 1-172.

Whitley, G.P. (1957). Ichthyological illustrations. Proceedings of the Royal
Zoological Society of New South Wales, 1955-56: 56-71.

Winterbottom, R. 1974. A descriptive synonymy of the striated muscles of the
Teleostei. Proceedings of the Academy of Natural Sciences of Philadelphia
125: 225-317.

Bull. nat. Hist. Mus. Lond. (Zool.) 61(1): 11-90

Issued 29 June 1995

Studies on the deep-sea Protobranchia
(Bivalvia); the Subfamily Yoldiellinae

J.A. ALLEN
University Marine Biological Station, Millport, Scotland KA28 0EG

H.L. SANDERS
Woods Hole Oceanographic Institution, Woods Hole, Mass., 02543, U.S.A.

F. HANNAH
University Marine Biological Station, Millport, Scotland KA28 0EG

CONTENTS

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NAG ICIMCMICIISU MEW, SPECIES) agents cen tanaetias« deta + «sais -(smnnenaceanuinans sigur sunaad@etearnssscerh (ques scaeses
Woldellaunconspicua inconspicua Veriill S)BUSHE M898 acc. nccsnccnveesooscsceecacnscusdacnescoucenavecsars
Oldie annGOnSpICHa G/TiCanal(MEWISUDSPECIES)) cee che cisesencaero+taene tee ereeccescn een ancaccesursecesneecersces
Yoldiella inconspicua profundorum (new subspecies) .............2cecseceececeeenececeecneceececeeeeeteneenees
Wo IdrellMarSenilrsensisi (MOWASPEGLES) eartere ee trtee sete oe «ne none dathchnceuseserecagsccnactdapsassrsrcerentocgedceteaer
Yoldiella curta Verrill & Bush, 1898 ........... Bene Pe an ctisncpareceseenseraaecensesesetee caneasdesed aeceaan meet
NOLEN Cr CXCA (ME WISPECIES) 9 1. saactcenedie teeta ceateee .« sscislenece= ce sec dewastectncncese siiadanedawenstenve cde ei
WMOTATCLIGAINETICANIGN (MEW: SPECIES) Me seeee tenn e: seta ace oe no si sacmeptacbne epee sucteeaen te ek Gar aceee eee ccee eee esecenes
Woldieliasvocircularish(@dhners 1960) sxc .ceensteact eee os wai eectaeaneedee ences Seat see cote oe eee ee eee eate aeeeeee re
MOL drel aro reritfatal (DEWISPECIES) eee. <sseciice « «+ rieesicteacsivs <> «nifralsalchiseiien wivuicieasaticus detente «ombeeeicacbeeceeceei te
MOldiella ovata (MEW SPECIES) be asiatencelicts-- ecees scseca cess assacter see swcetessdeaseeetes duceeentd aacebacdemecte doctors
Voldiellanscu) pias Metre ys. A819) x ts see oaign ue sae eacee = «<-> Pee ee ee eae eee ae ees
Woldiellagrerinevsia (iLidal gO aS 1) cx. censuswee s apc astespis+ “cia seltaaatandaan heer tatel-eee dete. see teach eeeeeme ances
Waldrallakenatal (MEW ISPECIES)) cecenteaser tecceutansese vec so- nee AMAR MP AE ee ORR wae eee
Ol diellarellAMMEW, SPECIES) jx snerierecectecniinies ins seianeiscuiaciesiows eo Rien naeasee ats eobae Soadsece gestae. enaecae te saetsh
Woldicllasfabuuld: (Mew SPECIES) Ween. ces ys eis coeiecrace a sioie eles ijesieetalciratiay/seie sonidos eteeaaes-taaleetess Sageeee
WOlGieiaveletial (Me WISPECISS) mera eerste seria ance A «on ws Ride dole pe apeeds okie weleaaes eeradias cee «Paves
Genus Portlandia (Moreh), 1857) se.cs.sscasceseve- tee enhesnssonnnw sot Meee some ce eretece teh aaeatesce oe tacece eee ees
Pparlandialentichian (MOUGK BISA2)) scr anaes cbs cident es os se peeaty eels sare be «cake ca vind bse rads so gaan ansea> =
OTL AMALGN OVGl (ME WESPCCIES) lire ate are lu 91 ete[iaeerts seine inp =16- 2 ce eeeamcnachete a elesedcae ast eee see eet ce Pe cea
Portlan dias tal (DEW {SPEGCIES) Wrest aac alata aise nies seis po siga sac ask laeomiicidee «isis eennguetBececeuaseeaseetes
Pontlandiaabyssorur (Koudsems 19/0) a ceen cecdce dace sr -eceeeaesasac cen seessdasMes asa ee -caceeeceeee fees
I AS CUSSIO Ure e sscea ee eee ale cin Oe cite cele lalla ote oes neu cetgels cielo [ese inlets ofa lao ian taLsnian oe sles inoptestastectes
IU CLE TENGE S MER RRNAE EaE REE Sere oct eee ce see aera heels sald demgnedinacrescidesetenerenchceiececesenad yedeetaecemects

© The Natural History Museum, 1995

12

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Synopsis. Four species of the genus Portlandia and twenty-nine species and five subspecies of the genus Yoldiella
from the deep Atlantic are described — many for the first time. The subfamily Yoldiellinae includes more species
than any other higher taxon of deep-sea protobranchs. The differences in morphology are for the most part subtle
and there are many closely related species. These close relationships have been analysed, the analysis taking into
account shell shape, hinge morphology, musculature and the extent and course taken by the hindgut. Taken
together with geographical and depth distribution a pattern of evolution is derived. This supports the view that the
derivation of the deep-water species of Yoldiella in the Atlantic has been derived mainly via downslope migration
and speciation rather than by deep water migration from the Southern Ocean.

INTRODUCTION

This is the eighth paper in a series on the biology and ecology
of the deep-sea protobranch bivalves of the Atlantic (Allen &
Sanders, 1973, 1982; Allen & Hannah (1989); Rhind & Allen
(1992); Sanders & Allen 1975, 1977, 1985. Our interest is
widespread and includes, ecology and adaptations to life at
great depths, morphology, reproduction, distribution and
evolution. Here we turn to the subfamily Yoldiellinae and
what has become our major and most difficult task of all our
protobranch studies to date. Nowhere have the problems of
elucidating evolutionary trends and specific and subspecific
divisions within the Protobranchia been so acute as in this
large group. Of necessity descriptive and taxonomic studies
have played a major role in all our studies. This is because so
many species from the deep oceans are new. Now that we
have studied more than three quarters of the protobranch
material in our collections, it has become obvious that major
questions on the evolution of the subclass — particularly those
taxa in the deep ocean — remain to be answered. We have
described (Sanders & Allen, 1985) intra- and inter-
population variations in various species and the difficulties in
separating even higher taxa with satisfactory, clear cut,
definitions (Allen & Hannah, 1986). These difficulties can be
no better illustrated than in our studies on the Yoldiellinae. A
considerable effort has been put into the analysis of the most
subtle differences in shape and form of the many species of
the subfamily. As a consequence we have decided to record
our observations in two parts. In the first, here, we describe
36 species in a way that has become standard for this series of
papers, describing those population variations that are perti-
nent to description and taxonomy. In the second, we will
report in detail on diversity and the quantitative aspects of
the ecology of sibling species which are distributed widely in
many Basins of the Atlantic.

Species of the subfamily Yoldiellinae, are among the most
common protobranchs of the deep sea and many are recorded
in the literature. The difficulties we have experienced in
accurately distinguishing the species is not new and confusion
is apparent in both past and recent literature and in museum
identifications of this group.

Descriptions of genera and higher taxa are based on the
recent studies of Shileiko (1985) and Allen & Hannah (1986),
but complemented from the results of this study. Holotypes
have been lodged in either the Natural History Museum,
London, or the Museum National d’histoire Naturelle, Paris.
The paratypes, together with the remainder of the specimens
collected, for the time being are in the care of JAA, but at the
conclusion of the studies will be lodged in appropriate
Museums.

Measurements of height, length, width and postumbonal

length have been taken and in the case of larger samples
ratios have been plotted. For species of which we have few
specimens the measurements have been tabulated. While
these record the variation in the major axes, they do not
measure subtle variation in shell outline and curvature. Much
time has been spent on computerized analysis of shell shape
and on this work we hope to report later but, to date, this has
not improved on visual recognition from comparative accu-
rate drawings. We prefer drawings to photographs for their
clarity.

In recognizing subspecies we comply with the ICZN.
Subspecies occur at different depth ranges and/or different
basins. In a few cases we recognize ‘forms’, infrasubspecific
units which, in compliance with ICZN, cannot clearly be
distinguished in their distribution patterns but which may
indicate a species in the process of subspeciation.

ABBREVIATIONS TO TEXT FIGURES

AA anterior adductor muscle ME mantle edge

AN anus MT major typhlosole

AS anterior sense organ NV nerve

BG ‘byssal’ gland OE oesophagus

CG cerebral ganglion PA posterior adductor muscle
CS combined siphon PG pedal ganglion

DD digestive duct PL palp

EG digestive diverticula PM pallial muscles

DH dorsal hood PP palp proboscis

ES exhalent siphon PR_ pedal retractor muscle
FA feeding aperture PSA posterior sorting area
FM _ pedal muscles RM longitudinal muscle
FT foot SC statocyst

GC gland cells SE siphonal embayment
GD _ duct of gland SF sole of foot

GI gill SS __ style sac

GS | gastric shield ST stomach

HG hind gut SY fold of sensory organ
HT heart TE tentacle

IF inner muscular fold TM transverse muscle

IS__ inhalent siphon TS tooth of gastric shield
LI ligament VG visceral ganglion

Family Nuculanidae Adams & Adams 1858

Shell elongate, usually moderately compressed, may be ros-
trate, shell gape if present, restricted to short posterior
margin where siphons protrude, concentric sculpture usually
present which may be strongly incised, middle and inner shell
layers non-nacrous; teeth chevron-shaped; ligament internal
or external with resilium; combined siphons present, usually
a simple siphonal tentacle attached to the left or right side of
the siphonal embayment.

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

Subfamily Yoldiellinae Allen & Hannah 1986

Shell small, usually compressed, ovate or elongate subovate,
occasionally with ill-defined rostrum, not gaping, smooth, or
very fine concentric sculpture; ligament amphidetic, largely
internal; combined siphons with various degrees of tissue
fusion, siphonal embayment small; hind gut with various
configurations.

Genus Yoldiella Verrill & Bush 1897

TYPE SPECIES. By original designation, Yoldia lucida Lovén,
1846.

Shell small, fragile, usually slender, subovate, usually
glossy, no escutcheon or lunule, no carina, not gaping,
occasionally fine concentric sculpture, postero-ventral margin
may be slightly sinuate, postero-dorsal margin usually slightly
convex, maybe posteriorly angulate; umbo usually anterior
occasionally posterior or central; chevron-shaped hinge teeth
may or may not extend beyond the inner limit of adductor
muscles; no chondrophore; except for very small external
componant, ligament internal and amphidetic, but may
extend anteriorly and/or posteriorly to a small degree, hind-
gut with various configurations, mostly deep-water species
beyond shelf-slope break.

Note. A detailed account of an internal morphology is given under
Y. lata (p. 34).

Yoldiella lucida (Lovén 1846)

TYPE LOCALITY. Hammerfest, northern Norway (desig. A.
Waren, 1989).

TYPE SPECIMEN. Lectotype (desig. A. Warén, 1989), Swed-
ish Museum of Natural History, No. 1533.

Yoldia lucida Lovén 1846, p.34.

18

Fig. 1 Yoldiella lucida. Lateral views of
shells of different size seen from the
right side to show variation in shell
outline. Collected by R.V. Dana 1922; a
& b, 50°20’N 9°00’E, 350 m; c & d,
58°13'N, 9°34’E, 650 m. (Scale = 1.0
mm).

Leda lucida Jeffreys 1869, p. 173. pl.100, Fig. 1; Jeffreys
1879, p. 578;Locard 1898, p. 351, 362.

Portlandia lucida Sars G.O. 1878, p.37. pl. 4, Figs. 8a, 8b;
Norman 1893, p. 364; Posselt 1898, p. 36; Ockelmann
1958, 122, p. 29.

Yoldiella iris Verrill & Bush 1898, 20, p. 863-864, pl. 80. Fig.
1,2, pl. 82, Fig. 11; Type specimen U.S., Natl. Mus.
159722.

Yoldiella lucida Tebble 1966, p. 29, Fig. 156.

MATERIAL.

Cruise Sta Depth No Lat Long Gear Date
(m)

NORWEGIAN BASIN

Thor 273 610
350 50 58°20'0N 99°00'0E

Dana 2896 60 58°13’0N 09°34'0E

NORTH AMERICAN BASIN

Chain 58 105 530 124 39°56'6N 71°03‘6W ET 5.5.66

Chain 88 207 805S— 264 39°51’3W 70°54’3W ES 21.2.69
811 — 39°51'0W =70°56'4W

WEST EUROPEAN BASIN

Incal DS03 609 2 ST°ST'ON 10°43'0W CP 16.7.76

DS04 619 1 57°58'0N 10°43‘0W CP 16.7.76

Museum material examined is listed in the text.

Specimens of Yoldiella lucida have most subtle differences
in shape that taxed the descriptive powers of our predecessors
and as they do ours. Jeffreys (1879) recognized three varieties
(lucida, declivis and truncata) while Locard (1898), accepting
the form figured by Sars (1878) as the type, recognized five
varieties (truncata — the form figured by Jeffreys, intermedia,
minor, depressa, and ventricosa).

Three thousand miles to the west, and in the same year as
Locard, Verrill & Bush (1898) described what they thought to

14

be Y. lucida from the North West Atlantic and two closely
related north-east Atlantic species (Yoldiella iris and Yoldi-
ella inflata).

We have examined material from the Zoological Museum,
Copenhagen, specimens identified by Vermll & Bush, the
Jeffreys collections in London and Washington and Scandinavian
material from a variety of sources which includes specimens
identified by Dr. Kurt Ockelmann in his study of Greenland
material. To prevent even more confusion we accept as our
baseline that specimens from Eastern Arctic and northern
temperate shallow water populations of the east Atlantic adja-
cent to the type locality and which are described in the earliest
accounts, as Yoldiella lucida s.s. We have also examined West
Atlantic material some of which was misidentified and which we
recognize as Y. lucidas.s. namely:-

MCZ No. 137266 labelled Yoldiella inflata, S. Block
Island, 180— 190 m.

MCZ No. 227737 labelled Yoldiella lucida V & B, off
Bradlees Bank, 120 m.

MCZ No. 159722 (unlabelled).

MCZ No. 202847 (which includes specimens from a num-
ber of Stations mixed together).

MCZ No. 78292 labelled Sta. 2697, off Halifax N.S., 377
m.

USNM No. 73172 labelled Y. lucida, 200 m.

USNM No. 202847 labelled Gulf of Maine. Mixture from
several stations.

Specimens misidentified as Y. iris in the U.S. National
Museum, but which are clearly Y. lucida s.s. :-

USNM No. 74517 labelled Gulf of Maine, 172 m.
USNM No. 159717 labelled Gulf of Maine, 40m.
USNM No. 159718 labelled Gulf of Maine, 134 m.

Samples of Y. lucida from the Skaggerak, loaned by the
University Museum, Copenhagen, agree well with the figure
of Sars (1878) and which Locard (1898) considered as the
type (Fig. 1). These are also very similar to the shallow water,
North American specimens referred to as Y. lucida and Y. iris
by Verrill & Bush (1898). These latter specimens are very
slightly more inflated but otherwise identical to the specimens
taken from North American Basin Stations 105 and 207 (Figs.
4&7).

Thus, Y. lucida s.s. is found in Arctic and North Atlantic
waters at shelf and upper slope depths, including the northern
part of North America Basin, off Nova Scotia, Norwegian
Sea, Greenland, Iceland, Skagerak and West European
Basin. Depth range: 38-811 metres.

SHELL DESCRIPTION (Figs. 1-4 & 15, Table 1). Shell elon-
gate, ovate, moderately inflated, inequilateral, irregular con-
centric ridges, partially opaque, umbos moderately large,
inwardly directed, proximal dorsal margin close to umbos
depressed in many but not all specimens, antero-dorsal and
postero-dorsal margins raised to form sharp, low, keel on
either side of umbo, distally antero-dorsal margin extends in
almost straight line to point opposite anterior limit of hinge
plate then curves to the anterior margin, ventral margin long,
even curve, distal postero-dorsal margin slopes in almost
straight line, close to dorsal limit of posterior margin maybe
slightly upturned, limit of posterior margin supramedial,
slightly truncate, postero-ventral margin not sinuous, but
may be slightly flattened; hinge plate moderately broad,

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 2 Yoldiella lucida. Lateral views of shells from off the
northeast coast of America; a, USNM No. 73172, 200 m; b,
USNM No. 74517 labelled Yoldiella iris, Gulf of Maine, 172 m; c,
USNM No. 159717 labelled Yoldiella iris, 40 m. (Scale = 1.0
mm).

except where narrow below umbo, anterior plate elongate
extends opposite anterior margin of adductor muscle, poste-
rior plate relatively short, does not reach posterior margin of
posterior adductor muscle, hinge teeth chevron-shaped, mod-
erately stout, maximum of 14 recorded in each series, occa-
sionally anterior series has one more tooth than posterior;
ligament amphidetic, moderately elongate, anterior and pos-
terior lobes extend ventral to hinge plate.

The above description is of a fully grown animal. In smaller

Fig. 3 Yoldiella lucida. Outline drawings of shells from the right
side from Sta. 105 to show change in shape with increase in size.
(Scale = 1.0 mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

15

Fig. 4 Yoldiella lucida. Hinge plates of a, right valve of a specimen from Sta. 105 (North America Basin); b & c, right and left valve of
specimens from ‘Thor’ Sta. 28 to show variation in the shape of the ligament. (Scales = 0.5 mm).

specimens the antero-dorsal margin is more convex, the
postero-dorsal margin is normally angulate and the posterior
margin more rounded (Fig. 4). As in many species of
Yoldiella older specimens tend to become subrostrate.

Prodissoconch length: 198 »m. Maximum recorded shell
length: 6.9 mm.

INTERNAL MORPHOLOGY. (Figs. 5 & 6) The mantle margin
has a well-developed anterior sense organ. Posteriorly the
siphons are combined, the inhalent siphon being open ven-
trally. A well-developed feeding aperture lies immediately
ventral to the siphons with numerous gland cells present in
what is a broadened region of the inner muscular lobe. A long
siphonal tentacle originates usually on the left side of the
siphonal embayment near the base of the siphon. The adduc-
tor muscles are large and unequal in size. The posterior
muscle is oval in cross-section and between 1/2 and 1/3 the
size of the anterior which is more circular in section.

The visceral and cerebral ganglia are relatively large,
club-shaped, with an exceptionally thick connecting commis-
sure. The pedal ganglia are also large but more round in
outline, and each with a large statocyst dorsal to it. The foot
is anteriorly directed with well-developed pedal retractor
muscles. The byssal gland is moderate in size. The gills are
well-developed with between 16~23 gill filaments, the num-
ber depending on the size of the individual. Posteriorly the
gill axes attach to the junction between the two siphons. The

labial palps are moderately large extending approximately a
third across the body with between 13-23 palp ridges on the
inner face. The palp proboscides are well-developed and are
long and muscular. A wide ciliated oesophagus opens into a
large stomach, the dorsal hood of which lies close to the
dorsal margin of the body. There is a large style sac which
penetrates the lower posterior half of the foot. The hind gut
forms a single loop on the right side of the body. It has a
typhlosole along its entire length. There are two digestive
diverticula to the left and one on the right of the stomach.
Material similar to that present in the stomach was observed
in the left hand digestive diverticulum. The kidney is rela-
tively small in comparison with Yoldiella species from deeper
waters. The sexes are separate, and the gonads overlie the
viscera dorsally and laterally. 203 ova were present in a
specimen of 3.4 mm total length.

Yoldiella obesa obesa (Stimpson 1851)

TYPE LOCALITY. Original not known; Type locality here
designated, St. Georges Bank, U.S. Fish Comm. Sta. 2072,
2.10.1883, 43°53’N, 65°35’W, Beam Trawl, 858 fms.

TYPE SPECIMEN. Holotype believed lost in Chicago fire.
Neotype here selected, USNM No. 38419.

Leda obesa Stimpson 1851, p. 113; Stimpson 1851, p. 10, pt.
II, Fig. 1; Tryon 1873, p. 184, pl. 38, Figs. 500, 501.

16 J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig.5 Yoldiella lucida. Lateral view of the internal morphology from the right side with detail of the posterior part of the mantle cavity and
the partially expanded siphon. (Scales = 0.5 mm).

Yoldia obesa Gould 1870, p. 155, Fig. 463; Verrill 1874, p. 46, Verrill 1874, p. 352, p. 368; Verrill 1874, p. 396.
p. 412, p.503; Smith & Harger 1874, p. 18, p. 23; Yoldia lucida Verrill 1881, pl. XLIV, Fig. 1; Vermill 1884, p. 279
(in part); Verrill 1885, p. 576 (in part); Bush 1893, p. 233.
Yoldiella lucida Verrill & Bush 1897, p. 55. Fig. 14; Verrill &
Bush 1898, p. 861, 862, 863, pl. 77, Fig. 2, pl. 80, Fig. 3.
Yoldiella inflata Verrill & Bush 1897. p. 56. Figs. 3, 4. 11;
Verrill & Bush 1898, p. 864, 865, pl. 80, Fig. 8, pl. 82, Fig.
5G:

MATERIAL.

Cruise Sta. Depth No Lat Long Gear Date
(m)

NORTH AMERICAN BASIN

Atlantis II 62 2496 6 36°26.0'N 70°33.0'W ET 21. 8.64
12 64 2886 8 38°46.0'N 70°06.0'W ES 21. 8.64
72 2864 11 38°16.0'N 71°47.0'W ES 24. 8.64
Chain 58 103 2033 28 39°43.6'N 70°37.4;W ET 4. 5.66
AtlantisII 128 1254 11 39°46.5'N 70°45.2'W ES _ 16.12.66
30 131 2178 51 39°38.5'N 70°36.5'W ES 18.12.66
— 39°39.0’'N 70°37.0'W
Chain88 210 2024 11 39°43.0'N 70°46.0'W ES 22/
Fig. 6 Yoldiella lucida. External view of the stomach as seen from 2064 — 39°43.2’N 70°49.5'W 23. 2.69
a, antero-frontal; b, right lateral; c, left lateral aspects. (Scale =
0.5 mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

50
Dn eee ee
30
80
H/L a ae Mo, ane e erty
e a. Pgh Gen c ou: ss
60 iy

length/total length, W/L width/length — all against length.

Also examined:

U.S. Fish Comm. USNM 73163 labelled Y. /ucida (Lovén),
Cisco Bay 1873 m; Station 2072 USNM 38419 labelled
Yoldiella inflata (young), off Georges Bank 1569 m;
USNM 35189 labelled Yoldiella inflata off Georges Bank
2360 m.

| Restricted to the North American Basin at lower slope
| depths. Depth range 1254-2886 metres.

| SHELL DESCRIPTION (Figs. 8-9 & 15). Shell moderately frag-
| ile, elongate, ovate, with fine concentric ridges, inequilateral;
umbo moderately inflated, anterior, inwardly directed; dorsal
margin in the region of the umbos in all but a few specimens,
slightly concave, dorsal margins form sharp raised edge,
antero-dorsal margin smooth curve joining anterior margin
with no angulation, ventral margin smooth, relatively deep
| curve, postero-dorsal margin in large specimens almost
Straight, subangulate with posterior margin, extreme poste-
rior limit of shell dorsal to horizontal midline; hinge plate
moderately strong, parallel to margin, chevron-shaped teeth
fairly stout, anterior and posterior hinge plates with same
number of teeth, occasionally with one additional tooth on
posterior plate; ligament amphidetic, circular in lateral view,
extends ventral to hinge plate but without chondrophore,
short anterior and posterior secondary external periostracal
extensions.

Prodissoconch length: 215 ~m. Maximum recorded shell
length: 4.45 mm.

)
|

17

. e Oe
e 5 e
e ° fe)
= ie) e . e
e . 1}
os oe Bs ° =
te e
e e
e e

Length (mm)

| Fig. 7 Yoldiella lucida. Comparison of the shell proportions of three populations; small closed circles, Gulf of Maine USNM 202847; large
closed circles, a subsample from ‘Thor’ Sta. 28; open circles, a subsample from Sta. 105. H/L height/length, PL/TL postero-umbonal

Fig. 8 Yoldiella obesa obesa. Neotype: USNM No. 38419. Lateral
view of a right valve and a hinge plate of left valve, also a dorsal
view of the umbos showing the outline of the prodissoconch.
(Scale = 1.0 mm).

18 J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Table 1 Yoldiella lucida; numbers of anterior and posterior hinge teeth in specimens from different localities.

Biscay North Atlantic ‘Thor’ N. European
Length Teeth No. USNM. Length Teeth No. Length Teeth No.
(mm) Ant. Post. No. (mm) Ant. Post (mm) Ant. Post.
INCAL DS 02 2.28 6 6 738419 3.07 8 8 2.50 8 7
INCAL DS 01 2.42 7 7 159718 3) 7) 10 10 ell 8 9
INCAL DS 01 2.75 7 7 74517 3535 11 10 2.91 9 9
INCAL DS 01 2.95 8 8 202847 3.84 11 11 3.44 9 9
INCAL CP 01 3.24 9 9 202847 3.94 11 11 3.49 9 10
INCAL DS 01 3.28 8 8 202847 4.03 11 10 4.31 11 11
INCAL CP 01 3.49 8 8 202847 4.51 12 12 4.39 12 12
INCAL DS 02 3.57 9 9 202847 4.67 11 11 4.67 11 11
POLYGAS
DS 26 3.61 11 11 73172 4.93 12 12 5.02 11 12
INCAL DS 01 Sai, 10 10 202847 5.98 12 14 5.02 14 14
INCAL DS 01 4.31 10 10 202847 6.02 15 14 5.10 13 14
5.10 11 12
5.10 11 12
5.10 13 13
9 12 11
5.61 13 13
Very similar to Y. lucida (for points of difference see also well-developed and provided with numerous gland cells.
p. 19), past records e.g. (Verrill & Bush, 1898; Warén, 1989) The broad and, in preserved specimens, convoluted part of
testify to this. the inner muscular lobe which forms the feeding aperture,

extends anteriorly beyond the limit of the aperture and is

INTERNAL MORPHOLOGY (Fig. 10). The internal morphology
is similar to that of Y. lucida. There is a well-developed
ciliated anterior mantle sense organ. The inhalent and exhal-
ent siphons are combined for most of their length, and the
inhalent siphon is not fused along its ventral margin. The
siphonal tentacle is well-developed and usually attached to
the left of the base of the siphons. The feeding aperture is

Fig. 9 Yoldiella obesa obesa. Lateral views of shells from the right
side to show change in shape with increasing size and detail of
hinge plate. Specimens from Sta. 62 North America Basin. Note
adductor muscles and hind gut loop seen through the transparent Fig. 10 Yoldiella obesa obesa. Lateral views of the internal
shell. (Scale = 1.0 mm). morphology from right and left side. (Scale = 1.0 mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

heavily ciliated throughout. The adductor muscles are rela-
tively large and unequal in size. The posterior muscle is oval
in outline and is approximately 1/2 to 1/3 the size of the
anterior muscle.

The gills are well-developed with up to 16 gill filaments
depending on the size of the individual. The number of ridges
on the inner surface of the labial palp varies between 12-17.
The palps extend over approximately half the body width and
have long, moderately thick palp proboscides. The visceral
and cerebral ganglia are elongate and each large pedal

_ ganglia has a large statocyst dorsal to it. The foot is large and
directed anteriorly, with well-developed retractor muscles.
| There is a moderately large byssal gland in the heel. The
_ oesphagus is wide and opens into a voluminous stomach. The
' style sac penetrates into the lower portion of the foot. The
hind gut forms a single loop on the right side of the body and
| has a typhlosole along its entire length. The digestive diver-
_ticula, as in other species, are situated one to the right and
_ two to the left of the stomach. The ducts to the left hand
| diverticula enter the stomach close together, ventral to the
| gastric shield one posterior to the other. The kidney is small.
| Gonads overlie the lateral and dorsal sides of the viscera and
| the sexes are separate.

| Y. obesa s.s. and Y. lucida differ as follows:- (1) The
ventral margin of Y. obesa is slightly deeper and more
|} rounded. (2) The anterior adductor of Y. obesa is relatively
| smaller when compared with a specimen of Y. lucida the
same size. (3) The internal ligament of Y. obesa is smaller,
| shorter and more rounded. (4) The umbo is more anterior in
|Y. obesa and slightly larger and more prominent. (5) The
| posterior margin is more rostrate and tapered in Y. obesa and

Fig. 11 Yoldiella obesa incala. Lateral view of shells from the right
side to show change in shape with increasing size, and detail of
the hinge plate of a right valve. Specimens from Incal Sta. DS02.
(Scales = 1.0 mm).

19
less truncate. (6) The posterior section of the loop of the hind
gut is less curved and almost vertical in Y. obesa.

Yoldiella obesa incala (new subspecies)

TYPE LOCALITY. R.V. Jean Charcot, Cruise INCAL, Sta.
DS01, East of Rockall Island, 15.7.1976, 57°59’N, 10°40’W,
Epibenthic Trawl, 2091 m.

TYPE SPECIMEN. Holotype: Museum National d’Histoire
Naturelle, Paris, Paratypes: in collection held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
WEST EUROPEAN BASIN
Jean Charcot DS25 2096 3 44°908.2’N 4°15.7'W DS. 1.11.71
(Polygas) DS26 2076 3 44°908.2'N 4°15.0’'W DS 1.11.71
Jean Charcot DS51 2430 1 44°01rS'N, | 4215S YW & IDS 125, 2274
(Biogas IV)
(Biogas V) CP07 2170 4 44°09.8’N 4°16.4';W CP 21. 6.74
(Biogas VI) DS80 4120 2 46°29.5'N 10°29.5'°W DS _ 27.10.74
DS86 1950 7 44°04.8'N 4°18.7'W DS _ 31.10.74
DS87 1913 5 44°05.2'N 4°19.4';W DS __ 1.11.74
(Incal) DSO1 2091 518 57°59.0'N 10°40.0'W DS _ 15. 7.76
DS02 2081 452 57°58.0'N 10°49.0'W DS 16. 7.76
CPO01 2068— 35 57°57.0'N 10°43.0'W CP 16. 7.76
2040
CP02 2091 87 57°580'N) 1094310" W "CR 16: 7276

Restricted to the West European Basin at lower slope depths.
Depth range: 1913-2170 mm.

SHELL DESCRIPTION (Figs. 11 & 12). Y. obesa incala is similar
to Y. obesa s.s. and requires little description other than to
identify points of differences with the North American sub-
species.

50

LE SE ae one
* ‘
‘80

H/L Os . eee erate

2
Length(mm)

Fig. 12 Yoldiella obesa incala. Variation in the ratios of height
HI/L, width W/L and postero-umbonal length PL/TL to length
against length of a sample from Incal Sta. CP01 from the Bay of
Biscay.

20

The subspecies has been long confused with Y. lucida s.s.
(Warén, 1989) and there is little doubt that records of the
latter species from deeper than 1000 m are of Y. obesa incala.
In large specimens in particular, elongation and narrowing of
the posterior margin together with a slightly upturned distal
end of the postero-dorsal margin, resembles Y. lucida. A
population from Rockall although similar in outline is rela-
tively smaller and less inflated than the Biscay populations.

Prodissoconch length: 187-208 »m. Maximum recorded
shell length: 4.8 mm.

INTERNAL MORPHOLOGY. Mantle structures are similar to
those in Y. obesa s.s., for example the ventral margin of the
inhalent siphon is open ventrally, however the feeding aper-
ture is not particularly well-developed. The posterior adduc-
tor muscle is oval in shape and half the size of the crescent-
shaped anterior muscle. There is a large stomach and style
sac, and a single loop of the hind gut on the right side of the
body. The byssus gland is moderately small. The palp probos-
cides are long and palp ridges number up to 21. The gills are
well-developed with up to 17 plates present.

Points of distinction are as follows:- (1) The umbo in Y.
obesa incala is slightly more medial than in Y. obesa s.s. (2)
The posterior dorsal section of the hind gut loop takes a
slightly more anterior course than in Y. obesa s.s. (3) The
anterior adductor is relatively more ventral in position than in
Y. obesa s.s. (4) Although the ligament is relatively short
and, in many specimens rounded as it is in the North
American subspecies, in some specimens the anterior and
posterior ends are slightly swollen giving a ‘dumb bell’
appearance.

In a specimen 3.03 mm length, 281 ova were counted, with
a maximum diameter 119 ym. In two specimens less mature,
240 and 387 ova were present in animals measuring 3.06 mm
and 4.23 mm respectively.

Fig. 13 Yoldiella similirus. Lateral views of shells from the right
side and a hinge plate of a left valve. Specimens from Sta. 236
Argentine Basin. (Scale = 1.0 mm).

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Yoldiella similiris (new species)

TYPE LOCALITY. R.V. Atlantis II, Cruise 60. Sta. 236,
Argentine Basin, 11.3.1971, 36°27.0’S, 53°31.0’W-36°28.1'S,
53°32.3’W, Epibenthic Trawl, 409-518 m.

TYPE SPECIMEN. Holotype: BM(NH) 1992028. Paratypes: in
collection held by J.A. Allen.

MATERIAL.
Cruise Sta’ Depth No Lat Long Gear Date
(m)
ARGENTINE BASIN
AtlantisII 236 409- 86 36°27.0'S 53°31.0'W ES 11.3.71
60 518 — 36°28.1'S 53°32.3’W
237 993— Si) 360322618) 55 280 Wee Ese lino
1011

Restricted to shallow slope depths in the Argentine Basin.
Depth range: 497-1011 metres.

SHELL DESCRIPTION (Figs. 13, 15-17). Shell small, subovate,
moderately inflated, smooth, fine irregular concentric lines,
inequilateral; umbos small, anterior of midline, inwardly
directed; antero-dorsal margin convex, curves evenly to ante-
rior margin, ventral margin convexity increases slightly poste-
riorly, posterior margin obliquely subtruncate, postero-dorsal
margin approximately straight, gently sloping, posteriorly —
forming a blunt angle, dorsal margins on either side of umbo
form an acute edge; hinge plate well-developed, anterior
slightly arched, posterior almost straight, 11 anterior and 12
posterior strong angular teeth in individual 3.75 mm long;
ligament amphidetic, small secondary external periostracal
extensions on either side of umbo, internal primary ligament
moderately large, extends below hinge line.

Prodissoconch length: 198 1m. Maximum recorded shell
length: 3.75 mm.

The shell and internal morphology of Y. similiris is very
similar to that of Y. lucida s.s. The following differences are
noted:- The umbo of Y. similiris is slightly more anterior, this

Fig. 14 Yoldiella similiris. Lateral view of internal morphology
from the right side. (Scale = 1.0 mm). For identification of parts
see Fig. 5.

DEEP-SEA PROTOBRANCHIA (BIVALVIA) 21

Fig. 15 Yoldiella spp. Dorsal
views of shells of a, Y. obesa
(eo obesa; b, Y. similiris and c, Y.
lucida. (Scale = 1.0 mm).

is particularly noticeable in the larger specimens. The height
to length ratio is greater in Y. similiris.

INTERNAL MORPHOLOGY (Fig. 14). The internal morphology
is very similar to that of Y. /ucida. The adductor muscles are
unequal in size, the posterior muscle is oval and approxi-
mately 1/3 the size of the anterior. There are a smaller
number of gill filaments (maximum of 11 observed) and the
labial palp ridges vary between 11-13. The stomach and style
sac appear relatively smaller than those in Y. /ucida and the
hind gut, while making a single loop on the right side of the
body, is not as deep as it is in the latter species. A maturing
female (3.4 mm total length) had 203 ova with a maximum

50

WL: .’ 4 <7 ale tess :
30
Be ° 28

H/L ap RS oo.

Length (mm)

Fig. 16 Yoldiella similiris. Lateral views of a series of shells to
show changes in shape with growth. Specimens from Sta. 236
Argentine Basin. (Scale = 1.0 mm).

Fig. 17 Yoldiella similiris. Variation in the ratios of height H/L,
width W/L and postero-umbonal length to length against length of
a subsample from Sta. 236. Argentine Basin.

22

diameter of 83 wm (probably less than half their eventual
maximum size).

Yoldiella hanna (new species)

TYPE LOCALITY. R.V. Atlantis II Cruise 42, Sta. 186, Cape
Basin, 15.5.1968, 22°57’S, 13°05’E, Epibenthic Trawl,
439-481 m.

TYPE SPECIMEN. Holotype BM(NH) 1992022, Paratypes: in
collection held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
CAPE BASIN
AtlantisII 186 439- 21 22°57.0'S 13°05.0’'E ES 16.5.68
42 481
188 619- 7 23°00.0'S 12°58.0'E ES 16.5.68
622

The species is restricted to the upper slope in the Cape Basin.
Depth range: 439-622 metres.

SHELL DESCRIPTION (Fig. 18). Shell small, ovate, relatively
inflated, inequilateral (posterior umbonal length 52-57% of the
total length), sculpture, fine concentric lines at ventral margin,
pale straw-coloured periostracum; umbos slightly raised,
inwardly directed; dorsal margin slightly convex, antero-dorsal

Fig. 18 Yoldiella hanna. Lateral views from the right side of three
shells and an internal view of a right valve to show detail of the
hinge plate. Specimens from Sta. 186 Cape Basin. (Scale = 1.0
mm).

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 19 Yoldiella hanna. Lateral view from right side of the
internal morphology. Specimen from Sta. 186 Cape Basin. (Scale
= 1.0 mm). For identification of parts see Fig. 5.

margin slopes gradually and smoothly to anterior margin, dorsal
part of which more convex, ventral margin long, smooth, curve,
postero-dorsal margin almost straight maybe slightly concave,
slopes gently to blunt-angled supramedial posterior margin;
hinge plate very strong, wide, except centrally under umbo; 10
strong chevron-shaped teeth on each side of ligament in largest
specimen; ligament amphidetic, goblet-shaped, extends below
margin of hinge plate.

Prodissoconch length: 200 4m. Maximum recorded shell —
length: 3.2 mm.

INTERNAL MORPHOLOGY (Fig. 19). Well-developed combined
siphons are present with a well-developed feeding aperture
ventral to them. The siphonal tentacle lies to the left. The
posterior adductor muscle is very small and elongate oval in
cross-section. The anterior adductor muscle is crescent-shaped

60 °.

Length (mm)

Fig. 20 Yoldiella spp. Comparison of the shell proportions of Y.
hanna (closed circles) and Y. artipica (open circles). Height H/L,
width W/L and postero-umbonal length PL/TL to length against
length.

|

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

and at least three times as large as the posterior. The foot is
large, anteriorly directed and with a large byssal gland. The
cerebral and visceral ganglia are relatively large, club-shaped
with a stout commissure. The pedal ganglia are also large but

| more rounded. The labial palps are well-developed and extend
across approximately half of the body and have up to 14 broad
_ internal ridges present on their inner face. The palp proboscides

are moderately long. The gills are small with up to 18 alternating

filaments.

The stomach is large and laterally distended. The proximal

| part of the hind gut penetrates deep into the foot, ventral to
_ the pedal ganglia. It forms a single loop on the right side of
_ the body.

In external appearance the shells of the species can be

| easily confused with Yoldiella artipica (p. 25) although Y.

hanna is slightly more inflated and the height/length ratio is

| slightly greater (Fig. 20). Nevertheless, hinge and teeth differ
| markedly, as does the hind gut configuration.

ny oldiella capensis (new species)

| TyPE LOCALITY. R.V. Atlantis II, Cruise 42, Sta. 189, Cape
| Basin,
1007-1014 m.

| TYPE SPECIMEN. Holotype BM(NH) 1992026, Paratypes: in
| collection held by J.A. Allen.

15.5.1968, 23°00'S, 12°45’E, Epibenthic Trawl,

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
CAPE BASIN
| AtlantisII 188 619 70 23°00.0'S 12°58.0'E ES 16.5.68
42 622
189 1007— 918 23°00.0'S 12°45.0'E ES 15.5.68
1014
190 974 15 23°05.0'S 12°45.0°'E AD 17.5.68
979

| Only found at slope depths of the Cape Basin. Depth range:

619-1014 m.

SHELL DESCRIPTION (Figs. 21 & 22). Shell, subovate, moder-
ately inflated, inequilateral, posteriorly somewhat wedge-
shaped, smooth with a few fine concentric lines, periostracum
pale straw-coloured; umbos slightly raised, inwardly directed;
antero-dorsal margin convex, slopes steeply from umbo to
anterior margin, dorsally posterior margin produced into
rounded point, postero-dorsal margin long, varying from
slightly convex in smaller specimens (usually) to slightly
concave, slopes gradually to posterior margin, hinge plate
strong, relatively long, plates approach shell margin below
umbo, anterior plate arched with up to 9 erect chevron teeth,
posterior plate extends to anterior margin of adductor,
Straighter than anterior, with up to 11 teeth and does not
extend beyond posterior margin of adductor; ligament
amphidetic, bilobed in lateral view, extends below hinge line.

Prodissoconch length: 170 1m. Maximum recorded shell
length: 8.0 mm.

With increasing size this species becomes more posteriorly
elongate and the distal posterior dorsal limit more pointed
(Figs. 21 & 22). Furthermore the posterior dorsal margin
becomes less curved with increasing size, so much so that

23

Fig. 21 Yoldiella capensis. Lateral view from the left side of the
largest shell taken from Sta. 189 Cape Basin and hinge detail of
right valve of the same specimen. (Scale = 1.0 mm).

Fig. 22 Yoldiella capensis. Lateral views of shells from the right
side to show change in shape with growth. Specimens from Sta.
188 Cape Basin. (Scale = 1.0 mm).

without a size series it would be difficult to equate small
specimens with large. Other shell ratios (H/L and W/L)
remain more or less constant as length increases (Fig. 23).

INTERNAL MORPHOLOGY (Fig. 24). Specialization of the
mantle includes a well-developed anterior sense organ and
combined exhalent and inhalent siphons. The latter are
well-developed with thick muscular walls. A siphonal tentacle
lies to the left of the siphons. There is a feeding aperture
ventral to the siphons which is much folded in the contracted
state. The adductor muscles are relatively small. The poste-
rior muscle is narrow and elongate, while the anterior is 2 to 3
times larger and crescent-shaped.

The gills are well-developed with up to 20 plates. The labial
palps are moderate in size. They extend between 1/4 — 1/3
distance across the body and have long and slender palp
proboscides. The nervous system is well developed with large
club-shaped visceral and cerebral ganglia and massive com-
missures which link them. Large, round, pedal ganglia each
have a large statocyst full of refractile granules dorsal to

24

WIL

1 2 3
Length(mm)

Fig. 23 Yoldiella capensis. Variation in the ratios of height H/L,
widthW/L, and postero-umbonal length PL/TL to length against
length of a subsample of specimens from Sta. 188 Cape Basin.

Fig. 24 Yoldiella capensis. Lateral view from the right side of the
internal morphology. Specimen from Sta. 188, Cape Basin. (Scale
= 1.0 mm). For identification of the parts see Fig. 5.

them. The foot is large, well-developed with an anteriorly
directed attitude. The stomach and style sac are also large.
The hind gut forms a single loop on the right side of the body.
This species bears some resemblance to Yoldiella lucida and
to Y. bilanta (Fig. 137).

Yoldiella bilanta (new species)

TYPE LOCALITY. R.V. Atlantis II, Cruise 42, Sta. 192, Cape
Basin, 17.5.1968, 23°05.0'S, 12°31.5'E, Epibenthic Trawl,
2117-2154 m.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH ;

TYPE SPECIMEN. Holotype: BM(NH) 1992027, Paratypes: in
collection held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
CAPE VERDE BASIN
AtlantisII 138 1944— 1 10°36.0’N 17°52.0'W ES _ 4.2.67
31 1976
14i1ee 2131 2 10°30.0'N 17°51.5'W ES _ 5.2.67
142 1624 1 10°32.0’N 17°51.5'W ES _ 5.2.67
1796
144 2051— 22 10°36.0'N 17°49.0'W ES 5.2.67
2357
CAPE BASIN
AtlantisII 191 1546— 1025 23°05.3’S 12°31.5'E ES 17.5.68
42 1559
192 2117— 1697 23°02.0'S 12°19.0'E ES 17.5.68

2154

Found in the Cape Verde and Cape Basins at lower slope
depths. Depth range: 1446-2357 metres.

SHELL DESCRIPTION (Fig. 25). Shell slender, elongate ovate,
inequilateral, smooth with few fine concentric lines, perios-
tracum pale straw-coloured; umbos anterior to midline,
slightly inflated, orthogyrate; antero-dorsal margin moderat-
ley convex, curves evenly to anterior margin, ventral margin

long, smooth curve, anterior curvature slightly more convex ~

than posterior, posterior margin narrow, slightly produced
with supramedial rounded angle, postero-dorsal margin
slopes gradually more or less straight or slightly concave from
umbo to posterior limit of hinge plate, then more acutely to
posterior margin; hinge plate, strong, long, relatively narrow
and straight with 9 anterior and 10 posterior teeth in largest
specimen; anterior and posterior plates approach margin
below umbo; ligament amphidetic, bilobed goblet-shaped,
extends below hinge line, short anterior and posterior exter-
nal extensions of fused periostracum.

Prodissoconch length c 190 ~m: Maximum recorded shell
length: 8.0 mm.

The morphology of the shell is similar to that of Yoldiella
capensis (Fig. 21). With increasing size the shell becomes
more posteriorly elongate and the postero-dorsal margin
becomes more straight so that the slight angulation at the
level of the limit of the posterior hinge margin becomes much
less obvious (Figs. 25 & 26). There is little change with
growth in the height and length and width to length ratios.
The hinge plate is more narrow and teeth less robust than in
Y. capensis and the lateral ‘dumb bell’ outline of the ligament
is very different from the more rectangular and deeper
ligament of the latter species.

INTERNAL MORPHOLOGY (Fig. 27). The most conspicuous
difference from Y. capensis is the size and shape of the
adductor muscles which are larger and more rounded in
Yoldiella bilanta The labial palps are relatively small and
extend approximately 1/3 distance across body. They bear up
to 16 palp ridges. The gill plates are relatively narrow and
number up to 18. The single loop of the hind gut has a greater
diameter but is not as ventrally deep as that in Yoldiella
capensis. Similar features to Yoldiella capensis include well-
developed combined siphons, nervous system, and a large,

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

1 2 3
Length(mm)

anteriorly dirécted foot. The lateral papillae of the foot are
not so well-developed as those in Y. capensis.

The specimens from the Cape Verde Basin are somewhat
| smaller and have a slightly deeper shell than those from the
| Cape Basin. There appear also slight differences in the size of
| the posterior adductor muscle and the gills, but these may be
|a result of distortion following preservation. Such small
| differences are within the normal variation of yoldiellid
: species and at most represent a population difference.

Yoldiella artipica (new species)

TYPE LOCALITY. R.V. Atlantis II, Cruise 42, Sta. 200,
Angola Basin, 22.5.1968, 9°43.5’S, 10°57’0E — 9°41.0’S —
10°55.0’E, Epibenthic Dredge, 2644-2754 m.

25

Fig. 25 Yoldiella bilanta. Lateral views
of three shells from the right side and
internal view of a left valve to show the
hinge-plate. Specimens from Sta. 192
Cape Basin. (Scale = 1.0 mm).

Fig. 26 Yoldiella bilanta. Variation in
the ratios of height H/L, width W/L,
and postero-umbonal length PL/TL to
length against length of samples from
Sta. 191 (open triangles) and Sta. 192
(closed circles) Cape Basin.

TYPE SPECIMEN. : Holotype: BM(NH) 1992021, Paratypes:
in collection held by J.A. Allen.

MATERIAL.
Cruise Sta. Depth No Lat Long Gear Date
(m)
ANGOLA BASIN
Atlantis II 200 2644- 25 9°43.5’S 10°S7.0'E ES 22.5.68
42 2754 — 9°41.0'S 10°55.0'E
Walda DS20 2514 2 2°32.0'S 8'18.1'E ES --.71

Restricted to the abyssal rise of the south east Atlantic.
Depth range: 2514-2754 m.

26

Fig. 27 Yoldiella bilanta. Lateral view of the internal morphology
from the right side of a specimen from Sta. 192 Cape Basin.
(Scale 1.0 mm). For identification of parts see Fig. 5.

SHELL DESCRIPTION (Fig. 28). Shell small, ovate, not
inflated, inequilateral (posterior umbonal length 54-58% of
total length), in larger specimens antero-ventrally, very fine
concentric lines form broadly spaced ridges, periostracum
pale yellow, umbo slightly raised, inwardly directed; dorsal
margin slightly convex, antero-dorsal margin curves moder-
ately steeply and evenly with anterior margin, ventral margin
smooth curve, postero-dorsal margin slopes gently from
umbo to extended posterior margin; ligament amphidetic,
small, rounded, not extending below hinge plate, with small
secondary anterior and posterior external extensions of fused
periostracum ; hinge plate moderately long, relatively nar-
row, small chevron-shaped teeth, 7 anterior and 8 posterior.

Prodissoconch length: 200 1m. Maximum recorded shell
length: 2.78 mm.

Shell measurements (mm) & ratios are as follows:-

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Length Height Width H/L W/L PL/TL
2.00 1.46 - 0.73 - 0.54
2.69 1.89 - 0.70 - 0.56
2.78 1.98 - 0.71 ~ 0.58
1.64 1.13 0.53 0.69 0.33 0.54
223 ils) 0.84 0.70 0.38 0.57
2.60 1.80 1.01 0.69 0.38 0.55
PAV 1.76 1.01 0.70 0.40 0.57

PL = postumbonal length

INTERNAL MORPHOLOGY (Fig. 29). The combined exhalent
and inhalent siphons are large. The inhalent is somewhat
shorter than the exhalent and open ventrally. A large sipho-
nal tentacle originates on the left side of the siphonal embay-
ment close to the base of the siphon. The feeding aperture is
not particularly well-developed. There is a large anterior
sense organ. The posterior adductor muscle is long and
narrow, and only half the size of the bean-shaped anterior
muscle. The gills are well-developed with up to 20 gill plates.
The labial palps are relatively small with up to 12 internal
palp ridges with long, thin palp proboscides. The foot is
extremely long and slender with an extended sole fringed
with deep papillae. There is a large byssal gland in the heel
with large pedal ganglia dorsal to it. The visceral and cerebral
ganglia are of moderate size and club-shaped. The hind gut is
similar to that in Yoldiella lata (p. 32) with one complete turn
of a double loop to the right side. That in Y. artipica entends
further posteriorly than that in Y. /ata. As in many yoldiellid
species the body wall enclosing the hind gut loops overhangs
the palps on the right side (Fig. 29).

Yoldiella similis (new species)

TYPE LOCALITY. R.V. Atlantis II, Cruise 42, Sta. 197,
Angola Basin, 21.5.1968, 10°24’S, 9°09’E — 10°29’S, 9°04’E,
Epibenthic Trawl, 4559-4566 m.

Fig. 28 Yoldiella artipica. A dorsal and two right
lateral views of shells, and one lateral view of a left
valve to show detail of hinge-plate. Specimens from Sta.
200 Angola Basin. (Scale = 1.0 mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

27

Fig. 29 Yoldiella artipica. Lateral view from the right side of the
interal morphology of a specimen from Sta. 200 Angola Basin.
(Scale = 1.0 mm). For identification of parts see Fig. 5.

TYPE SPECIMEN. Holotype: BM(NH) 1992023, Paratypes: in
collection held by J.A. Allen.

MATERIAL
| Cruise Sta Depth No Lat Long Gear Date
(m)
_ ANGOLA BASIN
| AtlantisIT 196 4612- 1 10°19.0’S 9°04.0'E ES 21.5.68
42 4630
197954596, 35) 10229.0'S_ -9°04:0'E «ES. 21.568
198 4559- 12 10°24.0'S 9°09.0’E ES 21.5.68
4566 — 10°29.0’'S 9°04.0’E

| Found at abyssal depths in the Angola Basin. Depth range:
4559-4630 m.

SHELL DESCRIPTION (Fig. 30). Shell small, ovate, very
_ slightly inflated, inequilateral with posterior margin suprame-
| dially subrostrate, shell with very fine concentric lines, form-
ing ridging on ventral half of shell; periostracum pale yellow;
umbos slightly raised, internally directed; dorsal margin
| slightly convex, antero-dorsal margin slopes from umbo to
form even curve with anterior margin, ventral margin
| smoothly curved, posterior margin sharply curved where it
| meets postero-dorsal margin; hinge strong, moderately
broad, extends to anterior and posterior limits of the anterior
| and posterior adductors respectively; hinge teeth well-
| developed, equal numbers (maximum 9) on anterior and
posterior hinge plates, ligament amphidetic, moderately
large, goblet-shaped with very small external extensions.

Prodissoconch length: 173 wm. Maximum recorded shell
length: 2.92 mm.

The shell outline of this species is similar to that of
specimens of Y. jeffreysi from the Cape Verde Basin. The two
species can be distinguished by the larger ligament, broader
hinge and more rostrate posterior margin of Y. similis.

Fig. 30 Yoldiella similis. Lateral view of a shell from right side and
a hinge-plate of a left valve compared with the hinge-plate of a
specimen of Y. jeffreysi (a) Specimens from Sta. 197, Angola
Basin and Sta. 316, West European Basin, respectively. (Scale =
1.0 mm).

Shell measurements (mm) and ratios are as follows:-

Length Height Width H/L W/L PL/TL
1.0 0.71 0.78 0.71 0.38 0.42
1.93 1.39 0.80 0.72 0.41 0.50
1.72 1.43 0.80 0.83 0.46 0.54
2.90 2.06 1.39 0.71 0.48 0.55

PL = postumbonal length

INTERNAL MORPHOLOGY (Fig. 31). The internal morphology
of Y. similis is very similar to that of Y. jeffreysi. The exhalent
siphon is combined with the less well-developed inhalent
siphon. The adductor muscles are unequal in size, the ante-
rior being approximately three times larger than the poste-
rior. In lateral view the posterior adductor muscle is oval in
outline while the anterior is ‘bean-shaped’. The foot has a
long, extended, narrow sole with a large byssal gland in the
heel. The nervous system is extremely well-developed with
large visceral and cerebral ganglia with short stout commis-
sures joining them. Gills appear to have relatively few plates
(up to 10) and the number of palp ridges is also few (up to 12)
but the latter are relatively broad. This species has a large
stomach and the hind gut makes one complete double coil to
the right side of the body.

Yoldiella sinuosa (new species)

TYPE LOCALITY. R.V. Knorr, Cruise 25, Sta. 299, Surinam
Basin, 29.2.1972, 7°55.1'N, 55°42.0'W, Epibenthic Trawl,
1942-2076 m.

28

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 31 Yoldiella similis. Lateral view from the right side of the
internal morphology of a specimen from Sta. 197. Angola Basin.
(Scale = 1.0 mm). For identification of parts see Fig. 5.

TYPE SPECIMEN. Holotype: BM(NH) 1992025, Paratypes: in
collection held by J.A. Allen.

MATERIAL.

Cruise Sta Depth No Lat Long Gear Date
(m)

SURINAM BASIN

Knorr 25 299 1942- 54 7°55.1’N 55°42.0'W ES 29.2.72

2076

Restricted to one Station on the lower slope of the Surinam
Basin. Depth range: 1942-2076 m.

SHELL DESCRIPTION (Figs. 32 & 33). Shell small, ovate,
moderately inflated, slightly inequilateral, surface smooth in
small individuals, ridges on ventral half of shell of larger
specimens, periostracum pale yellow; umbo small, slightly
raised, internally directed; no lunule or escutcheon; dorsal
margin convex, anterior and posterior margins slope at
similar angle from umbo, anterior margin smooth curve,
posterior margin slight extended and slightly subrostrate,
postero-dorsal margin slightly angled at posterior limit of
hinge plate, postero-ventral margin sinuous, ventral margin
slightly more convex posteriorly; hinge plate strong, rela-
tively long, with up to 9 anterior and 10 posterior chevron-
shaped teeth, hinge plate narrow below umbo; ligament
amphidetic, internal, ‘goblet-shaped’.

With increasing shell length, there is little change in the
height to length ratio but there is a slight increase in the width
to length ratio. There is also gradual post-umbonal extension
with the sinuous nature of the postero-ventral margin becom-
ing more conspicuous (Fig. 35).

Prodissoconch length: 179 4m. Maximum recorded shell
length: 3.69 mm.

INTERNAL MORPHOLOGY (Fig. 34). The combined siphons,
the siphonal tentacle (usually on the left side), and the
anterior sense organ are as in Y. similis. The adductor

Fig. 32 Yoldiella sinuosa. Lateral view of a shell from the right
side and detail of the hinge-plate of a right valve. Specimens from
Sta. 299 Guyana Basin. (Scale = 1.0 mm).

Fig. 33 Yoldiella sinuosa. Outline drawings of shells of different
sizes from the right side to show change in shape with growth.
Specimens from Sta. 299 Guyana Basin. (Scale = 1.0 mm).

muscles are approximately oval, the anterior muscle being
the larger and being almost twice the size of the posterior.
The gills have up to 17 plates. The labial palps are moderate
in size and extend 1/3 way across the body. They have up to
15 moderately broad ridges and each bears a long palp
proboscis. The pedal ganglia are large, round, with large
statocysts dorsal to them. The cerebral ganglia are slightly
larger than the visceral, both are club-shaped and well-
developed. The foot is large with a large byssal gland. The
stomach is large with a small, narrow style sac. The hind gut
takes an ‘S-shape’ course to the right side of the body before
returning to the mid dorsal margin and thence to the anus.
There is a fine typhlosole present along the length of the hind
gut. Ingested material was seen in part of the left digestive
diverticulum. The kidney is well-developed, extending anteri-

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

PA

PP

orly on either side of the stomach, anterior to the lateral

| pedal retractor muscles. It also pentrates the foot posterior to
| the stomach.

Only male specimens were observed in which the testes
were restricted to the anterior dorsal and ventral limits of the

_ viscera.

Yoldiella sinuosa is characterized by its slightly sinuous
postero-ventral shell margin and the s-shaped course of the

| hind gut (see p. ).

| Yoldiella blanda (new species)
| TYPE LOCALITY. R.V. Atlantis II, Cruise 60, Sta. 247,

Argentine Basin, 17.3.1971,
Epibenthic Trawl, 5208-5223 m.

43°32.0'S, 48°58.1'W,

| TYPE SPECIMEN. Holotype: BM(HM) 1992020, Paratypes: in

a collection held by J.A. Allen.

MATERIAL.
Cruise Sta. Depth No Lat Long Gear Date
(m)
ARGENTINE BASIN
Atlantis II 242 4382- 119 38°16.9'S 51°56.1’E ES 13.3.71
60 4402
245 2707 1 36°55.7'S 53°01.1’E ES 14.3.71
247 5208- 106 43°33.0'S 48°58.1’'E ES 17.3.71
5523
252 4435 42 38°29.8'S 52°09.1’E ES 22.3.71
256 3906- 63 52°19.3’'S 52°19.3’E ES 24.3.71
3117
| 259 3305- 20 37°13.3’'S 52°45.0’'E ES 26.3.71
| 3317
|

Basin. Depth range: 2707-5223 m.
SHELL DESCRIPTION (Figs. 36 & 37). Shell ovate, laterally

| This species is restricted to abyssal depths in Argentine

29

AA
Fig. 34 Yoldiella sinuosa. Lateral view
from the right side of the internal
morphology of a specimen from Sta. 299
Guyana Basin. (Scale = 1.0 mm).
50
WL i
40
e ~ ©
2 a)
HL. ‘
70 e ® é e
e
60
60
PL/TL soe elt,»
i
e
e
40

a | | Se Oe a a) aes er ee),
1 2 3 4

Length(mm)

Fig. 35 Yoldiella sinuosa. Variation in the ratios of height H/L,
width W/L, and post-umbonal length PL/TL to length against
length of a subsample from Sta. 299 Guyana Basin.

compressed, fragile, slightly inequilaterial; umbos small, not
markedly raised or inflated, slightly anterior to the mid line;
antero-dorsal, anterior and ventral margins, evenly curved,
postero-dorsal margin slopes in almost straight line from
umbo, anterior and posterior limits dorsal to mid horizontal
line; hinge plate moderately strong, parallel to dorsal margin,
anterior and posterior series of teeth equal in number;
ligament amphidetic and extends slightly below hinge plate

30

with very small anterior and posterior marginal extensions of
fused periostracum.

INTERNAL MORPHOLOGY (Fig. 38). Exhalent and inhalent
siphons combined, the inhalent siphon is shorter than the
exhalent and open at the ventral margin. There is a siphonal
tentacle to the left side. The posterior adductor muscle is oval
and approximately half the size of the crescent-shaped ante-
rior muscle. The gills are moderately well-developed and the
plates number up to 16. The labial palp ridges number
between 14-18 depending on size of individual. The foot is
large with a conspicuous byssal gland. There is a single hind
gut loop to the right of the body which has a characteristic
course that approaches an ‘S’ in shape.

Prodissoconch length: 198 »#m. Maximum recorded shell
length: 5.61 mm.

Most closely resembles Y. sinuosa but without the postero-
ventral sinuosity (Fig. 139).

Fig. 36 Yoldiella blanda. Lateral view of a shell from the right side
and detail of the hinge-plate of a right valve. Specimens from Sta.
242 Argentine Basin. (Scale = 1.0 mm).

Fig. 37 Yoldiella blanda. Outline drawings of shells of differing
sizes from the right side to show change in shape with growth.
Specimens from Sta. 242 Argentine Basin. (Scale = 1.0 mm).

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 38 Yoldiella blanda. Lateral view from the right side of the
internal morphology of a specimen from Sta. 242 Argentine
Basin. (Scale = 1.0 mm). For identification of parts see Fig. 34.

Yoldiella biscayensis (new species)
TYPE LOCALITY. R.V. Chain, Cruise 106, Sta. 326, Bay of

Biscay, 22.8.1972, 50°04.9'N, 14°23.8’W, Epibenthic Trawl,
3859 m.

TYPE SPECIMEN. Holotype BM(NH) 1992024, Paratypes: in
collection held by J.A. Allen.

MATERIAL.

Cruise Sta. Depth No _ Lat Long Gear Date
(m)

WEST EUROPEAN BASIN

Jean Charcot

(Polygas) DS20 4226 10 47°33.0'N 9°36.7'W DS_ 24.10.72
DS21 4190 6 47°31.5'N 9°40.77;W DS 24.10.72
DS22 4144 21 47°34.1'N 9°38.4"W_ DS_ 25.10.72
DS23 4734 17 46°32.8'N 10°21.0'W DS _ 26.10.72
(Biogas II) DS31 2813) 11 47°32.5'N 9°04.2".W DS 19. 4.73
(Biogas III) DS44 3992 4 47°33.2'N 9°42.0'W DS 27. 8.73
DS41 3548 20 47°28.3’N 9°07.2".W DS _ 26. 8.73
DS45 4260 6 47°33.9'N 9°38.4°;W DS 27. 8.73
DS48 4203 2v 44°29.0'N 4°54.0'W DS _ 31. 8.73
(Biogas IV) DS53 4425. 11 44°30.4’N 4°56.3’W DS 19. 2.74

DS54 4659 =20 9 46°31.1'N 10°29.2’W DS 21. 2.74

DSS5 4125 76 47°34.9'N 9°40.9'W DS 22. 2.74
DS56 4050 1 47°32.7’N =: 9°28.2".W DS 23. 2.74
DSS59 2790 3. 47°31.7'N 9°06.2'W_ DS 24. 2.74
DS60 3742 24 47°26.8'N 9°07.2'W DS _ 24. 2.74
KR35 4140 1 47°26.0'N 9°08.7'W KR 25. 2.74
(Biogas V) DS66 3480 31 47°28.2'N 9°00.0'W DS 16. 6.74
DS67 4150 4 47°31.0'N 9°35.0'W DS _ 17. 6.74
DS68 4550 2+2v 46°26.7'N 10°23.9'W DS 19. 6.74
DS69 4510 1 44°21.9'N 4°52.4"W DS 20. 6.74
DS70 2150 1 44°08.8’N 4°17.4';W DS 21. 6.74
(Biogas VI) DS74 2777 7 47°33.0'N 9°07.8'W DS _ 22.10.74
DS75 3250 9 47°28.1'N 9°07.8'W DS 22.10.74
DS76 4228 101 47°34.8'N 9°33.3’W DS _ 23.10.74
DS77 4240 11 47°31.8'N 9°34.6'W DS _ 24.10.74

46°31.2'N 10°23.8'W DS 25.10.74
46°30.4'N 10°27.1'W DS 26.10.74
46°29.5'N 10°29.5'W DS 27.10.74
46°28.3’N 10°24.6'W DS 27.10.74
44°25.4'N 4°52.8'W DS 29.10.74

DS78 4706 47
DS79 4715 106
DS80 4720 19
DS81 4715 9
DS82 4462 27

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

DS85 4462
CP13 3134
CP16 4825
CP17 4706
CP19 4434
CP22 4475
Jean Charcot CP11 4823
(Incal) OS02 4829
WS03 4829

DS14 4254—
4348
DS15 4211

DS16 4268

OSO0S 4296-
4248

OS06 4316
int9.347°27.9'N
int3.90S07

4249

OS08 4327

WS07 4281

WS08 4287-—
4301
WS09 4277

WS10 4354

KRO7 2891
321 2890-
1868
3356-
3338
3859
4426-
4435
4632

' Chain 106
323

326
328

330

4°50.8'W
9°38.0'W
10°25.0'W
10°19.5’'W
4°51.3'W
4°54.8'W
15°14.6'W
15°15.7'W
15°23.3'W
— 15°22.5'W
9°35.4'W

2 44°23.2'N
2 47°34.4'N
1 46°27.3'N
2 46°30.8'N
1 44°24.9'N
2 44°22.9'N
1 48°20.4'N
2 48°19.2'N
3 48°19.2'N

7 47°32.8'N
9°39.1'W

— 9°38.5'W
9°33.4'W

6 47°33.6'N

13. 47°29.8'N
— 47°30.3'N

4 47°31.3'N 9°34.6'W
— 47°32.2'N
5+1v 47°27.3'N

9°34.7'W
9°36.2'W
9°36.0'W
12. 47°31.8'N 9°34.3'W
— 47°31.3'N
13. 47°29.8'N
— 47°29.5S'N
9 47°30.6'N
— 47°31.2'N
13 47°30.5'N
— 47°29.3'N
8 47°28.8'N
— 47°27.9'N
18 47°27.3'N
— 47°28.2'N
8 55°02.9'N
2 50°12.3'N

9°32.9'W
9°38.8'W
9°37.1'W
WSR iN"!
9°33.7'W
9°34.1'W
9°34.0'W

9°39.9'W
12°43.9'W
12°43.9'W
13°35.8'W
39 = 50°08.3'N 13°53.7'W
— 13°50.9'W
29 =50°04.9'N 14°23.8’W
13. 50°04.7'N 15°44.8'W

29 =50°43.S'N 17°51.7'W
— 50°43.3’N 17°52.9'W

DS
CP
CP
cP
CP
CE
CP.
OS
WS
DS
DS
DS

DS

OS

OS

OS

WS

WS

WS

WS

KR
ES

ES

ES
ES

ES

30.10.74
23.10.74
26.10.74
26.10.74
28.10.74
30.10.74
1. 8.76
2. 8.76
1. 8.76

7. 8.76

8. 8.76

9. 8.76

7. 8.76

Restricted to abyssal depths in West European Basin, the

ig. 39 Yoldiella biscayensis. Lateral view of a shell from the right
side and detail of the hinge-plate of a left valve. Specimens from
Sta. 326 West European Basin. (Scale = 1.0 mm).

31

majority of specimens were taken from depths greater than
3500 mm. Depth range: 2150-4829 m.

SHELL DESCRIPTION (Figs. 39-42). The form of the shell
varies somewhat from locality to locality.

Form A: The following description is based on specimens
found in Bay of Biscay at 4000 m and below.

Fig. 40 Yoldiella biscayensis. Outline drawings of shells of
different sizes from the right side to show change in shape with
growth. Specimens from Sta. 326 West European Basin. (Scale =
1.0 mm).

Fig. 41 Yoldiella biscayensis. Outline drawings of three shells of
similar size from the right side to show differences in shape.
Specimens a & b from Sta. BGVI DS 76 (4228 m); specimen c
from Sta. BG II DS 31 (2813 m). (Scale = 1.0 mm).

32

40 :
4 a
a
WIL + geek ome, at 2 aE paras Bis
Com a® 4®e eo a a ~,
30 ae °
80

Length (mm)

Shell compressed, ovate, fragile, inequilateral, post-
umbonal length 48-62% of total length, sculpture of fine
irregular concentric lines, pale yellow periostracum; umbo
slightly raised, posteriorly directed; antero-dorsal margin
convex curving gradually to broadly rounded anterior mar-
gin, ventral margin evenly curved, postero-dorsal margin
very slightly sinuous, slopes gradually to distal limit of hinge
plate, then more sharply inclined to posterior margin; hinge
plate moderately long, moderately well-developed with up to
10 well developed teeth on each side of ligament, occasionally
one additional tooth in posterior series; ligament amphidetic,
large, internal, goblet-shaped, extends ventral to and some-
what posterior to proximal limit of posterior hinge plate.

Prodissoconch length: 187-198 pm. Maximum recorded
shell length: 5.82 mm.

Form B: This was taken between approximately
3000-4100 metres and at about 4000 m may be mixed with
Form A. The shell is distinguished from Form A by a more
marked concavity in the proximal part of the postero-dorsal
margin and by a more rounded posterior margin. Internally
the hinge and ligament are similar. Form B on average is
somewhat smaller than Form A.

Maximum recorded shell length: 4.19 mm.

Form C: Recorded at 2076 and 1891 metre depths. This
form is intermediate between Forms A and B. It has an
almost straight postero-dorsal margin.

Maximum recorded shell length: 7.39 mm.

INTERNAL MORPHOLOGY (Fig. 43). The three forms are
anatomically indestinguishable from each other. Exhalent
and inhalent siphons are combined. There is a siphonal
tentacle that lies either to the left or the right of the siphons.
A feeding aperture is present ventral to the siphon embay-
ment and there is a well-developed anterior marginal sense
organ.

The adductor muscles have conspicuous ‘quick’ and
‘catch’ parts. The posterior muscle is approximately half
the size of the anterior and oval in outline. The anterior
muscle is crescent-shaped. The gills are well-developed
with a relatively large number (up to 28) of gill plates
alternating on either side of the gill axis. The labial palps

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 42 Yoldiella biscayenesis.
Variation in the ratios of height
H/L, width W/L, and
postero-umbonal length PL/TLL
to length against length of the
three shape forms illustrated in
Fig. 41. Solid circles, angulate
specimens from Sta. BG VI
DS76; solid triangles, rounded
specimens from Sta. BG VI
DS76; open squares, specimens
from Sta. BG II DS31; West
European Basin.

are relatively large and, depending on the size of the
animal, have up to 25 internal ridges. The palp proboscides
are long and thin. The foot is large with a large byssal
gland. There is a small single papilla posterior to the
aperture of the gland and the posterior surface of the foot
is well-supplied with secretory cells. As in other species,
there is histological evidence of a secretion being released
along the whole length of the sole of the foot. Large,
round, pedal ganglia are situated dorsal to the byssal gland.
The visceral ganglia are ‘club’-shaped and relatively slen-
der, the cerebral ganglia are slightly more inflated. The
oesophagus, stomach and style sac are similar to those
described for other Yoldiella species, the hind gut is
relatively broad and forms a single loop on the right side of
the body. The posterior section of the loop describes a
conspicuous and characteristic ‘S’-shaped course. There is
a typhlosole along the length of the hind gut. The digestive
diverticula are similar to those of other species of Yoldiella.
The sexes are separate. All individuals more than 3.0 mm
in length show some gonadial development. A specimen
4.9 mm in length contained 190 ova (maximum diam. 150
pm).

Most closely resembles Y. sinuosa and Y. blanda and
distinguished by more angulate postero-dorsal margin and
lack of postero-ventral sinuosity.

Yoldiella lata (Jeffreys 1876)

TYPE LOCALITY. Valorous Sta. 9, Davis Strait, 14.8.1875,
59°10'N, 50°25’W, dredge, 1750 fms.

TYPE SPECIES. Lectotype: U.S. Natl. Mus., No. 199695 as
here designated.

Specimens from Biogas Sta. DS87 are housed in the
Museum National d’Histoire Naturelle, Paris.

Leda lata Jeffreys 1876, p. 431 (in part).

As is pointed out under Y. jeffreysi (p. 63) when the
‘Valorous’ material was examined, it was found that two
species had been grouped together under the name Leda lata.
Furthermore, the original description is such that it is not

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

Fig. 43 Yoldiella biscayensis. Lateral view from the right side of
the internal morphology of a specimen from Sta. BG VI DS76
West European Basin. (Scale = 1.0 mm). For identification of the
parts see Fig. 34.

possible to identify which of the two is L. lata. We have
separated the two species and chosen lectotypes from the
‘Valorous’ material.

MATERIAL.

Cruise Sta. DepthNo Lat

(m)

Long Gear Date

WEST EUROPEAN BASIN

Sarsia 44 1739 «18 43°40.8’N 3°35.2'W ED 16. 7.67
65 1922 25 46°15.0’N 4°50.0'W ED 25. 7.67

La Perle

(BiogasI) DS06 3
DS07 2
DS11 2205 8 47°35.5'Ne 8°33.7' WDS 8. 8.72
S12 2180) 92 -47°28:5'N '835:5'W DS 9. 8.72
DS13 2165 5+1v 47°33.7'N 839.9'W DS __ 9. 8.72
OS01 2v OS

Chain 106 313 1491— 457 51°32.2’N 12°35.9'W ES 17. 8.72

1500
316 2173-1653 50°58.7’N 13°01.6’'W ES 18. 8.72
2209

Jean Charcot

(Polygas) DS15 2246 8 47°35.2'N 8°40.1'W DS _ 21.10.72
DS16 2325 1 47°36.1'N 8°40.5'W DS _ 21.10.72
DS18 2138 10 47°32.2'N 8°44.9'W DS 22.10.72
DS25 2096 255 44°08.2'N 4°15.7'W DS __ 1.11.72
DS26 2076 1095 44°08.2'N 4°15.0’'W DS _ 1.11.72

+17v

CV102108 2 47°30.7'N 8°40.6'W CV 22.20.72

Jean Charcot

(Biogas II) DS32 2138 26 47°32.2'N 8°05.3’W DS 19. 4.73

(Biogas III) DS35 2226 9 47°34.4’N 8°40.7'W DS_ 24. 8.73

8°36.5'W DS 24. 8.73
8°34.6'W DS 24. 8.73
79520 WW DS 92518219
4°15.6W DS 1. 9.73

DS36 2147 7+2v 47°32.7'N
DS37 2110 12+2v 47°31.8'N
DS38 2138 13 47°31.5'N
DS49 1845 177+ 44°05.0’N

DSS50 2124 ca 44°09.9'N 4°15.9'W DS | 1. 9.73
(BiogasIV) DSS51 2430 of 44°11.3’N 4°15.4".W DS 18. 2.74
DS52 2006 aa 44°06.3'N 4°22.4'W DS 18. 2.74
DS61 2250 ah 47°34.7'N 8°38.8'W_ DS 25. 2.74

33

DS62 2175 48+1v 47°32.8'N 8°40.0'W DS 26. 2.74

Jean Charcot DS64 2156 12+ 47°29.2'N 8°30.7'W DS _ 26. 2.74
14v

CV33 1913 1 44°904.6'N 4°18.1'W CV 18. 2.74

CPO01 2245 19+4v 47°34.6'N 8°38.8'W CP 25. 2.74

(Biogas V) DS70 2150 6 44°08.8’N 4°17.4‘;W DS _ 21. 6.74

CP07 2170 136+ 44°09.8'N 4°16.4‘W CP 21. 6.74
4yv

(Biogas VI) DS71 2194 19 47°34.4'N 8°33.8'W DS_ 20.10.74

DS86 1950 325+ 44°04.8’N 4°13.7'W DS _ 31.10.74

13v

DS87 1913 550+ 44°05.2'N 4°19.4'W DS __ 1.11.74
4v

DS88 1894 40+2v 44°05.2'N 4°15.77;W DS _ 1.11.74

CP08 2177 12+1v 44°33.2'N 8°38.5'W_ CP 20.10.74
CP09 2171 34 47°33.0'N 8°44.1'W CP 20.10.74
CP24 1995 23 44°08.1'N 4°16.2,;W CP 31.10.74

Jean Charcot CP01 2068- 29 55°57.0'N 10°55.0'W CP 16. 7.76

2040
(Incal) CP02 2091 4 57°58.4'N 10°42.8'W CP 16. 7.76
— 57°57.7'N 10°44.6'W
CP08 26441 SOST437N 13°13:5'W) ‘GP 27: 776
= 50°15.2'N 13°14.8'W
QS01 2634 2 50°14.4’N 13°10.9°'W OS 30. 7.76

— 50°15.2’N 13°11.0'W

SSN L098 WW. SDSe 1537270
— 57°59.2'N 10°41.3"'W

57°58.8'N 10°48.5'W DS _ 16. 7.76
— 57°58.5'N 10°49.2'W

567281 Nes tiie 7 Wee DS) S185 7276
— 56°17.6'N 11°12.0'W

56°26.6’N 11°10.5'W DS 18. 7.76
— 56°15.9'N_ 11°10.7'W

DSO01 2091 234

DS02 2081 262

DS05 2053.7

DS06 2494 86

WS01 2550- 6 50°19.4’N 13°08.0'W WS 30. 7.76
2539 — 50°19.3’N 13°06.9'W

WS02 2498— 4 50°19.3'N 12°55.8'W WS 30. 7.76
2505 — 50°20.0’N 12°56.0'W

CANARIES BASIN

Discovery 6701 1934 9 27°45.2'N 14°13.0'W ED 16. 3.68

SIERRA LEONE BASIN
Atlantis1I 139 2099- 1 10°33.0’N 17°53.0’'W ES 4. 2.67
31 2187

Largely restricted to lower slope and abyssal rise depths in

the northeastern Atlantic, but predominantly in the West
European Basin with a few specimens taken in the Canaries
and Sierra Leone Basins. Depth range: 1491-3220 m.

SHELL DESCRIPTION (Figs. 44 & 45). Shell transversely
ovate, fragile, inequilateral, postumbonal length 53-65%
of total length, moderately inflated, very fine concentric
lines forming ridges ventrally, more conspicuous in larger
specimens, irridescent, pale yellow/brown periostracum,
light and dark banding patterns; umbo moderately inflated,
inwardly directed; dorsal margin raised, anterior and pos-
terior margins smoothly curved with anterior margin more
convex than posterior, antero-dorsal margin short, joins
anterior margin in smooth curve, postero-dorsal margin
approximately straight, gradually inclining distally to pos-
terior limit of hinge plate, then curves sharply to posterior
Margin, posterior margin more convex dorsally, ventral
margin smoothly curved; hinge plate narrow, anterior and
posterior hinge plates merge with margin below umbo,
teeth small, chevron-shaped, number on each plate
depending on size (up to 9/10), usually with one or two less
on anterior hinge plate; ligament amphidetic, small, inter-
nal rectangular in cross-section.

Prodissoconch length: 198 wm (average) range 190-220
jm. Maximum recorded shell length: 4.43 mm.

34

Fig. 44 Yoldiella lata. Lateral view of a shell from the right side
and detail of the hinge-plate of a right valve. Specimens from Sta.
316 West European Basin. (Scale = 1.0 mm).

Fig. 45 Yoldiella lata. Right lateral and dorsal views of shells to
show changes in shape with growth. Specimens from Sta.
INCAL/DS 06 West European Basin. (Scale = 1.0 mm).

The shell is similar in shape to Yoldiella jeffreysi but more
fragile, more compressed, and has fewer hinge teeth on a
narrower hinge plate. With increasing length there is a
gradual increase in the posterior umbonal length/total length
ratio. There is little change in the height/length or width/
length ratios (Fig. 46). Hydroids are present on several
specimens at the antero-ventral shell margin.

INTERNAL MORPHOLOGY (Fig. 47). With the exception of the

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

6
W/L
40 2 mises
80
HIL ee at tage z-
60
6 CO a *2 edewee? awe oo Es
‘ ae bores Lage og : Zz :
fn a ; 25
40
15 No.
5
2 3 4

Length

Fig. 46 Yoldiella lata. Variation in the ratios of height H/L, width
W/L and postero-umbonal length PL/TL to length against length
of a sample from Sta. BG V CP 07 from the West European
Basin.

Fig. 47 Yoldiella lata. Lateral view from the right side of the
internal morphology of a specimen from Sta. BG III DS 50 West
European Basin (Scale = 1.0 mm). For identification of parts see
Fig. 34.

—_—

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

posterior margin, the mantle edge is little modified and
comprises three marginal lobes. The middle sensory lobe
forms a frill while the inner muscular fold is thickened, being
approximately twice the thickness of the general mantle
epithelium and is relatively broad. Posteriorly combined
exhalent and inhalent siphons are formed by fusion of the
opposing inner muscular lobes. The inhalent siphon is shorter
than the exhalent siphon and remains open ventrally. The
| siphonal wall is relatively thin. A narrow band of longitudinal
, muscles lies immediately internal to the basal membrane of
the outer and inner epithelia. Internal to the longitudinal
fibres are circular and transverse muscle fibres with sub-
epithelial gland cells scattered throughout the layer. Between
the exhalent and inhalent siphons, there is a pair of haemo-
‘coelic channels within the muscle layer (Fig. 48a). The
proximal end of the gill axes join laterally at the junction
between the siphons. The gill axes probably act as a channel
to guide faecal rods into the lumen of the exhalent siphon.
|The anus lies immediately dorsal to the inner margins of the
siphon.

The siphonal tentacle, is developed from the middle sen-
sory lobe of the mantle and originates from a pocket in the
siphonal embayment at the base of the siphon, usually on the
eft side. It consists of an elongate finely-tapering cone which
jin transverse section comprises a large central nerve sur-
rounded by several muscle fibres (Fig. 48b). These in turn are
‘surrounded by a layer of connective tissue with gland cells
‘interspersed throughout and a layer of epithelial cells with
‘fine granular contents.

Ventral to the siphon is a broad, deep, specialized area of
ithe mantle — the feeding aperture. Ventral to the inhalent
/aperture there is an inner secondary muscular fold. Peripheral
‘to this the inner muscular layer is enlarged both in thickness
‘and width. In preserved specimens this area is highly convo-
luted. In life the muscular and sensory layers are probably
jextended beyond the shell margin as a flap on each side. The
epithelial cells in this region are densely ciliated (Fig. 48a).
| The adductor muscles are unequal in size with the ‘quick’
and ‘catch’ parts obvious. The posterior adductor is round in

ig. 48 Yoldiella lata. Transverse sections through a, the inhalent,
exhalent and feeding apertures; b, the base of the siphonal
tentacle; c, the anterior sense organ of a specimen from Sta. BG
III DS 49 West European Basin. (Scales = 0.1 mm).

35

outline. The anterior is approximately twice the size of the
posterior and crescent-shaped. At the mantle margin ventral
to the anterior adductor muscle there is a well-developed
anterior sense organ. This is derived from the middle sensory
fold which is greatly extended to form two flaps. The epithe-
lial cells of the outer flap are ciliated. Internal to this the
epithelium is glandular (Fig. 48c). Underlying this epithelium
is a thick layer of connective tissue containing the pallial
nerve.

The gills lie parallel to the postero-dorsal shell margin.
They are well-developed with up to 22 alternating gill plates
(the number relates to the size of the individual).

The labial palps are moderately large and, also depending
on the size of the animal, have up to 24 ridges on their inner
face. The palp proboscides are relatively slender. The palps
are wedge-shaped and extend 1/4~-1/2 way across the body.
The mouth is set posterior to the anterior adductor muscle.

The foot is well-developed, and of typical nuculanid form.
The divided sole is elongate with papillate edges. At the tips
of the papillae, lying between muscle fibres are glandular
cells with ducts to the surface of the foot. Secretions from
these may be used to lubricate the movement of the foot
through the sediment. A large well-developed byssal gland is
present in the heel of the foot (Fig. 49a). The byssal gland is
spherical, composed of large, hyaline cells, surrounded by
muscle fibres. It opens medially at the posterior limit of the
divided sole and several glandular epithelial cells surround
the opening. A very small medial papilla lies posterior to the
opening at the heel of the foot. Histochemical tests have been
carried out to identify type secretion from the gland. Tests
did not confirm any protein component but this could have
been masked by other muco-substances. The main compo-
nent at the centre of the byssal gland appears to be a
keratin-sulphate (PAS-ve with no glycol groups, and carboxyl
groups). It has the character of cartilage. (Secretions from the
gland cells in the foot do, however, possess carboyxyl
groups).

The posterior pedal retractor muscles comprise a thick,
wide, strap that inserts on the shell on either side of the hind
gut anterior to the posterior adductor muscle. A small
postero-lateral pedal retractor muscle is present on either
side of the stomach. Three major pairs of anterior pedal
retractor muscles pass from the neck of the foot anteriorly to
insert posterior to the anterior adductor muscle on either side

GS PSA

Fig. 49 Yoldiella lata. a, transverse section through the heel and
‘byssal’ gland of the foot and b, right and left lateral external
views of the stomach and style sac of specimens from Sta. Polygas
DS 26 West European Basin. (Scales = 0.1 mm).

36

of the oesophagus and lateral to the hind gut loop.

The cerebral ganglia are relatively small and oval while the
visceral ganglia are elongate and relatively thickened. The
pedal ganglia are large and elongate/oval, conjoined, and
each has a large dorsal statocyst associated with it and which
contains numerous small refractile crystals.

The mouth is displaced posteriorly to a small extent. It
opens into a relatively long, ciliated oesophagus which curves
first anteriorly to the posterior face of the anterior adductor
before passing posteriorly to the stomach. The oesphagus is
positioned almost centrally rather than dorsally within the
body. The longitudinal axis of the stomach and style sac lies
diagonally within the body, the style sac penetrating into the
upper half of the foot. The stomach and style sac are large,
taking up much of the central body space. Externally the
stomach is brown in colour. A large gastric shield covers part
of the dorsal wall and extends laterally, mainly to the left
side. To the right of it are approximately nine sorting ridges.
The organization of the stomach appears similar to that
described for Ledella (Allen & Hannah, 1989). The stomach
is cradled by the pedal retractor muscles. The digestive
diverticla are in three parts which for the most part lie
anterior to the stomach but also to the right and left of the
body. A duct from the right diverticulum joins the stomach
on the right anterior face just ventral to the oesophageal
opening. Immediately posterior to this, a duct from one of
the two left diverticula enters the stomach on the lower left
side (Fig. 49b). The third diverticulum opens into the stom-
ach immediately below the tooth of the gastric shield. The
epithelial cells which line these ducts contain highly refractile
golden granules. In the case of the third diverticulum the duct
is reduced to a small collar of cells around the aperture.
Material was present in the lumen of this diverticulum but not
in the other two diverticula.

From the style sac, the hind gut passes posteriorly into the
foot, as far as the pedal ganglia. It then passes anterior for a
short distance between ganglia and byssal gland and then
takes a dorsal course parallel to the posterior margin of the
foot to a point immediately ventral to the ligament. It then
forms a loop on the right side of the body which skirts the
posterior face of the anterior adductor and then passes
dorsally and posteriorly parallel to shell margin to the anus.
A deep typhlosole is present along the entire length of the
hind gut. Slight variations in the arrangement of the loop of
the hind gut are seen in this species. Material in the gut
consists of fine clay particles and skeletal fragments of various
kinds.

The sexes are separate. The gonads overlie the lateral and
dorsal sides of the viscera of the body. Gonads are present in
all specimens over 2 mm in length. Numbers of ova are
relatively high and vary with the size of the animal. A
maximum of c.730 ova were counted in a specimen 3.54 mm
in length. The maximum observed diameter of the ova range
from 126-190 wm. There is some indication of an annual
reproductive cycle. Ova of maximum size were present in
October and February. April and June samples show little
ovarian development, but increasing maturity was observed
in July and August. The gonadial apertures open into the
supra-mantle cavity, close to those of the kidney, anterior to
the posterior pedal retractor muscle.

The kidney is well-developed. It lies anterior to the poste-
rior pedal retractor muscle and extends on either side of the
stomach, tapering to its anterior limit close to the lateral
pedal retractor mucles. The kidney epithelium is a single

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

layer of cuboid cells. The heart, through which the hind gut
passes is well-developed in this species with numerous muscle
fibres in a relatively thick-walled ventricle.

Distinctive features include the hind gut on the right hand
side of body forming an extended single loop which turns
back on itself; the light and dark banding pattern of shell, and
the more rounded posterior adductor muscle. The hind gut,
although similar, is simpler in form than that in Yoldiella

Jeffreysi.

Yoldiella frigida Torrell 1859
TYPE LOCALITY. Spitzbergen, Ice Sound 55 m.

TYPE SPECIMEN. Holotype not known; lectotype (desig. A.
Warén, 1989) Swedish Museum of Natural History No. 1986.

Yoldia frigida Torrell 1859, p.148, pl. 1, Fig. 3; Friele 1878, p.
222; Leche 1878, pl. 1, Fig. 6 a-d; Sars G.O. 1878, pl. 4,
Fig. 1la—b; Friele 1879, p. 266.

Leda frigida Jeffreys 1870, p. 440; Jeffreys 1879, p. 570.

Portlandia frigida Norman 1893, p. 344, p. 364; Posselt 1898,
p. 34-35; Friele & Grieg 1901, p. 15; Hogg 1905, p. 112;
Jensen 1905, p. 320; Grieg 1909, p. 534; Ohdner 1915, pl.
1, Figs. 30-32;

Portlandia frigida Grieg 1916, p. 8; Ockelmann 1958, pl. 1.
Fig. 14.

Portlandia (Yoldiella) frigida Soot-Ryen 1939, p. 9; Clarke
1963, p. 100, pl. 2, Figs. 6-8.

Yoldiella frigida Soot-Ryen 1958, p. 10; Warén, 1989; Figs.
TE& F&1I0G&H.

MATERIAL.

Cruise Sta. Depth No Lat Long Gear Date
(m)

NORTH AMERICAN BASIN

Atlantis 4 400 304 39°56.6'N 70°39.9'W AD 30. 8.62

Chain 58 105 530) 121 39°56.6.N 71°03: WED 525-68

Chain 88 207 + 805— 153 39°51.3’N 70°54.3’W ES 21. 2.69
811 — 39°51.0'N 70°56.4'W

WEST EUROPEAN BASIN

Thalassa 2425 700 1 48°28.9'N 09°44.0'W_ PBS 25.10.73

Jean Charcot DS04 619 1 S7223:0'N TRO70 WR DS® 172 Gaia

Incal DS03 609 16 57°25.1'N 1103 7 OWES DSieil7: 6:74

In addition to the above, material from Ingolf Sta. 115 (det
Ockelmann), Spitzbergen, Franz Joseph Fjord, East Green-
rand (USNM No. 219726 det. Odhner) and off Martha’s
Vineyard Sta. 934 (USNM No. 193343 det. Verrill & Bush)
has been examined.

Predominantly a North Atlantic high arctic species from
shelf seas and upper slope depths (Warén (1989). Clarke
(1963) reports it as being present at abyssal rise depths but
there must be some doubt about this. There is some indica-
tion that in north temperate seas at the southern extremity of
its range the population is found deeper at the shelf slope
break, thus suggesting a relationship with temperature.
Greenland, Jan Mayen, Spitzbergen, Novaya Zemlya, Kera
Sea, West Siberia, Iceland, North American and West Euro-
pean Basins (see Ockelmann (1958) for other more doubtful
records). Depth range: 5-811 m.

SHELL DESCRIPTION (Fig. 50 & 51). Shell subelliptical,

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

approximately equilateral (postumbonal length 45-50% of
total length), smooth, moderately thin, pale yellow irrides-
cent periostracum, with banding patterns, narrow dark bands
and broader light bands between; umbo slightly raised
directed somewhat posteriorly; dorsal margins proximally
straight, distally curving to anterior and posterior margins,
posterior and anterior dorsal margins very slightly angulate at
limit of hinge plate, limit of anterior margin submedial, limit
of posterior margin supramedial, postero-ventral margin rela-
tively flattened, very slightly sinuous particularly in larger
specimens; hinge plate relatively short, not extending beyond
inner margin of adductor muscles, 5 anterior teeth and 6
posterior teeth in specimen 2.5 mm, ventral part of chevron
reduced; ligament amphidetic, large elongate internal part,
with small external extensions either side of umbo; adductor
muscle scars distinct, particularly anterior, line of attachment
of pallial sinus also visible. Hydroids may be present, dor-
sally, close to umbo and on the posterior and postero-ventral
margins.

Prodissoconch length: 166-208 jm. Maximum recorded

ig. 51 Yoldiella frigida. Lateral views of left valves showing detail
of the ligament and the hinge-plates; a, a specimen from Stor
Fjord Spitzbergen (USNM No. 219726); b, a specimen from Sta.
105 North America Basin. (Scale = 1.0 mm).

37

Fig. 50 Yoldiella frigida. Lateral
views of shells from the right side.
Specimens a & b from Franz Joseph
Fjord, E. Greenland; c, from Sta.
INCAL DS 04 Bay of Biscay; d, from
off Martha’s Vinyard (USNM No.
193343). (Scale = 1.0 mm).

shell length: 3.98 mm, (Sta. 207), however, specimens from
Greenland donated by Ockelmann measure 6.1 mm.

Comparing height/length, width/length and post-umbonal
length/total length ratios of specimens from the North Ameri-
can Basin (Sta. 105 & 207) and West European Basin (Incal
DS03), it was found that in the latter case the average for
each of the three parameters is slightly less than those from
the two North American Stations and whose ranges for the
most part overlap (Figs. 52 & 53). Specimens from Sta. 105
appear to be somewhat wider than specimens from Sta. 207.

It is highly likely that many records of Y. frigida from the
North Atlantic relate to other species. This is a view also
expressed by Warén (1989). For example, Y. frigida (USNM
No. 193343) figured by Verrill & Bush (1898) and here (Fig.
50), closely resembles Yoldiella nana again confirming the
observations of Warén (1989). (Y. nana has a straighter
dorsal margin, a slightly more inflated umbo and fewer hinge
teeth). Further comparing USNM No. 193343 with high arctic
specimens of Y. frigida the former has a larger umbo, and
thicker hinge. In addition, we believe that many shallow
water specimens of Yoldiella inconspicua from the North
American Basin have been misidentified as Y. frigida. Subtle
differences seen here, as in many other deep-sea protobranch
bivalves, are particularly difficult to disentangle (Fig. 53) and
in the case of Yoldiella this particularly applies to species
found at shallower slope depths.

INTERNAL MORPHOLOGY (Fig. 54). Mantle structures include
a well-developed anterior sense organ and long slender
combined siphons with a single lumen. A siphonal tentacle is
present originating, but not without exception, on the left
side of the inner limit of the siphon embayment. The adduc-
tor muscles are unequal in size the anterior being the some-
what larger and oval in outline whereas the posterior muscle
is more circular in outline. The gill axes attach ventrally to the
siphon and with the exception of the ventral margin, the
latter must be assumed to be largely exhalent in origin. There
is a well-developed feeding aperture ventral to the siphon.
The gills are well-developed, the number of filaments range
from 12-22 according to the size of the specimen. The palps
are large and extend over approximately half the length of the
body. There are between 11 and 20 relatively broad palp
ridges and the palp proboscides are also well-developed. The

38
50 “
a A
4 4
ns 3 “
W\L a 4 a
hs “« ae is : ~4 a
a A . On 4 a a
30
80

Length (mm)

foot which tends to be largely hidden by palps is also
relatively large with well-developed retractor muscles. This is
particularly true of the anterior series. There is a moderate-
sized byssal gland. The pedal ganglia are elongate each with a
large statocyst dorsal to it. The visceral ganglia lie relatively
ventral in position, distant from the posterior section of the
hind gut. The visceral and the cerebral ganglia are not
particularly large.

The gut describes a single loop on the right side of the
body. The size of the hind gut and the path it describes on the
right side of the body is highly characteristic of a species even
in those with a single loop and is particularly useful in
distinguishing, for instance, Y. frigida from Y. inconspicua
(Figs. 53 & 63). The oesophagus is particularly wide in
cross-section, the stomach is large with two sorting areas
clearly visible to the right side of the gastric tooth. The more
ventral of the two, which has not been observed in other

J.A. ALLEN, H.L. SANDERS AND F. HANNAH .

Fig. 52 Yoldiella frigida. Variation in ratios of
height H/L, width W/L and postero-umbonal length
PL/TL to length against length of samples from;
open triangles, Sta. 105 North America Basin;
closed triangles, Sta. INCAL DS 03 Bay of Biscay;
closed circles Sta. 207 North America Basin.

Fig. 53 Yoldiella frigida. Outlines of shells
from the right side from three Stations to
show variation in shape within and between
populations and with increasing size.
Specimens from a, Sta. Ingolf 115 (det
Ockelmann); b, Sta. 105 North America
Basin; c, Sta. INCAL DS 03 Bay of Biscay.
(Scale = 1.0 mm).

species, is broad and close to the aperture of the right
digestive duct. Ingested material was seen in the dorsal part
of the left digestive diverticulum. The duct to the right
digestive diverticulum overlies the dorsal section of the hind
gut. The kidney is well-developed and the pericardial cavity is
large.

The sexes are separate. The developing gonads surround
the hindgut loop in the usual manner. In one sectioned
specimen (2.3 mm total length) 59 ova were present with a
maximum diameter of 120 um. The ova were present ventral
and internal to the digestive diverticula, with fewer numbers
overlying the viscera dorsally.

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

Yoldiella robusta (new species)

TYPE LOCALITY. R.V. Atlantis II, Cruise 60, Sta. 280,
36°18.0'S,

Argentine Basin, 29.3.1971,
Epibenthic Trawl, 256-293 m.

53°23.9'W,

ae ed NN | | gee y

meteor pert
\

ig. 54 Yoldiella frigida. Lateral view from the right side of the
internal morphology of a specimen from Sta. 105 North America
Basin. Inset is a right-frontal external view of the stomach. (Scale
= 1.0 mm). For identification of parts see Figs. 34 & 49.

‘ig. 55 Yoldiella robusta. Lateral view of a shell from the right
| side and detail of hinge-plate of a left valve. Specimens from Sta.
| 280 Argentine Basin. (Scale = 1.0 mm).

39

TYPE SPECIMEN. Holotype : BM(NH) 1992030, Paratypes: in
collection held by J.A. Allen.

MATERIAL.

Cruise Sta. Depth No Lat Long Gear Date
(m)

ARGENTINE BASIN

Atlantis II 280 256- 3495 36°18.0’'S 53°23.9'W ES 29.3.71

293

Restricted to one Station at the shelf/slope break of the
Argentine Basin. Depth range: 256-293 m.

SHELL DESCRIPTION (Figs. 55 & 56). Shell subovate, moder-
ately inflated, irregular concentric growth lines and small
ridges best developed posteriorly, periostracum pale yellow;
umbos anterior to mid line, slightly raised and inwardly

Fig. 56 Yoldiella robusta. Outlines of shells from the right side to
show change in shape with growth. Specimens from Sta. 280
Argentine Basin. (Scale = 1.0 mm).

Fig. 57 Yoldiella robusta. Lateral view from the right side of the
internal morphology of a specimen from Sta. 280 Argentine
Basin. (Scale = 1.0 mm). For identification of parts see Fig. 34.

40

directed; proximal antero-dorsal margin horizontal for very
short distance before curving steeply to anterior margin,
proximal postero-dorsal margin horizontal or slightly convex,
distally slightly angled opposite posterior limit of hinge,
postero-ventral margin slightly sinous giving a relatively
narrow rounded medial tip to posterior margin, ventral
margin smooth convex curve. Hinge plate short, moderately
narrow with up to 9 small chevron-shaped teeth on each
series, one additional tooth on posterior plate. Ligament
large, amphidetic, bilobed in lateral view, with short, narrow,
secondary external extensions anterior and posterior to
umbo.

Prodissoconch length: c. 200 jm. Maximum recorded shell
length: 4.0 mm.

Shell measurements (mm) & ratios are as follows:-

Length Height Width H/L W/L PL/TL _ Teeth(A/P)
1.89 il iil 0.70 On O87 sil -
2.58 1.76 0.98 0.68 0.38 0.53 -
2.62 1.80 0.94 0.69 0.36 0.56 -
2.81 1.89 1.09 O67 10:39 0255 -
3.05 2.05 1.23 0.67 0.40 0.54

3.44 2.21 0.64 - - 0.57 8/9
3.55 2.38 1.44 0.67 0.40 0.55 -
3.61 2.46 0.68 - - 0:53 7/8
3.81 2.67 1.52 0.70 0.40 0.56 =
3.98 2.62 1.58 0.66 0.40 0.59 =

INTERNAL MORPHOLOGY (Fig. 57). The internal morphology
is very similar to that described for Y. frigida. The siphons are
combined and there is a siphonal tentacle to the left. The
anterior sense organ is well-developed. The adductor muscles
are relatively large, the posterior being more round and
slightly smaller than the anterior muscle. The foot is also
large with a moderately-sized byssal gland. The gills are well
developed and suspended more ventral within the mantle
cavity than other species. There are approximately 15 alter-
nating gill filaments. The labial palps extend across about half
the body and have 12-16 internal ridges and well-developed
palp proboscides. The visceral and cerebral ganglia are
relatively large. The viscera are also more ventral in position
than in most other Yoldiella species. The pedal ganglia are
large and elongate. The hind gut forms a single loop on the
right side of the body.

Yoldiella extensa (new species)

TYPE LOCALITY. R.V. Atlantis II, Cruise 60, Sta. 245,
Argentine Basin, 14.3.1971, 36055.7'S, 53001.4’W,
Epibenthic Trawl, 2707 m.

TYPE SPECIMEN. Holotype: BM(NH) 1992036, Paratypes: in
collection held by J.A. Allen.

MATERIAL.

Cruise Sta. Depth No Lat Long Gear Date
(m)

ARGENTINE BASIN

Atlantis] 245 2707 26 36°55.7'S 53°01.4,°W ES 14.3.71

60

Only found at the one Station at abyssal depth in Argentine
Basin. Depth range: 2707 m.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 58 Yoldiella extensa. Lateral view of a shell from the right side
and detail of the hinge-plate of a right valve. Specimens from Sta.
245 Argentine Basin. (Scale = 1.0 mm).

SHELL DESCRIPTION (Figs. 58 & 59). Shell oblong-ovate,
moderately robust, moderately inflated, nearly equilateral,
very fine concentric lines, periostracum pale yellow; umbo
inflated, raised slightly, directed posteriorly; dorsal margin
slightly convex, antero-dorsal margin gradually slopes to
broad rounded anterior margin, ventral margin long, curva-
ture similar to that of dorsal margin, posterior margin slightly
extended with faint suggestion of postero-ventral sinus; liga-
ment amphidetic, moderate size, elongate, but slightly asym-
metric in lateral view with posterior part slightly longer and
wider than anterior, slight external extension on either side of
umbo; hinge plate narrow, relatively short, with small eden-

Fig. 59 Yoldiella extensa. Outlines of shells from the right side to
show change in shape with growth (see also Fig. 58). Specimens
from Sta. 245 Argentine Basin. (Scale = 1.0 mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

tulous space below umbo, anterior plate follows curvature of
antero-dorsal margin, posterior plate with slight convex arch,
equal number of teeth on each plate, teeth with prominent
dorsal arm and reduced ventral arm.

Prodissoconch length: c 170 wm. Maximum recorded shell
length: 4.62 mm.

Shell measurements (mm) & ratios are as follows:-

Length Height Width H/L W/L PL/TL
2.56 1.81 1.26 0.70 0.49 0.51
2.60 1.76 1.22 0.68 0.47 0.53

INTERNAL MORPHOLOGY (Fig. 60). The siphons are com-
bined, with a single lumen present. A siphonal tentacle
occurs on the right side of the siphonal embayment. The
adductor muscles are unequal in size the anterior being one
and a half to two times larger in cross-section than the
posterior. Both are more or less oval in shape. The gills are
well-developed with 13-16 plates. The labial palps are small
in the contracted state, extending about 1/4 distance across
body, and have 8-10 moderately narrow palp ridges. In
contrast the palp proboscides are well-developed. The pedal
ganglia are large and elongate, similarly the visceral and
| cerebral ganglia are also well-developed. They are club-
| shaped with the cerebral ganglia being slightly larger than the
visceral. The foot is long and narrow, with a large byssal
| gland in the heel. The stomach is large and there is a single
| hind gut loop on the right side of the body.

| Yoldiella inconspicua inconspicua Verrill & Bush 1898

| TYPE LOCALITY. Off Martha’s Vineyard, 318 fm. Original
| specimens were taken from North American Basin between
| Lat. 42°33’N, Long. 69°58.5'W and Lat. 35°12.10’N and
| 74°57.15'W by the U.S. Fish Commission Steamers Fish
| Hawk and Albatross.

Fig. 60 Yoldiella extensa. Lateral view from the right side of the
internal morphology of a specimen from Sta. 245 Argentine
Basin. (Scale = 1.0 mm). For identification of the parts see Fig.
34.

4]
TYPE SPECIMEN. Holotype: U.S. Natl. Mus. No. 48867.

Yoldiella inconspicua Verrill & Bush 1898, p. 869, pl. 79,
Figs. 5-3.
Yoldiella nana Warén 1989 (in part), p. 227.

MATERIAL.

Cruise Sta Depth No Lat Long Gear Date

(m)

WEST EUROPEAN BASIN

Sarsia S44 1739 11 43°40.8’N
S50. 2379 = 343 43°46.7'N

Jean Charcot DS20 4226 2 47°33.0'N

(Polygas) DS21 4190 3 47°31.5'N
DS22 4144 3 47°34.1'N
DS28 4413 2 44°23.8'N

335-2. W BD 16) 7767
3°38.0'W ED 18. 7.67
9°36.7'W DS 24.10.72
9°40.77;W DS 24.10.72
9°38.4'W DS 25.10.72
4°47.5'W DS _— 2.11.72

Chain 106 316 2173— 20 S0°58.7’'N 13°01.6'W ES 18. 8.72
2209
318 2868- 72 50°04.9'N 14°23.8'W ES 20. 8.72
2890 — 50°05.3'N 14°24.8"W
323) 3338> 16) 9S0;0833'Ne 133537 WES 21. 8372
3356 — 50°08.3’N —13°50.9'W
326 3859 3 50°04.9'N 14°23.8'W ES 22. 8.72
— 50°05.3'N — 14°24.8’W
330 4632 30 50°43.5'N 17°51.7'W ES 24. 8.72
— 50°43.4’N = 17°52.9"W

11°04.0'W ES 4. 7.73
S°072'W DS 26. 8.73

Challenger 10 2540 1 56°36.0'N
Jean Charcot DS41 3548 5 47°28.3'N

(Biogas III)
(BiogasIV) DS55 4125 58 47°34.9'N 9°40.9'W DS 22. 2.74
(Biogas V) DS66 3480 3 47°28.2'N 9°00.0'W DS 16. 6.74
DS67 4150 1 47°31.0'N 9935:0'W) (DSesli. 6.74
(Biogas VI) DS71 2194 1 47°34.3'’N 8°33.8'W DS 20.10.74
DS75 3250 1 47°28.1'N 9°07.9'W DS 22.10.74
DS76 4228 26 47°34.8'N 9°33.3'W DS 23.10.74
CP13 4134 4 47°34.4'N 9°38.0'W CP 23.10.74
DS77 4240 1 47°31.8'N 9°34.6'W DS 24.10.74
DS82 4462 1 44°25.4'N 4°52.8'W DS 29.10.74
(Incal) DS01 2091 2 57°39.8'N 10°39.8'W DS 15. 7.76
— 57°59.2'N —_10°41.3'W
DS02 2081 4 57°58.8'N 10°48.5'W DS 16. 7.76
— 57°58.5'N —10°49.2'’W
C003 2466 12, 56°38:0°N) . 11564.0'W (CP) 1776
— 56°37.3’N _11°07.8’W
C004 2483— 9) 56733.22N — 11STIS’W CP 17. 7.76
1513 — 56°32.5'N 11°12.4'W
DS05 2503 270 56°28.1'N 11°11.7'W DS 18. 7.76
— 56°27.6'N 11°02.0’W
DS06 2494 277 56°26.6'N 11°10.5’'W DS 18. 7.76
— 56°25.9'N__11°10.7’W
DS07 2884 280 55°00.7'N 12°31.0'W DS 19. 7.76
— §5°01.0'N 12°21.0’W
DS08 2891 58 55°02.0'N 12°34.6'W DS 19. 7.76
— §2°01.9'N 12°33.4’W
CPO05 2884 149 55°00.4’N 12°29.4’W CP 19. 7.76
— 55°00.9’N = 12°31.1’W
CP06 2888- 218 55°02.3’N 12°40.3'W CP 19. 7.76
2893 — 55°02.6’N = 12°41.7’W
KR06 2891 1 55°02.9'N 12°43.77;W KR 20. 7.76
— 55°02.2’N —12°39.0’W
KRO7 2891 1 55°02.9'N 12°43.9'W KR 20. 7.76
— 55°02.2'N 12°39.0'W
CP07 2895 488 55°03.4’N 12°46.2";W CP 20. 7.76.
— 55°04.4'N 12°46.7'W
DS09 2897 867 55°07.7'N 12°52.6'W DS _ 20. 7.76
— 55°08.1’N = 12°53.2'’W
CP08 2644 135 50°14.7’N 13°13.5’'W CP 27. 7.76
— 50°15.2’N = 13°14.8’W
DS10 1719 = 21 50°12.7’N 13°16.6'W: DS 27. 7.76
— 50°13.2'N 13°16.4'W

42
QS01 2634 322 50°14.4’N 13°10.9'W OS 30. 7.76
— 50°15.2'N —13°11.0"W
WS01 2550 221 50°19.4'N 13°08.1'W WS 30. 7.76
— 50°19.3'N —13°06.9"W
WS02 2498 287 50°19.3’N 12°55.8'W WS 30. 7.76
— 50°20.0'N 12°56.0'W
CP10 4823 2748°25:5'N SOW ICR: 31776
— 48°26.3'N 15°09.8’W
WS03 4829 1 48°19.2’N 15°23.3,;W WS 1. 8.76
(broken valve) — 48°19.1'N 15°22.5'W
WS07 4281 5 47°30.6'N 9°37.1'W WS 7. 8.76
— 47°31.2'N = 9°35.7'W
DS14 4254- 1 47°32.6'N 9°35.7';W DS _ 7. 8.76
4307 — 47°32.9'N —-9°35.1'W
OS06 4316—- 2 46°27.3'N 9°36.2',W OS _ 9. 8.76
4307 — 47°27.9'N —_9°36.0'W
DS16 4268 7 47°29.8'N 9°33.4‘;W DS _ 9. 8.76
— 47°30.3'N
QS07 4249 2 47°36.8’N 9°34.3'W OS 10. 8.76
— 47°31.3'N = 9°34.3"W
QS08 4327 1 47°29.8'N 9°39.2'W QOS 11. 8.76
— 47°29.5'N —-9°38.8"W.
NORTH AMERICA BASIN
AtlantisII 62 2496 25 39°26.0’N 70°33.0'W ET 21. 8.64
12 64 2886 80 38°46.0’'N 70°06.0'W ET 21. 8.64
72 2864 120 38°16.0'N 71°47.0'W ET 24. 8.64
73 1330- 1 39°46.5'N 70°43.3’'W ET 25. 8.64
1470
Chain 50 76 2862 53 39°38.3'N 67°57.8'W ET 29. 6.65
78 3828 3 38°00.8’N 69°18.7'W ET 30. 6.65
85 3834 32 37°59.2’N 69°26.2’W ET 5S. 7.65
87 1102 17 39°48.7'N 70°40.8’'W ET 6. 7.65
AtlantisII 126 3806 14 39°37.0'N 66°47.0'W ET 24. 8.66
24 — 39°37.5'N 66°44.0’W
Chain 106 334 4400 5 40°42.6'N 46°13.8'W ES 30. 8.72
— 40°44.0'N 46°14.6’W
335 3882- 12 40°25.3'N 46°30.0'W ES 31. 8.72
3919
Knorr 35 340 3264- 95 38°14.4’N 70°20.3'W ES 24.22.73
3356 — 38°17.6'N 70°22.8'W

We have examined the Verrill & Bush material housed in
the U.S. National Museum. With the exception of specimen
No. 49390, which is more inflated and slightly more inequilat-
eral than is typical of specimens of Y. inconspicua and which
is probably Y. frigida, the Verrill & Bush collection is
correctly described.

Warén (1989) synonymizes Y. inconspicua with Y. nana.
We disagree with this view. Y. nana is essentially a high
latitude species from the shelf and upper slope while Y.
inconspicua s.s. is restricted to temperate latitudes at lower
slope to abyssal depths in the North American and West
European Basins.

Depth range: 1102-4829 m, but most common between
2400-3000 m.

SHELL DESCRIPTION (Fig. 61). The shell is very well
described by Verrill & Bush (1898). We would add that
although the shell is small, compressed and an ovate wedge-
shape, the width, height and thickness of shell for any given
length is variable (Fig. 62). Some specimens have a slightly
sinuous postero-ventral margin. In many specimens the gut
and internal organs are visible through a transparent shell, in
others light and dark banding patterns are present on thicker
shells. Hydroids may be present over most of the shell
margin. The large elongate, slightly ‘saddle-shaped’, internal
ligament is characteristic. The hinge teeth are small, the total
number ranges from 6 in an individual 2.42 mm in length to
13 in an individual of 3.44 mm in length. There is normally an

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 61
from the right side and detail of hinge from a right valve; a,
variation in shape of three specimens from Sta. 76 North America
Basin with b, a specimen from Sta. INCAL CP 06 West European
Basin. (Scale = 1.0 mm).

Yoldiella inconspicua inconspicua. Lateral views of shells

additional tooth in the anterior series (3/3—7/6). Although the
shape varies between individuals there is an overall trend of a _
slight increase in posterior umbonal length and in width with ©
increasing shell length. The height ratio remains more or less :
constant (Fig. 62). Analysis of shell length of two samples |
from the east and west Atlantic showed marked size differ-
ences (Fig. 64) probably indicative of the differences in time
of successful settlements. Individual peaks in the sample from
the Bay of Biscay perhaps might indicate an annual breeding
event and a lifespan of 5 or 6 years.
Shell measurements (mm) & ratios are as follows:-

Length Height Width H/L WIL PLITL ©
4.20 2.87 1.66 0.68 0.39 0.53 ’
4.03 2.78 1.54 0.69 0.38 0.51
3.79 2.71 1.53 0.71 0.40 0.50
3.66 2.45 1.43 0.66 0.39 0.53
3.58 2.50 1.38 0.70 0.38 0.50
3.41 2.75 . 0.72 0.53
1.49 1.02 0.54 0.69 0.36 0.48
0.99 0.76 S 0.77 e 0.48
0.93 0.62 s 0.67 S 0.50

INTERNAL MORPHOLOGY (Fig. 63). The morphology of the
mantle is essentially the same as in other nuculanoid species.
There are typically three mantle lobes. Specialization of the
mantle margin includes an anterior sense organ, in position

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

50
WIL

30

80

Length (mm)

| Fig. 62 Yoldiella inconspicua inconspicua. Variation in the ratios
of height H/L, width W/L and postero-umbonal length PL/TL to
length against length of a sample from Sta. INCAL CP 08 West

European Basin.

vertically below the anterior adductor muscle. A large sipho-
nal tentacle is usually attached on the right side of the
innermost wall of the siphonal embayment, but was recorded
/on the left in a few specimens. Siphons are combined and
form a wide tube with a single lumen. The feeding aperture is
/not particularly well-developed, being represented in Y.
|inconspicua by two small flaps ventral to the combined
| siphon. The inner muscular layer is not expanded anterior to
| these, as it is in some other Yoldiella species. The adductor
'muscles are large and approximately equal in size. The
| posterior muscle is circular and the anterior more ‘crescent-
‘shaped’.

| The gills are well-developed and filaments number from

Fig. 63 Yoldiella inconspicua inconspicua. Lateral view from the
right side of the internal morphology of a specimen from Sta. 72,
North Atlantic Basin. (Scale = 1.0 mm). For identification of the
parts see Fig. 34.

43

Numbers

1 2 3 4
Length (mm)

Fig. 64 Yoldiella inconspicua inconspicua. Length frequency
histograms of two samples from a, Sta. 72 North America Basin
and b, Sta. INCAL CP 08 West European Basin.

12-23, the latter in a large individual 3.55 mm in length. The
distal gill filaments lie close to the siphon and the gill axis
attaches to the siphon ventro-laterally. The labial palps are
moderately large with between 12-22 ridges on the internal
face. The palp proboscides are long and thin. The cerebral
ganglia are large, the visceral ganglia are moderate in size and
the pedal ganglia are large and elongate oval in shape. They
lie more dorsal in position in the foot than in most other
Yoldiella species. The foot is divided and fringed with papil-
lae and has a large byssal gland in the heel.

The oesophagus is distended and wide in diameter. The
longitudinal plane of the stomach lies at an angle to the
vertical sagittal plane, and anteriorly is displaced to the right
while posteriorly it lies to the left. The stomach is large but
somewhat laterally flattened. The style sac is small. The first
section of the hind gut which lies within the foot is wide in
section from thence it curves dorsally parallel to the posterior
margin of the foot. At its dorsal limit of its course there are
two small kinks to the left of the body (possibly a result of
contraction in preserved specimens) before it crosses to the
right side to form a single anterior loop the outline of which is
characteristic of the species. The diameter of the gut appears
to vary somewhat between populations, western Atlantic
specimens appear to have a more slender hindgut although
the dimensions fall within the overall range of eastern Atlan-
tic specimens. A typhlosole is present along the whole length
of the gut. A considerable amount of food material is
frequently present in the left hand digestive diverticulum.
The digestive diverticula extend well posterior within the

body. A large well-developed kidney extends anteriorly close
to the posterior limit of the diverticula.

The sexes are separate. The gonads lie dorsal and ventral
to the viscera and anteriorly they occur internal to the
digestive diverticula. A female 2.8 mm in length collected in
July (Incal CP 07) contained 255 closely packed ova with a
maximum diameter of 132 pm.

Yoldiella inconspicua africana (new subspecies)

TYPE LOCALITY. R.V. Atlantis II, Cruise 42, Sta. 201,
Angola Basin, 23.5.1968, 9°25’S, 11°35’E to 9°29’S, 11°34’E,
Epibenthic Trawl, 1964 m.

TYPE SPECIMEN. Holotype BM(NH) 1992039, Paratypes: in
collection held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
SIERRA LEONE BASIN
AtlantisIT 145 2185 29 10°36.0’'N 17°49.0'W ES 6.2.67
31 147 3984 5 10°38.0’'N 17°52.0'W ES 6.2.67
CAPE BASIN
AtlantisIT 192 2117—- 55 23°02.5'S 12°19.0'E ET 17.5.68
42 2154
194 2864 28 22°54.0'S 11°55.0’E ET 17.5.68
ANGOLA BASIN
201 1964 109 9°25.0’'S 11°35.0'E ET 23.5.68
— 9°29.0'S 11°34.0'E
CAPE VERDE BASIN
Discovery 8521' 3058- 2 20°46.9'N 18°53.4‘.W WS 25.6.74
3053 — 20°47.6'N = 18°53.5'W
8521° 3070- 1 20°47.9'N 18°53.4'W WS 26.6.74
3964 — 20°48.6'N 18°53.4'W
8521! 3113— 34 13°47.8’N 18°14.0'W WS 4.7.74
3119 — 13°48.0’'N 18°14.8'W
8532° 2958- 12 13°48.2'N 18°08.0'W WS _ 5.7.74
2952 — 13°47.6'N _18°07.5'W
Walda DS20 2514 5 ZB2O'S 8°18.1'W DS

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 65 Yoldiella inconspicua africana.
Outlines a, of a shell from the right side
and a left valve to show detail of the
hinge-plate from Sta. 8532° Cape Verde
Basin and b, two shells from the right
side from Sta. 201 Angola Basin. (Scale
= 1.0 mm).

Y. inconspicua africana is found off the West coast of Africa
in the Sierra Leone, Cape Verde, Angola and Cape Basins at
lower slope to abyssal depths. Depth range: 1964-3119 m.

SHELL DESCRIPTION (Fig. 65). Shell very small, short, rela-
tively compressed, ovate wedge-shape, very slightly inequi-
lateral, except for some very fine lines shell surface smooth,
periostracum pale yellow, iridescent; umbos just anterior to
midline, slightly raised; antero-dorsal margin slightly convex,
nearly horizontal close to umbos, then curving to slightly
pointed anterior margin, ventral margin broadly rounded,
often slightly swollen posteriorly, ascending steeply to poste-
rior supramedial angle, postero-dorsal margin nearly hori-
zontal close to umbo, then slightly convex, sloping gradually
to posterior margin; posterior hinge plate narrow; teeth
small, oblique v-shape, 4-5 on each hinge plate; ligament
short, amphidetic, relatively wide, yoke-shaped, slight exter-
nal extension on either side of umbo.

Prodissoconch length: 166 ~#m. Maximum recorded shell
length 2.56 mm (Discovery Sta. 8521°).

INTERNAL MORPHOLOGY (Fig. 66). The siphons are com-
bined but with a single lumen. The combined siphon is large
and long, with relatively thin muscular walls. The single
tentacle is large, attached to the right or the left at the base of
the siphonal embayment. The feeding aperture is moderately
well-developed with a small internal secondary fold. The
mantle epithelial in the area of the feeding aperture and
anterior to it is well supplied with gland cells, similar to those
present in Yoldiella curta (p. 47). The anterior sense organ
lies far anterior and is covered dorsally by an extended and
particularly well-developed hood derived from the middle
sensory lobe. The adductor muscles are slightly unequal in
size; the larger anterior muscle is bean-shaped and the
posterior is round in outline. The gills are well-developed
with up to 12 filaments. The labial palps are relatively large
with up to 15 broad ridges on the inner face. The palps extend
across approximately one third of the body. The palp probos-
cides are long and relatively slender. The foot is well-
developed with a deeply divided sole. The byssal gland is
moderately small. The ganglia are large. Both cerebral and

ee

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

| Fig. 66 Yoldiella inconspicua africana. Lateral view from the right
side of the internal morphology of a specimen from Sta. 8532°
Cape Verde Basin. (Scale = 1.0 mm). For identification of parts
see Fig. 12.

visceral ganglia are ‘club’-shaped, the visceral being the
larger. The ovoid pedal ganglia lie central in the foot, just
dorsal and anterior to the byssal gland. A large statocyst lies
dorsal to each pedal ganglion.

The mouth is set posterior to the anterior adductor muscle.

The oesophagus is relatively narrow while the stomach is
large with a small style sac which does not penetrate the foot.
The duct of the right digestive diverticulum passes dorsally
| around the hind gut to join the stomach close to the oesoph-
ageal aperture. One of the two left diverticula opens ventral
to the gastric tooth while the second opens adjacent to that
| from the right. No ingested material was observed in the
| diverticula. From the style sac, the hind gut passes between
| the pedal ganglia and the byssal gland before turning dor-
| sally. It makes a very small flexure on the left side immedi-
| ately before crossing to the right side at the dorsal limit of the
body posterior to the ligament. On the right side of the body
| the hind gut makes a broad sweeping loop as far as the
| posterior face of the anterior adductor before passing dorsally
| and posteriorly to the anus.
The gonads overlie the posterior, dorsal and dorso-lateral
| Sides of the viscera and extend antero-dorsally with part lying
/ centrally between the digestive diverticula. The kidney is
|small in comparison to the other species of Yoldiella
| described here, extending no further than the posterior edge
| of stomach. The heart is relatively large.

This subspecies differs from Y. inconspicua s.s. in having a
more elongate outline and a more pointed posterior shell
margin. The dorsal margin is slightly more convex, the
height/length ratio is slightly greater. The hind gut differs
) slightly in the course taken to the right of the body (Figs. 60 &

45

Fig. 67 Yoldiella inconspicua profundorum. Lateral view of shell
from the right side and detail of the hinge-plate of a left valve.
Specimens from Sta. 242 Argentine Basin. (Scale = 1.0 mm).

Yoldiella inconspicua profundorum (new subspecies)

TYPE LOCALITY. R.V. Atlantis II, Cruise 60, Sta. 256,
Argentine Basin, 24.3.1971, 37°40.9'S, 51°19.3'W,
Epibenthic Trawl, 3906-3917 m.

TYPE SPECIMEN. Holotype: BM(NH) 1992038, Paratypes: in
collection held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
ARGENTINE BASIN
AtlantisI] 242 4382- 50 38°16.9'S 51°56.1'W ES 13.3.71
60 4402
243 3815— 1 37°36.8’S 52°23.6’'W ES 14.3.71
3822
245) E2107 2. S658, SSs014owWr 2ESe 14327
247 5208— 1 43°33.0’'S 48°58.1'W ES 17.3.71
5223
252 4435 23 38°29.8’S 52°09.1’W ES 22.3.71
256 3906- 19 37°40.9'S 51°19.3’W ES 24.3.71
3917
259 3305— 5 37713°3'S. 5224510. W ES 92613371
3317
264 2041- 6 36°12.7’'S 52°42.7'W ES 28.3.71
2048

Restricted to the Argentine Basin at lower slope and abyssal

depths. Depth range: 2041-5223 m.

SHELL DESCRIPTION (Fig. 67). Shell small, equilateral, mod-
erately inflated, subovate, fine concentric lines; umbos
slightly raised, inwardly directed; shell outline somewhat
variable, anterior and posterior dorsal margins straight proxi-
mal to umbo, curve distally to broadly rounded anterior and
slightly pointed posterior margins, ventral margin smooth
curve; anterior and posterior hinge plates relatively narrow

46

Fig. 68 Yoldiella inconspicua profundorum. Lateral view from the
right side of the internal morphology of a specimen from Sta. 242
Argentine Basin. (Scale = 1.0 mm). For identification of the parts
see Fig. 34.

(but broader than in Y. inconspicuous s.s.) short, with up to 7
teeth on each side; ligament amphidetic, large, slightly elon-
gate, short external secondary extensions of fused perios-
tracum on either side of umbo.

The shell shape of this subspecies while similar to that of Y.
inconspicua s.s. is slightly more rounded, in particular the
posterior limit of the shell margin is medial instead of
supramedial. The hinge plate is broader and the teeth stron-
ger in Y. i. profundorum.

Maximum recorded shell length: 3.31 mm.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Shell measurements (mm) & ratios are as follows:-

Length Height Width H/L W/L PL/TL
2.90 2.09 1225 0.72 0.43 0.52
Sheil EPS) 1.47 0.68 0.44 0.52

INTERNAL MORPHOLOGY (Fig. 68). Very similar to that of Y.
inconspicua s.s. and Y. argentinea. In Y. i. profundorum, size
for size, the posterior adductor muscle is more elongate than
in Y. inconspicua s.s., the palps are slightly smaller and the
ridges are broader but not as long and slightly fewer in
number (12-14 internal ridges).

Yoldiella argentinensis (new species)

TYPE LOCALITY. R.V. Atlantis II, Cruise 60, Sta. 237,
Argentine Basin, 11.3.1971, 36°32.6'S, 53°23.0’W,
Epibenthic Trawl, 993-1011 m.

TYPE SPECIMEN. presently housed in the Sanders collection,
Woods Hole Oceanographic Institution, Paratypes: in collec-
tion held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
ARGENTINE BASIN
Atlantis II 236 497-518 3 36°27.0'S 53°31.6’W ES 11.3.71
60 237 =993- 26 36°32.6'S. 53°23.0'W BS” 113.71
1011
239 1661— 26 36°49.0'S 53°15.4,;W ES 11.3.71
1679

Y. argentinensis has only been recorded from Argentine

Fig. 69 Yoldiella argentinensis.
Lateral view of the shells from the
right side to show variation in shape;
detail of the hinge-plate of a left
valve; dorsal view of a shell.
Specimens from Sta. 237 Argentine
Basin. (Scales = 1.0 mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

Basin at shelf slope break and upper slope depths. Depth
range: 497-1679 m.

SHELL DESCRIPTION (Fig. 69). Shell small, subovate,
approximately equilateral, relatively compressed, ‘boat-
shaped’ in outline except for fine concentric lines, smooth,
iridescent; umbos inflated, inwardly directed; anterior and
posterior dorsal margins concave proximal to umbo, distal to
umbo slightly convex, slope gradually to anterior and poste-
rior margins, anterior margin slightly produced, ventral mar-
gin broadly curved, more convex posteriorly to slightly
produced, posterior margin; horizontal axis supramedial;
hinge plate short, teeth small, ‘V-shaped, proximal teeth
very small, difficult to discern, 6 anterior and 6 posterior in
individual 3.66 mm long; ligament amphidetic, large with
long, narrow, secondary external anterior and posterior
extensions.

Prodissoconch length: 150 1m. Maximum recorded shell

_ length: 4.03 mm.

The shape of the shell is similar to that of Yoldiella
inconspicua s.s. The main differences include the marked
proximal concavities of the dorsal margin, the more inflated
umbos and detail of the hinge.

| INTERNAL MORPHOLOGY (Fig. 70). There is a wide combined
siphon with a single lumen. The siphonal tentactle lies to the
left side of the siphonal embayment. A feeding aperture is
present ventral to the combined siphon. The adductor
/muscles are slightly unequal in size, in cross-section the
posterior is broadly oval in outline while the larger anterior
/ muscle is ‘bean’-shaped.

The foot is well-developed and there is a large byssal gland
‘in the heel. The gills are well-developed with between 10-17
| alternating filaments (the number varying with the size of the
)individual). The labial palps are of moderate size, extending
| approximately one third across the body with between 10-18

internal ridges. The hind gut forms a single loop on the right
\side of the body (Fig. 70).

Yoldiella curta Verrill & Bush 1898

TYPE LOCALITY. North America Basin, 40°16.5’N,
(67°05.3'W. 1290 fm. Original specimens were taken from the
\North America Basin between 41°11.5’N, 66°12.3’W and

Fig. 70 Yoldiella argentinensis. Lateral view from the right side of
the internal morphology of a specimen from Sta. 237 Argentine
Basin. (Scale = 1.0 mm). For identification of the parts see Fig.
34.

47

39°38'N — 70°22'W by U.S. Fish Commission Steamer Alba-
tross, 5.10.1883, Beam Trawl.

TYPE SPECIMEN. Holotype U.S. Natl. Mus. No. 38457.

Phaseolus ovatus Verrill 1884, p.230.

Yoldia jeffreysi Verrill 1884, p.229; Verrill 1885, p. 576.

Yoldia curta Verrill & Bush 1898, p. 868, pl. 97, Fig. 8;
Warén 1989, p. 244.

Yoldiella miniscula Verrill & Bush 1898, p. 870, pl. 79, Figs.
2, 7; Warén 1989, p. 244, Figs. 14 A & B.

Although a very well-defined species in that it is the only
known Yoldiella with the hind gut configuration described
below (p. 51), in its shell shape it is very similar to a number
of other species. We believe that Y. miniscula Verrill & Bush
is synonymous with it, indeed Verrill & Bush (1898) state this
latter very minute species may, with a larger series, prove to
be the young of one of the preceding species they describe.
These latter include Y. inconspicua, Y. pachia, Y. lucida, Y.
iris, Y. inflata, Y. subangulata, Y. lenticula and Y. curta.
Verrill & Bush (1898) were correct in their supposition and
here we illustrate specimens of Y. miniscula to show their
similarity (Fig. 71).

Note. The holotype specimen of Y. miniscula (U.S. Natl. Mus.
No. 38415) is from the same Station as the holotype of Y. curta.

MATERIAL.
Cruise Sta DepthNo Lat Long Gear Date
(m)

WEST EUROPEAN BASIN

La Perle DS06 1

(BiogasI) DS07 4
DS12 2180 1 47°28.5'N 8°35.5'W DS 9. 8.72
DS13 2165 15 47°33.7'N 8°39.9'W DS _ 9. 8.72

Jean Charcot

(Polygas) DS25 2096 16+6v 44°08.2’N 4°15.7';W DS © 1.11.72
DS26 2076 2 44°08.2’N 4°15.0'W DS 1.11.72
CV10 2108 2v 47°30.7'N 8°40.6'W CV 22.10.72

(Biogas II) DS32 2138 4 47°32.2'N 8°05.3’W DS 19.4.73

(BiogasIII) _DS36 2147 5 7 4732.7'N 8°365'W DS 24. 8:73

(1 empty 4 dried)

DS38 2138 7+2v 47°32.5'N

DS49 1845 4+4v 44°05.9'N

(Biogas IV) DS52 2006 2+4v 44°06.3'N
DS61 2250 1 47°34.7'N

DS62 2175 10 47°32.8'N

DS63 2126 19+2v 47°32.8'N

DS64 2156 7+4v 47°29.2'N

8°35.8'W DS 25. 8.73
4°25.6'W DS 1. 9.73
4°22.4';W DS 18. 2.74
8°38.8'W DS 25. 2.74
8°40.0'W DS 26. 2.74
8°35.0'W DS 26. 2.74
8°30.7'W DS 26. 2.74

(Biogas V) CP07 2170 3 44°09.8’N 4°16.4,W CP 21. 6.74
(Biogas VI) DS71 2194 7 47°34.3'N 8°33.8’W DS 20.10.74
DS86 1950 28 44°04.8’N 4°18.7"W DS 31.10.74
DS87 1913 7 44°05.2’N 4°19.4;W DS 1.11.74
(13+8v)
DS88 1894 5 44°05.2’N 4°15.4;W DS 1.11.74
Jean Charcot CP08 2177. 3 = 44°33.2’N_—-8°38.5'W_CP__20.10.74
(Biogas VI) CP09 2171 1 47°33.0’'N 8°44.1'W CP 20.10.74

Soe IN| 0255.00W CB 65e7. 476
— 57°56.0'N 10°55.0’'W

57°58.4'N 10°42.8'W CP 16. 7.76
— ST°S7.7'N_ 10°44.6'W

57°59.7'N 10°39.8'W DS _ 15. 7.76.
— 57°59.2'N 10°41.3"W

57°58.5'N 10°48.5'W DS 16. 7.76
— S7T°58.5'N 10°49.2'W

Jean Charcot CP01 2068- 38

(Incal) 2040
CP02 2091 4
DS01 2091 468

DS02 2081 544

DS05 2503 2 56°28.1‘'N 11°11.7'W DS _ 18. 7.76
— 56°27.6'N 11°12.0’'W
DS06 2494 6 56°26.6'N 11°10.5'W DS 18. 7.76

— 56°25.9'N 11°10.7'W

48
OS01 2634 5 50°14.4’N 13°10.9'W OS 30. 7.76
— 50°15.2'N 13°11.0’'W
Sarsia S-65 1922 19 46°15.0'N 4°50.0'W ED 25. 7.67
33/2 1537- 8 43°41.0’N 3°36.0'W AD 13. 7.67
1830
S44 1739 68 43°40'8’N 3°35'2'W ED 16. 7.67
Chain 106 313 1500- 106 5132 24Ne 112335: 94 Wa ES elise 2
1491
316 2173- 91 50°58.7’N 13°01.6’'W ES 18. 8.72
2209 — 50°57.7'N_ 13°01.3'W
NORTH AMERICA BASIN
Atlantis II 73 1470— 699 39°46.5’'N 70°43.3’W ET 25. 8.64
12 1330
Chain 50 87 1102 354 39°48.7'N 70°40.8'W ET 6. 7.65
Chain 58 103 2202 303 39°43.6’N 70°37.4'W ET 4. 5.66
AtlantisII 115 2031-249 39°39.2'N 70°24.5'W ET 16. 8.66
24 2051
AtlantisII 128 1254 8 39°46.5'N 70°45.2'W ES 15.12.66
30 Bil Wiley 5S) 39°38.5'N 70°36.5'W ES 18.12.66
— 39°39.0'N 70°37.1’'W
Chain 88 207 805— 239 39°51.3'N 70°54.3’W ES 21. 2.69
811 — 39°51.0’N 70°56.4'W
210 2024 4 39°43.0’N 70°46.0'W ES 22/23.2.69
1064 — 39°43.2’N 70°49.5’'W
ARGENTINE BASIN
Atlantis II 239 1661-— 36 36°49.0’'S 53°15.4’'W ES 11. 3.71
60 1679
240 2195- 1 367432448 SS c102 IW) SES? 92393271
2323
264 2041- 4 STORIES SPUR aS AS, STAAL
2048
GUYANA BASIN
Knorr 25 293 2456-— 49 8°58.0'N 54°04.3'W ES 27. 2.72
1518
295 1000— 25 8704.2'N 54°21-3°W ES 28) 2.72
1022
SIERRA LEONE BASIN
AtlantisII 142 1124 45 10°30.0'N 17°51.5'W ES 5. 2.67
31 1796
145 2185 1 10°36.0'N 17°49.0'W ES 6. 2.67
BRAZIL BASIN
159 §834— 1 7°58.0’'S 34°22.0'W ES 18. 2.67
939
167 943— 4 7-58.0'S) 34°17/0'W ES: 20 2567
1007 — 7°50.0'S
169 587 6 8°03.0'S 34°23.0'W ES 21. 2.67
— 8°02.0'S 34°25.0’W
CAPE BASIN
Atlantis II 189 11007496 23°00.0’'S 12°45.0'E ES 16. 5.68
42 1014
190 974— 7 23°05.0’'S 12°45.0’E AD 17. 5.68
979
191 1546- 2 23°05:025 122311549 SES) “1728568
1559
Walda DS10 1432 1 18°40.0'S 10°56.3’E DS

Y. curta is probably the most widely distributed species of
Yoldiella. It occurs throughout the Atlantic at slope and
abyssal rise depths. Depth ranges:

BrazileBasin goer cere cere ce ree cone seca 587-1007 m
Cape: Basin: wean sseaadaseen terete cctanees tbe 974-1559 m
Guyana Bassinet fee. mentee eerccca enn. see 1000-1518 m
ANION BEINN Ghagdeonasnoarisoéedeoanoobovgacc 1661-2048 m
SHAE CONS BHASIM.coooapseosdencpoboocnooonsoc: 1624-2185 m
North Aimenicanys aSimmereseeecreeeeeresceetee ce 805-2178 m
West:European: Basin soins agscso- <r tecumtar 1537-2634 m

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

SHELL DESCRIPTION (Figs. 71 & 72). Verrill & Bush (1898)
give a full and accurate description of this species. Y. curta
has a small, smooth, ovate, semi-transparent shell through
which the characteristic, indeed unique, course of the hind
gut is usually visible. The latter is usually seen as a double
strand on the right side of the body with a single loop on the
left extending out from the posterior margin of the anterior
adductor. The hinge is short with a large internal ligament.
The outline of the shell is variable both within (Fig. 73) and
between populations. There is no lunule or escutcheon. The
hinge plate is very short and slender and does not reach to the
level of the inner margin of the adductor muscles. The
number of teeth on each hinge plate varies with size up to a
maximum of 7. There is usually one more tooth on the
posterior hinge plate. The shell may be confused with other
species e.g. Y. inconspicua, Y. lucida and Y. frigida, even
though the hinge and the hind gut configuration are charac-
teristic.

In general, with increasing length, the height/length ratio
increases thus producing a more rounded outline (Figs. 74 &
75). The outlines of the anterior and posterior margins are
variable. The anterior margin may be smoothly rounded or
slightly angular, while the posterior margin is slightly
extended with the degree of curvature and the position of the
posterior limit in relation to the mid horizontal shell axis
somewhat variable. With increasing size shells become more
inequilateral with the posterior end slightly elongate. There is
also a gradual increase in the width to length ratio.

Prodissoconch length: 170 wm. Maximum recorded shell
length: 4.77 mm.

INTERNAL MORPHOLOGY (Figs. 76 a & b). As in other
Yoldiella species the mantle is little modified and for the most
part unfused. The inner muscular lobe is relatively broad and
somewhat folded in preserved specimens. Posteriorly the
inner muscular layer fuses to form a relatively wide, thin-
walled combined siphon with a single lumen. In one whole
mount several faecal pellets lying one on top of the other
were seen within the siphon and in one series of sections two
faecal pellets one in the dorsal half and one in the ventral half
of the siphonal lumen were seen. The gill axes attach laterally
close to the ventral margin of the siphon, thus any inhalent
componant is probably restricted to the ventral margin. A
large siphonal tentacle originates from a pocket in the sipho-
nal embayment at the base of the siphon, usually on the left
side. A small secondary muscular mantle fold marks the inner
limit of the feeding aperture which in life must extend beyond
the shell margin as a pair of flaps. Here the inner mantle lobe
is increased in thickness and width and in the preserved
contracted state is much folded. The epithelium on the
ventral side of the secondary muscular fold, on the dorsal side
of the muscular fold, as well as the epithelium between the
two, is well supplied with acidophilic gland cells. The gland
cells within the epithelium between the folds extend anteri-
orly beyond the limit of the feeding aperture for a short
distance. The anterior sense organ is well-developed. Over it
the middle sensory lobe forms a long thin flap which is
well-supplied with glandular epithelial cells. The adductor
muscles are large, approximately equal in size with the ‘quick’
and ‘catch’ parts clearly visible. The anterior adductor if
anything is slightly the larger, it is ‘crescent’-shaped in cross
section while the posterior muscle is approximately circular.
The gills lie comparatively ventral in position within the

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

mantle cavity. The outer and inner demibranchs are attached
to the mantle and body respectively by tissue junctions.

| Depending on the size of the individual there are 11-16

alternating gill plates. The labial palps are moderately large,

| extending approximately 1/3 across the body and have
| between 13-19 wide ridges on their inner faces.

The foot is large and well-developed. A large byssal gland

| is present, with little obvious internal structure as seen in
| whole mounts. At the point where the gland opens to the
| exterior the surrounding tissue contains acidophilic gland and
| basiphilic cells. As in other species of Yoldiella there is a

broad band of posterior retractor muscles. The latero-
posterior retractor muscle is inserted ventrally on lateral body
wall. There are three major pairs of anterior pedal retractor

| muscles which, from the neck of the foot, pass to either side
| of the oesophagus and insert on the shell posterior to the

anterior adductor muscle. Another smaller pair of anterior

| retractor muscles insert immediately posterior to the anterior

| adductor and pass within the ventral visceral margin at

| epithelia to the region of the mouth (Fig. 76b). Posterior to

| these a second pair of muscles pass vertically from the dorsal
shell margin to the labial palps.

The cerebral ganglia are relatively large, as are the
elongate-oval pedal ganglia. The visceral ganglia are smaller,
cylindrical and lie close to the antero-ventral margin of the
posterior adductor muscle.

The mouth is a short distance posterior to the anterior
adductor muscle. The oesophagus curves dorsally and anteri-
orly before turning posteriorly to enter the stomach approxi-
mately midway on the left side of the anterior face. No
sorting ridges are visible externally on the right side of the
stomach. There is a prominent gastric tooth at the left
antero-dorsal edge of the gastric shield. The latter extends
posteriorly over the left, the dorsal and part of the right wall.
The style sac is small, narrow and penetrates the foot to level
of pedal ganglia. The digestive diverticula are extensive
occurring on the right and left sides of the body and anterior
to the stomch. Fine material was observed in the lumen of the
left digestive diverticulum. A duct from the left side opens
into the stomach below the gastric tooth. The course of the
two other ducts are less clear, but from the evidence of other
species they probably open into the stomach anteriorly close
to the oesophagus. The hind gut sweeps across the ventral

49

Fig. 71 Yoldiella
curta. a, lateral
external and
internal view of
left valve of the
type specimen of
Y. curta (USNM
No.38457) and b,
the lateral
external and
internal view of
the right valve of
the type specimen
of Y. miniscula
(USNM No.
38415). Specimens
from the U.S. Fish
Commission Sta.
2084 North
America Basin.
(Scale = 1.0 mm).

Fig. 72 Yoldiella curta. a, lateral view from right side of shell,
detail of the hinge-plate of a right valve and dorsal view of a shell
from U.S. Fish Commission Sta. 2073 North America Basin,
compared with b, a lateral view of a shell from the left side and
detail of the hinge-plate of a right valve from Sta. Polygas DS 25
West European Basin. (Scale = 1.0 mm).

right side of the viscera to the anterior adductor muscle,
where it crosses to the left side of the body to form a single
loop before returning to the right alongside the outward

50

Fig. 73 Yoldiella curta. Outlines of shells of a similar size to show
intrapopulation differences in shape. Specimens from Sta. BG IV
DS 86 West European Bain. (Scale = 1.0 mm).

50
WIL . oe On e ‘. ;
e Uo “aS rare one
30
80
BLY om *.38* qeineae Saxe
60
60
PL/TL Segoe einle
50 °e « . oe e
ar oe = = raat Se we
1 2 3
Length (mm)

Fig. 74 Yoldiella curta. Variation in the ratios of height H/L, width
W/L and postero-umbonal length PL/TL to length against length
of a sample from Sta. INCAL CP 01 West European Basin.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 75 Yoldiella curta. Outlines of shells from the right side to

show change in shape with growth. Specimens from Sta. BG I DS
13 West European Basin. (Scale = 1.0 mm).

section of the mid dorsal margin of the body and from there
to the anus. A shallow typhlosole is present along the length
of the hind gut. Occasional aberrent specimens were
recorded with an extra length of gut being accommodated
within the general pattern (see Fig. 75).

The kidney is well-developed. There is a large pericardial
cavity but the heart is relatively small with few muscle fibres
in the wall of the ventricle. Gonads occur dorsally from the
anterior adductor muscle to just posterior to the stomach and
in the anterior part of the body internal to the digestive
diverticula and hind gut. The sexes are separate. In one
sectioned female (2.3 mm) 27 ova were counted and in other
specimens (1.8 and 2.1 mm) 69 and 70 ova were recorded.
respectively. The maximum dimension was 108 ym.

This is one of a few deep-water specimens which were
brought to the surface alive and from which it was posible to
make some observations on the ciliary currents of the mantle
and the contained organs. The ciliary sorting mechanisms of
the gills, palps, mantle and body were found to be similar to

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

those described by Stasek (1965) for Yoldia ensifera.

The ciliation on the frontal surface of the gill plates is
axially directed, there those on the posterior 2/3rds of the axis
move anteriorly and those on the anterior 1/3rd move poste-
riorly. Particles thus accumulate at a point which is close to
where the palp proboscides join the dorsal palp surface and
they are accepted onto the palp. The palp ridges sort in the
manner described by Stasek (1965). Rejected particles arrive
at the posterior-ventral margin of the palp. A main rejection
tract at the mantle edge to the inside of the muscular fold
carries these particles to a point of collection at the ventral
edge of the feeding aperture. Particles on the body eventually
pass to the dorsal margin of the palps and join with those
from the gills.

Hydroids were present on the shell margins of a number of
specimens.

Yoldiella perplexa (new species)

TYPE LOCALITY. R.V. Knorr Sta. 299, Guyana Basin,
29.2.1972, 7°55.1'N, 54°42.0'W, Epibenthic Trawl,
1912-2076 m.

| TYPE SPECIMEN. Holotype BM(NH) 1992031, Paratypes: in
_ collection held by J.A. Allen.

Fig. 76 Yoldiella curta. Lateral views from the right and left sides
of the internal morphology of a specimen from Sta. BG VI DS 71
West European Basin. (Scale = 1.0 mm). For identification of the
parts see Fig. 34.

51

Fig. 77 Yoldiella perplexa. Lateral view of shell from the right
side, lateral internal view of a right valve to show detail of
hinge-plate and a dorsal view of a shell. Specimens from Sta. 299
Guyana Basin. (Scale = 1.0 mm).

MATERIAL.

Cruise Sta Depth No Lat Long Gear Date
(m)

GUYANA BASIN

Knorr 25 293 2456- 22 8°58.0'N 54°04.3’'W ES 27.2.72
1518

299 1942—- 82 7°55.1’N 54°42.0'W ES 29.2.72

2076

Recorded from the lower slope of the Guyana Basin. Depth
range: 1456-2076 m.

SHELL DESCRIPTION (Fig. 77). Shell small, ovate, moderately
inflated, inequilateral, smooth with some fine irregularly
spaced concentric lines at ventral margin, periostracum pale
yellow; umbos moderately developed, slightly raised,
inwardly directed anterior to midline; proximal dorsal margin
straight in small specimens, slightly convex in larger, antero-
dorsal margin curves gradually to wide anterior margin,
dorsal part slightly more convex than ventral, ventral margin
deeply convex, postero-dorsal margin slopes gradually to
steeply rounded posterior margin, postero-ventral margin
relatively straight and in some specimens very slightly sinous;
hinge plate moderately broad, not narrow below umbo, hinge
teeth strong, few in number, 6 anterior and 6 posterior in
individual 2.6 mm; ligament amphidetic, moderate in size,
not extending to ventral edge of hinge plate, round in lateral

52

Fig. 78 Yoldiella perplexa. Lateral view from the right side of the
internal morphology of a specimen from Sta. 299 Guyana Basin.
(Scale = 1.0 mm). For identification of parts see Fig. 34.

view with little external periostracal extension anterior and
posterior to the umbo.

Prodissoconch length: 177 14m. Maximum recorded shell
length: 3.11 mm.

INTERNAL MORPHOLOGY (Fig. 78). The combined siphons
are well-developed. There is a long thin siphonal tentacle
attached to the left ventral side of a moderately deep siphonal
embayment. The anterior sense organ is well-developed. The
adductor muscles are relatively large; the anterior is bean-
shaped and slightly larger than the posterior and which is
more round.

The gills bear up to 16 alternating plates which are distant
from the internal limit of the siphons and to which they are
joined by long extended axes. The labial palps are relatively
small and extend approximately 1/4 distance across body.
There are up to 13 palp ridges on the inner face. The palp
proboscides are moderately well-developed.

The foot is also well-developed and anterior in position in
preserved specimens. There is a small byssal gland and the
pedal ganglia dorsal to the gland are also small. The visceral
ganglia are slender and elongate while the cerebral ganglia
are slightly larger and more ‘club’-shaped. The stomach and
style sac are large. The hind gut describes a shallow single
loop to the right side of the body. No gonads were seen in the
whole mounts.

Y. perplexa is characterized by a rounded high shell which
has a slightly sinuous postero-ventral shell margin. Unlike
other high rounded species such a Y. americana and Y.
subcircularis it has a simple and not particularly extensive
hind gut loop to the right and thus has characters intermedi-
ate between Y. lucida or Y. obesa and species such as Y.
subcircularis.

Yoldiella americana (new species)

TYPE LOCALITY. R.V. Chain Sta. 84, North America Basin,
4.7.1965, 36°24.4'N, 67°56.0’W, Epibenthic Trawl, 4749 m.

TYPE SPECIMEN. Holotype BM(NH) 1992032, Paratypes: in
collection held by J.A. Allen.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

MATERIAL.
Cruise Sta. Depth No Lat Long Gear Date
(m)
NORTH AMERICA BASIN
AtlantisII 70 4680 190 36°23.0’'N 67°58.0'W ET 23. 8.64
12

Chain 50 77 ~—- 3806 1 38°00.7’N 69°16.0'W ET 30. 6.65
80 4970 75 34°49.8'N 66°34.0'W ET 2. 7.65
81 5042 9 34°41.0'N 66°28.0'W ET 2. 7.65
83 5000 4 34°46.5'N 66°30.0'W ET 3. 7.65
84 4749 235 36°24.4'N 67°56.0'W ET 4. 7.65
85 =. 33834 4-37°59:2‘N (69°26.2'W IEE 95.4765
86 3843 «268 «37°59.0'N 69°18.5'W ET 5. 7.65

AtlantisII 92 4694 3 36°20.0'N 67°56.0'W ET 13.12.65

17 93 5003 6 34°39.0'N 66°26.0'W ET 14.12.65

AtlantisII 121 4800 66 35°50.0'N 65°11.0'W ET 21. 8.66

24 122 4833 25 34°50.0'N 64°57.5'W ET 21. 8.66
— 35°52.0'N 64°58.0'W

123 4853 94 37°29.0'N
124 4862 76 37°26.0'N

64°14.0'W ET 22. 9.66
63°59.5'W ET 22. 8.66

— 37°25.0'N 63°58.0'W
125 4825 37 37°24.0'N 65°54.0'W ET 23. 8.66
— 37°26.0'N 65°50.0’'W
AtlantisII 175 4667— 145 36°36.0'N 68°29.0'W ES 29.11.67
40 4693 — 36°36.0'N 68°31.0'W
Chain 106 330 4632 243 50°43.5’N 17°51.7';W ES 24. 8.72
— 50°43.4’N = 17°52.9'W
331 4793 10 41°13.0’'N 41°36.7'W ES 29. 8.72
— 41°13.2'N 41°38.7'W
ARGENTINE BASIN
AtlantisII 242 4382- 8 38°16.9'S 51°56.1'W ES 13. 3.71
60 4402
247 5208- 293 43°33.0'S 48°58.1.W_ ES 17. 3.71
5223
252 4435 1 38°29:8'S; S2:09;" Wa BS) 225 3.7
GUYANA BASIN
Knorr 25 287 4980— 12 13°16.0'N 54°52.2’,W ES 24. 2.72
4034 — 13°15.8’N 54°53.1"W
288 4417— 45 11°02.2'N 55°05.5’'W ES 25. 2.72
4429 — 11°03.8'N —55°04.8"W
Biovema DS03 5150 3 10°47.1'N 42°40.77;W DS 16.11.77
— 10°47.1'N 42°40.3"W
DS05 5100 =28 10°45.9'N 42°40.2'W DS _ 18.11.77
— 10°46.8'N 42°39.8'W
CO04 5100 1 10°45.9'N 42°40.2".W CP 18.11.77
— 10°45.9'N 42°39.3'W
DS09 5875 Ty 1A536:54N S275 SAW DS) 251i
— 11°37.1'N 32°51.3'W
DS11 5867 2 11°37.5'N 32°43.8'W DS _ 26.11.77
— 11°37.6'N 32°52.8'W
SIERRA LEONE BASIN
AtlantisII 139 2099- 1 10°33.0’N 17°53.0'W ES 4. 2.62
31 2187

Y. americana is found mainly in the deep western Atlantic
with a few records to east of the Atlantic Ridge at the western
edge of the Sierra Leone Basin. Otherwise the species is
recorded from the North America, Guyana and Argentine
Basins. Depth range: 2099-5867 m.

SHELL DESCRIPTION (Fig. 79). Shell small, ovate, moderately
inflated, approximately equilateral; umbos moderately
raised, inwardly directed; dorsal margin slightly convex,
antero-dorsal margin slopes evenly to join broad curve of
anterior margin, ventral margin more convex posteriorly,

DEEP-SEA PROTOBRANCHIA (BIVALVIA) 38)

rior external extension of fused periostracum.

Prodissoconch length: 187 1m. Maximum recorded shell
length: 2.5 mm.

There is little change in the posterior umbonal length/total
length ratio or the height/length ratio with increasing size,
however, individuals tend to become more tumid with
increasing length. The width/length ratio is the most variable
feature when comparing populations from different areas
(Fig. 80). Thus, specimens from the west Atlantic are rela-
tively wider than those from the the east Atlantic although in
lateral view (Fig. 81) and in internal anatomy they differ
little. Populations are remarkably different in their size
range. This is probably indicative of single massive successful
spatfalls at different times (Fig. 82).

INTERNAL MORPHOLOGY (Fig. 83). The anterior sense organ
is moderately well-developed. The siphons are combined,
with the inhalent siphon open at the ventral edge. A siphonal
tentacle originates from the base of the siphonal embayment,
either on the nght or the left side. There is a feeding aperture
ventral to the siphons. Anterior to this the inner muscular
lobe is enlarged convoluted and heavily ciliated. The adduc-

Fig. 79 Yoldiella americana. Lateral view of shell from the right
side, lateral internal view of a left valve, enlarged view of a
hinge-plate of a left valve and a dorsal view of the shells.
Specimens from Sta. 80, North America Basin. (Scale = 1.0 mm).

50 A aN Aaa
Pees. ” awni’ss ois

30

80

Length (mm)

Fig. 80 Yoldiella americana. Variation in ratio of height H/L,
width W/L and postero-umbonal length PL/TL to length against
length of subsamples from Sta. Biovema DS 05, Cape Verde
Basin (closed circles) and from Sta. 247 Argentine Basin (open
triangles).

ClGieicie

dosterior margin rounded or with slight angulation forming

light subrostrum; hinge plate moderately strong, long strong _ Fig. 81 Yoldiella americana. Outlines of shells from the right side
hevron-shaped teeth, equal numbers on anterior and poste- to show change in shape with growth of specimens from Sta.

ior plates, 14/14 in a specimen 2.44 mm total length; Biovema DS 05 Cape Verde Basin (left) compared with
igament amphidetic, small, rectangular in shape, small poste- specimens from Sta. 247 Argentine Basin. (Scale = 1.0 mm).

54

tor muscles are relatively small and elongate and approxi-
mately equal in size. The gills are well-developed. The exact
number of gill plates is difficult to determine but at least 16
are present in the largest specimen. The labial palps are
relatively large with up to 17 ridges on the inner face and with
long palp proboscides. The foot is elongate with a relatively
narrow neck. Many subepithelial gland cells open on to the
sole of the foot. There is a relatively large byssal gland in the
heel. A small papilla is present immediately posterior to the
aperture of the byssal gland. The cerebral and visceral ganglia
are ‘club’-shaped and of moderate size. The pedal ganglia, in
the neck of the foot, are large and elongate.

The hind gut passes from the style sac to the /eft side of the
body and forms a loop then crosses to the right side where it
forms a similar loop before passing postero-dorsally to the
anus. The point of crossover from left to right is ventral to the
ligament or slightly posterior to it. A small typhlosole is
present along the length of the gut.

The sexes are separate. From whole mounts, it appears
that the ovaries contain relatively few large ova. The testes
occupy a large part of the anterior half of the body with the
digestive diverticula dorsal to them.

Y. americana has a similar rounded shape to Y. perplexa
although without the slight sinuous postero-ventral margin of
the latter species. Y. americana is the only species taken with
a single hind gut loop to the right and left of the body.

Yoldiella subcircularis (Odhner 1960)

Portlandia (Yoldiella) subcircularis Odhner 1960, p. 369, pl.
ILE Ne te

TYPE LOCALITY. Swedish Deep-Sea Expedition, Sta. 373,
North America Basin, collected by R.V. Albatross,

24.8.1948, 28°25'N, 61°0S’'W -— _ 28°05'N, 60°49’W,
$,500—-5,987 m.
60
“2
®
2
E40
z
20
1 2
Length(mm)

Fig. 82 Yoldiella americana. Length frequency histogram of a
sample from Sta. 84, North America Basin.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 83 Yoldiella americana. Lateral views from right and left sides
of the internal morphology of a specimen from Sta. 84, North
America Basin. (Scale = 1.0 mm). For identification of parts see
Fig. 34.

TYPE SPECIMEN. Holotype: Swedish Museum of Natural His-
tory, Stockholm.

MATERIAL.

Cruise Sta. DepthNo Lat Long Gear Date

(m)

WEST EUROPEAN BASIN

Jean Charcot DS20 4226 47°33.0'N 9°36.7'W DS _ 24.10.72
(Polygas) DS21 4190 47°31.5'N 9°40'7".W DS 24.10.72
47°34.1'N 9°38.4";W DS 25.10.72

46°31.1’N 10°29.2'W DS 21. 2.74
47°34.9'N 9°40.9"'W DS 22. 2.74
(Biogas V) S68 4550 46°26.7'N 10°23.9'W DS 19. 6.74
(Biogas VI) DS75 3250 1 47°28.1'N 9°07.8'W_ DS_ 22.10.74
DS76 4228 29+1v 47°34.8'N 9°33.3'W DS 23.10.74
DS77 4240 13+2v 47°31.8'N 9°34.6';W DS 24.10.74

(Biogas ITV) DS54 4659

7

1
DS22 4144 4
2
DS55 4125. 7
1

DS78 4706 4 46°31.2’N 10°23.8'W DS 25.10.74
DS79 4715 13 46°30.4’N 10°27.1'W DS _ 26.10.74
DS80 4720 4 46°29.5'N 10°29.5'W DS 27.10.74
DS81 4715 1 46°28.3’N 10°24.6'W DS 27.10.74
Jean Charcot QS02 4829 2 48°19.2’N 1 15°15.7W OS 2. 827G
(Incal) QS05 4296- 1 47°31.3’N 9°34.6'W OS 7. 8.76
4248 — 47°32.2'N —-9°34.7'W
QS06 4316 6 46°27.3'N 9°36.2",W OS 9. 8.76
— 47°27.9'N —9°36.0'W
QS07 4249 15 47°31.8'N 9°34.3".W OS 10.8.76
— 47°31.3'N — 9°34.3'W
QS08 4327 5 47°29.8'N 9°39.2'W OS 11. 8.76
— 47°29.5'N 9°38.8’W
WS03 4829 10 48°19.2'N 15°23.3.W WS 1. 8.76
— 48°19.1'N 15°22.5’W
WS07 4281 4 47°30.6'N 9°37.1'W WS _ 7. 8.76
— 47°31.2'N —9°35.7'W
WS08 4287 11 47°30.5'N 9°33.7"W WS _ 9. 8.76
WS09 4277 3 47°28.8'N 9°34.0'W WS 10. 8.76
— 47°27.9'N

_ DEEP-SEA PROTOBRANCHIA (BIVALVIA)

WS10 4354 10 47°27.3'N
— 47°18.2'N
Jean Charcot DS11 4823 1 48°18.8'N
(Incal) — 48°18.6'N
DS14 4254- 3 47°32.6'N
4248 — 47°32.6'N
DS15 4268 12+2v 47°29.8'N
— 47°30.3'N
CP11 4823 1 48°20.4’N
— 48°21.2’N
Chain106 330 4632 8 50°43.5'N
— 50°43.4'N
CANARIES BASIN
Discovery 6714 3301 3 27°13.0'N
GUYANA BASIN
Knorr 25 287 4980- 5 13°16.0'N
4934
288 4417-64 11°02.2'N
4429
Jean Charcot DSO3 5150 1 10°47.1'N
(Biovema) — 10°47.1'N
DS05 5100 6 10°45.9'N
— 10°46.0'N
CP04 5100 1 10°45.9'N
— 10°45.9'N
CAPE BASIN
Jean Charcot DSO5 4560 3 33°20.5'S
(Walvis) DS06 4585 7 3324S
DS07 5100 20 26°59.7'S
DS08 5225 3 26°59.6'S
DS09 5220 15 26°59.9'S
NORTH AMERICA BASIN
AtlantisII 70 4680 1 36°23.0'N
12
Chain 50 ZG 3828) 2 38°00.0'N
80 4970 45 34°49.8'N
81 5042 14 34°41.0'N
83 5000 2 34°46.5'N
85 3834 8 37°59.2'N
AtlantisII 92 4694 3 36°20.0'N
17
Atlantis II 93 5007 23 34°39.0'N
24 121 4800 2 35°50.0'N
AtlantisII 175 4667— 2 36°36.0'N

9°39.9'W

15°11.5'W
15°12.0’'W
9°35.7'W
9°35.1'W
9°33.4'W
9°33.4'W
15°14.6’W
15°13.7'W
51°07.0'W
51°09.0'W

15°41.0'W

54°52.2'W

55°05.5'W

42°40.7'W
42°40.3'W
42°40.2"W
42°39.8'W
42°40.2'W
42°39.3'W

2°34.0'E
2°32.0’W
107 VE
1°07.3'E
1°06.7'E

67°58.9'W

69°18.7'W
66°34.0'W
66°28.0'W
66°30.0'W
69°26.2'W
67°56.0'W

66°26.0'W
65°11.0'W
68°29.0'W

5
13

14
21
29

. 8.64

6.65
7.65
7.65
7.65
» 7.65
12.65

.12.65
. 8.66
11.67

55

This is a true abyssal species found only in the deepest part of
the Basins and, except for the Argentine & Norwegian Basins
is widely distributed throughout the Atlantic. Depth range:
3250-5225 m.

SHELL DESCRIPTION (Figs. 84 & 85). Shell small, moderately
thick, oval, inflated, inequilateral, posterior margin slightly
produced; surface with very fine irregular concentric lines,
fine radial striae present in some specimens, dorsal area
between umbo and posterior margin furrowed; umbos raised
slightly, orthogyrate; dorsal margin proximally straight on
either side of umbo, distally antero-dorsal margin joins
rounded anterior margin in broad curve which continues to
ventral margin, more convex posteriorly, distal postero-
dorsal margin curves more gradually to produce slightly
drawn out posterior margin, limit of posterior margin supra-
medial; hinge plate moderately broad, long, slightly curved,
up to 10 small ‘V-shaped, chevron teeth on each side of
ligament; ligament amphidetic, small, ‘goblet’-shaped, small
external extension of fused periostracum on either side of
umbo, small chondrophore present.

Prodissoconch length: 146-187 4m. Maximum recorded
shell length 4.4 mm.

The description of Portlandia (Y) subcircularis (Odhner,
1960) agrees well with that presented above.

With increasing length, the posterior part of the shell
becomes slightly more extended and at the same time the
postero-dorsal margin becomes more smoothly curved (Figs.
85 & 86). The width to length ratio also increases with
increasing size, but little proportionate change occurs in the
height to length ratio although this varies in individuals
between 0.72 and 0.84. The height/length ratios are the
highest recorded for any species of Yoldiella. In some larger
specimens the dorsal margin immediately anterior and poste-
rior to the umbos may become slightly concave and as a result
a small, shallow lunule and escutcheon may be formed.
Comparison of specimens from the different Basins show that
northerly (W. European) and southerly (Angola) populations
in the eastern Atlantic are similar in form as too are the North

Fig. 84 Yoldiella subcircularis.
Lateral view of a shell from the
right side and a right valve to show
the detail of the hinge-plate.
Specimens from Sta. 285 Guyana
Basin and Sta. 93 North Atlantic
Basin respectively. (Scale = 1.0
mm).

Nn

6

Fig. 85 Yoldiella subcircularis. Outlines of shells from the right
side to show variation in shape. Specimens taken from Sta. BG
VI DS 76, West European Basin. (Scale = 1.0 mm).

| e 00 e aia
50) 3 - ee 7 ; °a —- .
W\L ee ee?
40- : .
al
1 Oe :
80-4 cw. & omse ote 8 oe }
“AIL
70- 7
66. ee S e e OF A
coe ew eel = s 8 a
‘pure fh. . e -} Fr. s
50-
| =o == ee ae Lia
1 2 3 4
Length(mm)

Fig. 86 Yoldiella subcircularis. Variation in ratios of height H/L,
width W/L and postero-umbonal length PL/TL to length against
length of subsamples from Sta. BG DS 76, West European Basin
(closed circles) and Sta. Walvis QS 07, Cape Basin (closed
squares).

J,A. ALLEN, H.L. SANDERS AND F. HANNAH

America specimens. In contrast the small number of speci-
mens from the Guyana Basin differ slightly in having a lower
height/length ratio (significant at the 0.1 level).

INTERNAL MORPHOLOGY (Figs. 87 & 88). In Y. subcircularis
the volume of the mantle cavity is large compared with that of
the body. A well-developed sense organ lies at the far
anterior margin. The siphons are combined and the inhalent
siphon is shorter than the exhalent and is open ventrally.
Both siphons are thin-walled and a pair of lateral haemoco-
eles are present in the junction between them. There is a
moderately large siphonal tentacle, most frequently found on
the left side. The feeding aperture is well-developed with
areas of glandular mantle epithelium on each side which are
probably extended in life as paired, broad flaps. For some
distance anterior to these, the inner muscular lobe is particu-
larly well-ciliated and it may be that the feeding area either is
separated from the pedal gape by a ciliary junction or it is the
region where pseudofaeces are formed. The adductor
muscles are small, approximately equal in size, oblong in
shape and lie close to the dorsal and anterior and posterior
mantle margins respectively.

The gill plates are relatively few in number (18 in a
specimen 3.0 mm). Tissue junctions join the demibranchs to
the mantle dorsally. The gill plates are particularly muscular

Fig. 87 Yoldiella subcircularis. Lateral views from right and left
sides of the internal morphology of a specimen from Sta. 80 North
America Basin. (Scale = 1.0 mm). For identification of parts see
Fig. 34.

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

and well-developed. The labial palps are large, with up to 32
closely spaced ridges on each inner face. In some of the
largest specimens (> 3.3 mm) the anterior part of the palps
are frequently folded in on themselves. The palp proboscides
are relatively small and slender. The foot is conspicuous,
being long and thin, with a much extended narrow neck and a
long divided papillate sole. Gland cells are present along the
ventral part of the sole and surrounding the aperture of a
large byssal gland. Pedal retractor muscles are less conspicu-
ous in this species than in others described here. The central
haemocoele of the foot is extensive. The cerebral and visceral
ganglia are relatively small, circular in transverse vertical
| cross-section. The visceral ganglia are some distance anterior
to the posterior adductor muscle. The pedal ganglia lie at the
dorsal limit of the extended neck of the foot close to its
junction with the body, they are large and elongate with large
associated statocysts dorsal to them.

The mouth is displaced a short distance posteriorly from
the anterior adductor muscle. The stomach and style sac are
moderately large. Two broad sorting ridges can be seen on
the right anterior wall of the stomach. A prominent gastric
tooth is present on the left dorsal wall and posterior to it the
gastric shield extends over the left and most of the right walls
of the stomach. The major typhlosole extends along the right
side of the stomach. The digestive diverticula lie either side
and anterior to the stomach. The duct of the right digestive
diverticulum curves dorsally over the hind gut and enters the
stomach on the right anterior dorsal wall. A duct from one
left diverticulum enters the stomach far anterior on the left
dorsal side, and immediately posterior to it, a short duct from
a second left diverticulum enters the stomach, ventral to the
gastric tooth. Material similar to that in the stomach was
present in the lumen of the latter diverticulum, but this could
possibly have resulted from tissue contraction following fixa-
tion. The style sac is relatively large and extends into the
dorsal part of the foot. The hind gut has a shallow typhlosole
along its entire length, the impression of which can be seen on
extruded faecal pellets. The hind gut is arranged in two loops
on each side of the body. These are usually visible through

Fig. 88 Yoldiella subcircularis. Dorsal view of the hind gut of a
specimen from Sta. 80 North America Basin and a diagrammatic
view of the course of the hind gut as seen from the left side.
(Scale = 1.0 mm).

57

the shell antero-dorsally. There is also a double vertical
extension of the hind gut deep into the foot. Thus, design
advantage is taken of the exceptional length of the neck of the
foot to accommodate a significant part of the greatly
extended hind gut.

The sexes are separate. Sectioned specimens larger than
2.8 mm had maturing gonads. The number of ova ranged
from 37 (2.80 mm) to approximately 140 (2.91 mm) with a
maximum ovum diameter of 132 1m and 156 um respectively.
The gonads overlie the lateral and dorsal sides of the viscera.
Mature females were present in February, June, August and
October samples from the West European Basin. In two
specimens from (Sta. DS76, Sta. DS79 respectively both 2.91
mm total length), eggs were partially shed into the mantle
cavity and had a maximum dimension of 156 pm. Neverthe-
less, there is no evidence to show that eggs are retained and
incubated within the mantle cavity.

Although the kidney is relatively small, it extends forward
on either side of the stomach for a short distance.

Yoldiella biguttata (new species)

TYPE LOCALITY. R.V. Knorr Cruise 25, Sta. 299,Guyana
Basin, 29.2.1972, 7°55.1’N, 55°42.0’W, Epibenthic Trawl,
1942-2076 m.

TYPE SPECIMEN. Holotype: BM(NH) 1992029. Paratypes: in
collection held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
BRAZIL BASIN
AtlantisII 167 943- 5 7258.0'S 342170) WES” 2012767
31 1007 — 7°50.0'S 34°17.0’'W
ARGENTINE BASIN
Atlantisiiy” 245 92707 | 2936°55:7'S Wess: 0le4 WeES 1423371
60
GUYANA BASIN
Knorr 25 293 1456— 13 8°58.0'N 54°04.3'W ES 27.2.72
1518
295 1000- 2 8°04.2'N 54°21.3'W ES 28.2.72
1022
299 1942- 74 T55A1°N —55°42:0'W ES 92922472:
2076
301 2487- 44 8°12.4’N 55°50.2'W ES 29.2.72
2500
303 2849- 4 8°28.8'N 56°04.5'W ES. 1. 3.72
1853

Y. biguttata is distributed off the coast of eastern South
America at mid slope to abyssal depths in the Argentine,
Brazil and Guyana Basins. Depth range: 943-2853 m.

SHELL DESCRIPTION (Fig. 89). Shell small, inflated, equilat-
eral, fine concentric striae; umbos posterior to midline,
slightly raised, moderately large, inwardly directed; dorsal
margin raised, sharp-edged, particularly so anterior to umbo,
dorsal margin close to umbo straight, proximal antero-dorsal
margin curves to broadly rounded anterior margin, the limit
of which is slightly ventral to the midline, ventral margin
shallow curve, in some specimens almost straight centrally,
postero-ventral margin sinuous giving a characteristic oblique

58

configuration to shell in lateral view; hinge plate moderately
shallow, short, reaching no further than inner limit of adduc-
tor muscles, teeth few, anterior and posterior series either
equal or with one additional tooth in anterior series (5/6 in
largest specimen); ligament amphidetic, very large in relation
to size of shell.

Shell measurements (mm) & ratios are as follows:-

Length Height Width H/L W/L PL/TL
1.64 1.24 0.89 0.75 0.54 0.39
1.62 1.12 0.72 0.69 0.44 0.45
1.50 1.07 0.68 0.72 0.45 0.46
1.50 1.08 0.73 0.72 0.49 0.39
il 37 1.02 0.70 0.74 (5) 0.47
1.19 0.81 0.48 0.68 0.40 0.39
0.87 0.62 0.33 0.71 0.38 0.40

Prodissoconch length: 198 jm. Maximum recorded shell
length: 1.64 mm.

INTERNAL MORPHOLOGY (Fig. 90). The combined siphon is
thin-walled and with a single lumen. In that the gill axis joins
the ventral edge of the siphon the inhalent component can be
assumed to be largely absent. The siphon is not open ven-
trally. A single fine, elongate mantle tentacle originates on
the left ventral inner limit of the moderately deep siphonal
embayment. An area of secretory cells is present at the base
of the siphon. Ventral to the siphon is a well-developed
feeding aperture. The posterior adductor muscle is round in
cross-section while the anterior is slightly larger and
‘crescent’-shaped. The visceral and cerebral ganglia are well-
developed and joined by a stout commissure. The cerebral
ganglia are slightly the larger. The gills have a relatively small
number of plates (maximum number recorded 9) most of
which are carried posterior to the foot. Labial palp ridges
number 14-18, depending on the size of the specimen, and
are moderately large. The palp proboscides are also moder-
ately large. The foot papillae contain considerable numbers

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 89 Yoldiella biguttata. Lateral views
of shells from the right and left sides and a
right valve in inner lateral and dorsal view
to show variation in shape and hinge-plate
details. Specimens from Sta. 301 and Sta.
299, Guyana Basin. (Scale = 1.0 mm).

Fig. 90 Yoldiella biguttata. Lateral views from the right and left
sides of the internal morphology of a specimen from Sta. 299,
Guyana Basin. (Scale = 1.0 mm). For identification of the parts
see Fig. 34.

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

of secretory cells. There is a relatively large single posterior
papilla immeditely posterior to the aperture of the byssal
gland at the heel of the foot. The gland is large and is
surrounded by secretory cells. The hind gut is complex with
twinned loops on each side of the body, with two recurved
sections immediately anterior to the stomach. The main
lateral loops of the hind gut cross from one side of the body to
the other dorsally and immediately posterior to the resilium.
All the specimens examined had well-developed gonads
which makes it difficult to determine the exact course of the
gut, but from sections we are confident that it has a similar
design to that described for Yoldiella subcircularis. One
sectioned female (1.63 mm total length) from Sta. 299 has 18
large ova present (maximum observed dimension 114 pm).
Despite the similarity of the internal morphology to that of
Yoldiella subcircularis the shapes of the shell, hinge and
ligament are totally different in the two species (p. 55). As we
point out elsewhere the various types of hind gut morphology
do not necessarily correlate to a particular characteristic suite
of shell characters. All that can be said with certainty is that
| the hindgut of Yoldiella biguttata has configuration so far
| found only in the Yoldiellidae and only in species from the
abyss and abyssal rise.

Yoldiella ovata (new species)

TYPE LOCALITY. R.V. Knorr Cruise 25, Sta.300, Guyana
Basin, 29.2.1972, 8°14.2'N, 55°53.5'W, Anchor Dredge,
2524-2542 m.

| TYPE SPECIMEN. Holotype: BM(NH) 1992035. Paratypes: in
| collection held by J.A. Allen.

Fig. 91 Yoldiella ovata. Lateral views of a shell from the left side
and dorsally and an inner view of a left valve to show detail of the
hinge-plate. Specimens taken from Sta. 300, Guyana Basin.
(Scale = 1.0 mm).

59

MATERIAL.
Cruise Sta. Depth No Lat Long Gear Date
(m)
GUYANA BASIN
Knorr 25 300 2524- 208 8°14.2’N 55°53.5’W AD 29.2.72
2542
301 2487— 324 8°12.4’N 55°50.2".W ES 29.2.72
2500
303 2842—- 13 828.8’N 56°04.5'W ES 1 .3.72
2853

Restricted to the abyssal rise of the Guyana Basin. Depth
range: 2487-2853 m.

SHELL DESCRIPTION (Figs. 91 & 92). Shell small, ovate,
moderately inflated, inequilateral, transparent, dorso-
laterally smooth, ventrally, with very fine concentric lines
forming ridges; umbos slightly anterior of midline, moder-
ately inflated, orthogyrate; dorsal margin slightly convex,
antero-dorsal and postero-dorsal margins with similar curva-

\

Fig. 92 Yoldiella ovata. Outlines of shells from the right side to
show change in shape with growth of specimens from Sta. 300,
Guyana Basin. (Scale = 1.0 mm).

60

ture, ventral margin smoothly curved, posterior margin
somewhat more convex than anterior and very slightly
extended, anterior and posterior margins broad, rounded;
hinge plate elongate, moderately wide except below umbo
where narrow, moderately strong, acute taxodont teeth, 7 in
anterior and 8 in posterior series in specimen 2.3 mm;
ligament internal, amphidetic, slightly elongate, barely
extends below hinge plate.

Prodissoconch length: 172 1m. Maximum recorded shell
length: 2.9 mm.

INTERNAL MORPHOLOGY (Fig. 94). The inhalent and exhal-
ent siphons are combined, the inhalent is open along its
ventral margin. The siphonal tentacle is either to the right or
left. There is a feeding aperture and anterior to it a heavily
ciliated region of the inner mantle fold. There is a well-
developed anterior sense organ. The posterior adductor
muscle is small and oval, the anterior adductor, also oval, is
approximately twice the size of the posterior. The labial palps
are moderately large with up to 19 palp ridges and each has a
long thin palp proboscis. The gill which is small, dorsal in
position in preserved specimens, has up to 14 plates. The
visceral and cerebral ganglia are typically ‘club’-shaped, the
visceral being the smaller. The pedal ganglia are moderately
large, elongate and situated high in the foot. The foot is long
and thin with deep papillae fringing the sole. A large byssus
gland is present in the heel.

The stomach and style sac are small and the latter does not
penetrate far into the foot. The hind gut forms double loops
to the left and right of the body, recurring anterior to the
stomach, thus taking a similar course to Yoldiella subcircu-
laris and Yoldiella biguttata. A considerable amount of fine
material was present in the digestive diverticula of the left
side. The kidneys although moderately well-developed do not

50

W\L rae ee
40 ° yo
80

H\L e i : eo P ;
70 : &
60

PLITL sy ir ea
50 ° :

Length(mm)

Fig. 93 Yoldiella ovata. Variation in the ratios of height H/L,
width W/L and postero-umbonal length PL/TL to length against
length of subsample from Sta. 300, Guyana Basin.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 94 Yoldiella ovata. Lateral views from the right and left side
of the internal morphology of a specimen from Sta. 300, Guyana
Basin. (Scale = 1.0 mm). For identification of the parts see Fig.
34.

penetrate anteriorly into the visceral mass to any great
extent. The sexes are separate. One sectioned female (2.7
mm) contained approximately 50 ova with a maximum diam-
eter of 80 um.

With increasing length, the height/length, width/length and
particularly the posterior umbonal length/total length ratios
gradually increase (Fig. 93).

Yoldiella ovata is closely related to Y. biguttata, but can be
distinguished by its more ovate outline and not being flat-
tened at the postero-ventral shell margin.

Yoldiella insculpta (Jeffreys 1879)

TYPE LOCALITY. West of Ireland, H.M.S. Porcupine, 1869,
Sta. 16, 54°19’N, 11°50’'W, 816 fms.

TYPE SPECIMEN. Holotype: not designated. Lectotype: (here

|

DEEP-SEA PROTOBRANCHIA (BIVALVIA) 61

designated) BM(NH) 85.11.5.459.

Leda insculpta Jeffreys 1879, p. 580, pl. 46, Fig. 5; Dautzen-
berg 1889, p. 80; Dautzenberg & Fischer 1897, p. 204;
Locard 1898, p. 355.

We have examined the material designated as syntypes by
Warén (1980) and housed in the collections of the U.S.
National Museum and the Natural History Museum, London.

MATERIAL.
Cruise Sta DepthNo Lat Long Gear Date
(m)
WEST EUROPEAN BASIN
LaPerle DS11 2205 9+2v 47°35.5'N 8°33.7;W DS _ 8. 8.72
(Biogas) DS13 2165 23+6v 47°33.7’N 8°39.9’W_ DS 10. 8.72
Jean Charcot DS15 2246 5 47°35.2'N 8°40.1'W DS _ 21.10.72
(Polygas) DS16 2325 1 47°36.1'N 8°40.5'W DS 21.10.72
DS18 2138 4 47°31.2'N 8°44.9'W DS 22.10.72
(BiogasII) DS31 2183 4 47°32.5'N 9°04.2,;W DS 19. 4.73 setae er pec AP
DS32 2138 14 47°32.2'N_ 8°05.3'W__ DS _ 19. 4.73
(BiogasIII) DS35 2226 18+8v 47°34.4'N 8°40.7’W_ DS 24. 8.73 Fig. 96 Yoldiella insculpta. Lateral view of shell from the left side
DS36 2147 5 Sea N Saal DS 24. 8.73 and an inner view of right valve. Specimens from Sta. 16 and Sta.
DS37 2110 mA. 4PSLEN | 8'34.6'W DS 24. 8.73 17 Porcupine Expedition, W. of Portugal. (USNM No. 199773).
DS38 2138 2 47°32.5'N 8°35.8’W__DS__ 25. 8.73 (Scale = 1.0mm).
(Biogas IV) DS61 2250 10+2v 47°34.7'N 8°38.8'W DS 25. 2.74
DS62 2175 63 = 47°32.8’N__ 8°40.0'W__DS_ 26. 2.74
DS63 2126 23+ 47°32.8’N 8°35.0'W DS 26. 2.74
10v
DS64 2156 18+ 47°29.2’N 8°30.7'W DS 26. 2.74
12v
CP01 2245 8+4v 47°34.6'N 8°38.8’W CP 25. 2.74
(Biogas V) DS65 2360 1 47°36.1'N 8°40.5'W DS 15. 6.74
DS71 2194 6+2v 47°34.3’N 8°33.8’W_ DS 20.10.74
DS87 1913 1 44°05.2’N 4°19.4’W DS 1.11.74 aS
CP08 2177 11+2 44°33.2'N 8°38.5'W CP 20.10.74
CP09 2171 24+4v 47°33.0'N 8°44.0'W CP 20.10.74
Sarsia 65 1922 42 46°15.0'N 4°50.0'W ED _ 25.7.67
CANARIES BASIN
Discovery 6701 1934 1 27°45.2'N 14°13.0'W ED 16. 3.68
6704 2129 1 27°44.9'N 14°25.0'W ED 17. 3.68
6710 2670 2 27°23.6'N 15°39.6'W ED 19. 3.68
6714 3301 2 27°13.0'N 15°41.0'W ED 20. 3.68

Fig. 95 Yoldiella insculpta. Dorsal and lateral view of shell from
the right side and a lateral view of the hinge-plate of a left valve.
| Specimens from Sta. BG IV DS 62, West European Basin.
| (Scales = 1.0 mm).

Fig. 97 Yoldiella insculpta. Outlines of shells from the right side to
show variation in shape. Specimens from Sta. BG DS 63, West
European Basin. (Scale = 1.0 mm).

62

Distributed in the north eastern Atlantic, Bay of Biscay, off
northwest Africa and the Azores at abyssal rise depths.
Depth range: 1354-3301 m.

SHELL DESCRIPTION (Figs. 95-98). Shell ovate, moderately
inflated, smooth with very fine concentric lines near ventral
margin forming slight ridges in larger specimens, occasionally a
few faint radial lines are present in larger specimens; perios-
tracum pale straw-coloured, slightly iridescent; umbos very
slightly anterior of midline, moderately inflated; proximal dor-
sal margin straight or slightly concave on either side of umbo,
antero-dorsal and anterior margin merge in an even curve in
most specimens, distally postero-dorsal margin slopes down to
posterior margin; posterior margin with slight angulation, poste-
rior limit of anterior and posterior margins dorsal to horizontal
mid-plane, ventral margin a smooth curve; hinge plate relatively
narrow, barely extending to outer margins of adductor muscles,
anterior hinge line relatively straight, distally slopes away from
dorsal shell margin, chevron-shaped teeth small, posterior hinge
line slightly curved anteriorly, with same number of teeth in
each series (8/8 in a specimen 2.2 mm and 12/12 in a specimen
3.5 mm); ligament amphidetic, small, rounded, extends below
hinge plate.

Prodissoconch length: c. 190 1m. Maximum recorded shell
length: 3.65 mm.

There is a slight increase in the post-umbonal length as
length increases, otherwise the ratios of height/length and
width/length remain constant.

INTERNAL MORPHOLOGY (Fig. 99). The mantle edge is well-
developed, particularly the inner muscular fold which at its
dorsal edge, contains secretory cells beneath the main rejec-
tion tract. Postero-ventral to the siphons, both the middle
and inner folds are increased in size, convoluted and heavily
ciliated and form a feeding aperture. Exhalent and inhalent
siphons are combined, the latter being open ventrally. A pair
of lateral haemocoelic canals are present the junction

50 e e a e@ @e . be
wit * "st Sotehey
40
80
e ce ° “coen & co eo e
e e 3 e
H\L ae ec e

Length(mm)

Fig. 98 Yoldiella insculpta. Variation in the ratios of height H/L,
width W/L and postero-umbonal length PT/TL to length against
length of a sample from Sta. BG IV DS 62, West European
Basin.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

\

2

: \

Fig. 99 Yoldiella insculpta. Lateral views from the right and left
sides of the internal morphology of a specimen from Sta. S 65,
West European Basin. (Scale = 1.0 mm). For identification of
parts see Fig. 34.

between the siphons. The siphonal tentacle is large and found
equally to either the right or left side of the siphonal
embayment. There is a well-developed anterior sense organ.
Adductor muscles are slightly unequal in size, the anterior
being the larger.

Gill plates are relatively small and difficult to count in
preserved specimens (maximum recorded c. 18-20). The
labial palps are well-developed with approximately up to 26
closely spaced ridges on each inner face. The palps extend
between 1/3—-1/2 across the body and each bears a long thin
palp proboscis. The mouth is positioned a short distance
posterior to the anterior adductor muscle. The foot is rela-
tively large with a broad sole with a small papilla at the
posterior limit. There is a large byssal gland in the heel (Fig.
100). The arrangement of pedal retractor muscles is similar to
that of other species of Yoldiella. The cerebral ganglia are
relatively large, the visceral ganglia are small and elongate.
The pedal ganglia lie ventral to the style sac in the proximal
part of the foot and are moderately large. The stomach is
displaced slightly to the right of the body and is of moderate
size with a large style sac. The hind gut configuration is
unique. There are three loops to the right of the body and two

\
fi
Wi

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

loops to the left of the body with a further double loop
passing to the left of the stomach into the foot anterior to the
stomach. The loops do vary slightly in their extent and
curvature. This is similar to the condition in Y. subcircularis,
Y. biguttata and Y. ovata but with an additional loop on the
right. The hind gut has a typhlosole along its entire length.
Fine material similar to that present in the stomach was seen
in the digestive diverticula of a number of specimens. The
kidney is well-developed. Sexes are separate.

The maximum diameter of the ovum as observed in sec-
tions of females from samples taken in February and August
was similar (130 ym). There seems to be a wide variation in
the numbers of ova present. A female collected in August
(2.74 mm) had 255 ova while another collected in February
(3.09 mm) had only 65 ova. Maturing gonads were recorded
for all months sampled.

Yoldiella jeffreysi (Hidalgo 1877)

TYPE LOCALITY. H.M.S. Valorous Sta. 16, Iceland Basin,
west of Rockall Plateau, south Maury Channel, 55°10’/N,
25°58’ W, 23.8.1875, Dredge, 1785 fm.

TYPE SPECIMEN. Holotype: not designated; Lectotype: U.S.
Natl. Mus., No. 199696 as here designated.

Leda lata Jeffreys 1876, 1876. p. 431 (in part).

Leda jeffreysi Hidalgo 1877, p. 396; Jeffreys 1879, p. 579, pl.
46, Fig. 2; Dall 1881, p. 124; Smith 1885, p. 234; Dautzen-
berg 1889, p. 75; Dautzenberg & Fischer 1897, p.204;
Locard 1898, p. 353.

Portlandia jeffreysi Posselt 1898, p.36.

Yoldiella jeffreysi Verrill & Bush 1898, p. 866, pl. 81, Fig. 5,
pl. 83, Fig. 3.

A holotype of Y. jeffreysi was never designated but the

Fig. 100 Yoldiella insculpta. Transverse section through the
‘byssal’ gland. (Scale = 0.1 mm). Abbreviations see p. 12.

63

following USNM specimens were identified as syntypes by
Warén (1980):-

No. 1999695, Valorous Sta. 9; No. 199694, and No. 199696,
Valorous Sta. 16; No. 199700, Porcupine Sta. 20; No.
199701, Porcupine Sta. 16 & 17; No. 199698 Porcupine Sta.
30.

and in the BM(NH):—

No: 77 1e8:25,"-Valorous- -stas/" 95-42. 15, 16; No.
85.11.5.592-593, Porcupine Sta. 31; No. 85.11.5 366-367,
Porcupine Sta. 16; No. 85.11.5. 591, Porcupine Sta. 9.

Jeffreys original specimens were taken from the North
Atlantic (Valorous Stations 9, 12, 13 & 16 and from which he
described his species Leda lata (Jeffreys, 1896). Unfortu-
nately the material from these four “Valorous’ Stations, which
we have examined, contains two species of Yoldiella which
are, superficially, similar in form (p. ). Furthermore the 1876
description is so general that there is nothing to indicate
which of the two species Jeffreys chose when he described
Leda lata. Thus, we here accept Leda jeffreysi Hidalgo (1877)
as the first unequivocal specific designation and which Jef-
freys (1879) himself accepted two years later.

We have also examined specimens referred to as Yoldiella
Jeffreysi by Verrill & Bush (1898) (USNM, No. 4888) and
these clearly differ from Y. jeffreysis.s. in being stouter, with
a more inflated umbo, a broader hinge plate, with teeth of
different form and fewer in number.

Y. jeffreysi is a very widespread species. It occurs from the
base of the continental slope to the deepest abyssal depth. In
our samples it occurs in the Argentine, Guyana, North
America, West European, Canary, Cape Verde & Angola
Basins. It has also been recorded from the Gulf of Mexico
(2416-2868 m) and from off West Greenland (3200 m) and in
the Mediterranean off Palermo.

Depth range: 2040-4862 m. The depth distribution is
similar throughout the Atlantic.

MATERIAL.
Cruise Sta DepthNo Lat Long Gear Date
(m)
NORTH AMERICA BASIN
AtlantisI] 64 2886 7 38°46.0'N 70°06.0'W ET 21. 8.64
12 72 2864 6 38°16.0'N 71°47.0'W ET 24. 8.64
Chain 50 76), 82862) 32, 939738'3IN) 16725780 W) MET 2916:65
77 ~=—:3806 109 = 38°00.7'N__ 69°16.0'W_ ET 30. 6.65
78 3828 57 38°00.8’N 69°18.7'W ET 30. 6.65
84 4749 16 36°24.4'N 67°56.0'W ET 4. 7.65
85) 3834413) 3759'2,N) 69°26:2,W SET 52 765
AtlantisII 123 4853 4 37°29.0'N 64°14.0'W ET 22. 8.66
24 124 4862 1 37°26.0'N 59°59.5'W ET 22. 8.66
— 37°25.0'N 63°58.0'W
126 3806 138 39°37.0'N 66°47.0'W ET 24. 8.66
— 39°37.5'N 66°44.0'W
Atlantis II 175 4667— 53 36°36.0'N 68°29.0'W ES 29.11.67
40 4693 — 68°31.0’W
Chain 106 330 4632 155 50°43.5'N 17°51.7'W ES 24. 8.72
— 50°43.3’N 17°52.9'W
334 4400 49 40°42.6’N 46°13.8’'W ES 30. 8.72
— 40°44.0'N 46°14.6'W
335 3882- 28 40°25.3'N 46°30.0’'W ES 31. 8.72
3919
Knorr 35 340 32164-13 38°14.4’N 70°20.3’'W ES 24.11.73

64

3356 — 38°17.6'N
GUYANA BASIN
Knorr 25 287 4980- 5 13°16.0’N
4934 — 13°15.8’N
288 4417— 31 11°02.2’N
4429 — 11°03.8'N
291 3859-155 10°06.1’N
3868 — 10°06.6’N
306 3392- 38 9°31.1’N
3429
307 3862—- 30 12°34.4’N
3835 — 12°40.8'N
ARGENTINE BASIN
Atlantis II 259 3305- 30 37°13.3’S
60 3317
WEST EUROPEAN BASIN
Chain106 316 2173— 85 50°58.7'N
2209 — 50°57.7'N
318 2506 106 50°27.3'N
— 50°26.8'N
321 2290- 94 50°12.3'N
2968
323 3356- 6 50°08.3'N
3338 -
Jean Charcot DS23 4734 1 46°32.8'’N
(Polygas) DS25 2096 2v 44°08.2'N
(Biogas II) DS31 2813 11 47°32.5'N
(BiogasIII) DS37 2110 1 47°31.8'N
DS71 3546 4 47°28.3'N
(BiogasIV) DS51 2430 27 44°11.3’N
DS52 2006 1 44°06.3’N
DS53 4425 1 44°30.4'N
DS54 4659 1 46°31.3’N
DS58 2775 10 47°34.1'N
DS62 2175. 1 = 47°32.8'N
DS63 2126 1 47°32.8'N
DS64 2156 2 47°29.2'N
CV38 2695 2 47°30.9'N
(Biogas V) DS68 4550 1 46°26.7'N
(Biogas VI) DS71 2194 2 47°34.4'N
DS74 2777 2+2v 47°33.0'N
DS78 4706 1 46°31.1'N
DS79 4715 2 46°30.4'N
CP10 2878 1 47°29.6'N
(Incal) DSO01 2091 73 57°59.7'N
— 57°59.2'N
DS02 2081 67 57°58.8'N
— 57°58.5'N
CP01 2068- 10 57°57.7'N
2040 — 57°56.4'N
CP02 2091 2 57°58.4'N
— ST UN
CP03 2466 4 56°33.2'N
— 56°32.5'N
CP04 2483—- 2 56°33.2'N
2513 — 56°31.5’N
DS05 2503 123 56°28.1'N
— 56°27.6'N
DS06 1494 243 56°26.6'N
— 56°25.9'N
DS07 2884 267 55°00.7'N
— 55°01.0'N
DS08 2891 32 55°02.0'N
— 55°01.9'N
CPOS 2884 58 55°00.4'N
— 55°00.9'N
CP06 2888- 39 55°02.3’N
2893 — 55°02.6'N
CP07 2896 17 55°03.4’N
— 55°04.4'N
DS09 2897 274 55°07.7'N
— 55°08.1'N

70°22.8'W

54°52.2'W
54°53.1"W.
55°05.5'W
55°04.8'W
55°14.0'W
55°15.4'W
56°20.6'W

58°59.3'W
59°09.2'W

52°45.0'W

13°01.6'W
13°01.3'W
13°20.9"W
13°19.9"W
13°35.8'W

13°53.7'W
13°50.9'W
10°21.0'W
4°15.7'W
9°04.2'W
8°34.6'W
9°07.2'W
4°15.4'W
4°22.4'W
4°56.3'W
10°29.1'W
9°08.2"W.
8°40.0'W
8°35.0'W
8°30.7'W
8°59.5'W
10°23.9'W
8°33.8'W
9°07.8'W
10°23.8'W
10°27.1'W
9°04.5'W
10°39.8'W
10°41.3’W
10°48.5'W
10°49.2’W
10°55.0'W

10°42.8'W
10°44.6'W
11°11.3'W
11°12.4’W
11°11.3’W
11°12.4'W
11°11.7'W
11°12.0'W
11°10.5’'W
11°10.7'W
12°31.0'W.
12°32.0'W
12°34.6'W
12°33.4'W
12°29.4'W
12°31.1'W
12°40.3'W
12°41.7'W
12°46.4'W

12°52.6'W
12°53.2'W

24.

26.

26.

18.

19%

20.

PAN

Dee

SP

2.72

a Shi

= 3.2

3.71

8.72

8.72

8.72

8.72

26.10.72
1.11.72

19.
24.
28.
18.
18.
19.
Zils
23)
26.
26.
26.
24.
19.

4.73
8.73
8.73
2.74
2.74
1.74
2.74
2.74
2.74
2.74
2.74
2.74
6.74

20.10.74
21.10.74
25.10.74
26.10.74
21.10.74

1S:

16.

7.76

7.76

16.7.76

16.

17.

9h

18.

18.

19.

19.

19.

19.

20.

20.

7.76

7.76

7.76

7.76

7.76

7.76

7.76

7.76

7.76

7.76

7.76

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

CP08 2644 49 50°14.7’N 13°13.5'W CP 27. 7.76
— 50°15.2'N 13°14.8’W
DS10 2719 48 50°12.7’N 13°16.6'W DS 27. 7.76
— 50°13.2'N 13°16.4’W
OS01 2634 794+ 50°14.4’N 13°10.9'W OS 30. 7.76
52v — 50°15.2'N 13°11.0’W
WS01 2550- 39+2v 50°19.4’N 13°08.1'W WS 30. 7.76
2539 — 50°19.3'N 13°06.0’'W
WS02 2498- 416 50°19.3’N 12°55.8’'W WS 30. 7.76
2505 — 50°20.0'N 12°56.0’'W
CP10 4823. 1 48°25.5'N 15°10.7'W CP 31. 7.76
— 48°26.3'N 15°09.8'W
DS11 4823 1 8 48°18.8’N 15°11.5'W DS 1. 8.76
— 48°13.6'N 15°12.0'W
WS03 4829 5+1v 48°19.2’N 15°23.3'W WS 1. 8.76
CP11 4823 2 48°20.4’N 15°14.6'W CP 1. 8.76
— 48°21.1'W 15°13.7'W
OS02 4829 4 48°19.2'N 15°15.7;W OS 2. 8.76
DS14 4254- 1 47°32.8’'N 9°35.4";W DS_ 7. 8.76
4248
DS16 4268 1 47°29.8'N 9°33.4";W DS _ 9. 8.76
— 47°30.3'N 9°33.4’W
CANARY BASIN
Discovery 6704 2129 17 27°44.9'N 14°25.0'W ED 17. 3.68
CAPE VERDE BASIN
Discovery 8521 3053- 90 20°46.9’N 18°43.4".W WS 25. 6.74
3058 — 20°47.6'N 18°53.5'W
8521 3064- 52 20°47.9'N 18°53.4’'W WS _ 26. 6.74
307 — 20°48.6'N 18°53.4"W
8532 3112- 36 13°47.8’N 18°14.0'W WS _ 5. 7.74
3119 — 13°48.0'N 18°14.8'W
8532 2952- 20 13°48.2’'N 18°08.0'W WS _ 5. 7.74
2958 — 13°47.6'N 18°07.5'W
ANGOLA BASIN
AtlantisII 197 4592- 35 10°29.0'S 9°04.6’E ES 21. 5.68
42 4597
CAPE BASIN
Jean Charcot DS06 4585 1 33°34.5’N _2°32.9'E DS 31.12.78
(Walvis)

SHELL DESCRIPTION (Figs. 101-103). Shell ovate, somewhat
inflated, inequilateral, fragile, frequently with broad,
opaque, concentric banding which is variable in form; umbo |
inflated, anterior in position, orientated medially; dorsal shell
margins slope gently from umbo, antero-dorsal, anterior and
antero-ventral margins form a smooth curve, anterior limit of
which lies dorsal to the mid horizontal line, ventral margin
smoothly curved, with posterior limit of shell also dorsal to |
mid horizontal line, postero-dorsal margin slightly convex, |}
slope increases moderately sharply at posterior limit of hinge
plate to join posterior margin to form slight subrostration;
hinge plate, stout, with relatively large chevron-shaped teeth, |
posterior series with one or two more teeth than anterior |}!
series (maximum 10 in posterior series), teeth extend as far as |
the anterior and posterior margins of anterior and posterior |
adductor muscles respectively, hinge plate lies close to shell |
margin below umbo; ligament moderately sized, amphidetic,
‘saddle’-shaped in lateral view, extends below hinge plate.

Prodissoconch length: 187-198 4m. Maximum recorded
shell length: 3.7 mm.

The species has a high degree of variation in shape and
within a population may range from the form described above’
(the most common) to a more quadrangular outline in which)
the deepest part of the ventral margin instead of being}
approximately central is posterior to vertical line through the
beaks (Fig. 104). Such specimens may be more inflated.

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

65

Table 2 Yoldiella jeffreysi; ratios of Posterior Length (PL), Height (H), Width (W), to Total Length (TL) and other parameters.

Basin Max.recorded Pie
length (mm) (mean)
W. European 3.70 48/60
Incal DS07 (54)
Canary 3.49 52/58
Sta. 6704 (56)
Cape Verde 2.84 51/56
8521° (54)
N. America 2.54 47/55
Sta. 126 (51)
Guyana 2.96 48/58
Sta. 291 (54)
Argentine 2.95 44/60
Sta. 259 (53)

Fig. 101

Yoldiella jeffreysi. Lateral view of a shell from the left
side and an inner view of a right valve to show detail of the
hinge-plate. Specimens from Sta. 316, West European Basin.
(Scale = 1.0 mm).

The populations in different basins also differ somewhat in
overall shape (Fig. 105). In the Atlantic, more southerly
populations are somewhat less inflated and large specimens
have an extended posterior margin and thus a more inequilat-
eral shape. Overall, populations in the western Atlantic have
a similar shape to the majority of specimens from the
northern part of the West European Basin and they are
inflated to a similar degree. The more southern populations
are more extended posteriorly. Specimens from the Argen-
tine Basin are slightly more inflated but less so than those
from the northern West European Basin.

It is a general feature of all populations that the width/
length and post-umbonal length/total length increase with
increasing length. In contrast, there is little change in the
height/length ratio (Fig. 105).

The inter- and intra- variability in the shape of populations
of deep sea protobranchs has been noted in many taxa and
most recently for the family Malletiidae (Sanders & Allen
1985). Yoldiellid species are no exception and Y. jeffreysi is

H/TL W/TL ProdissoconchN
(mean) (mean) length (wm)
69/75 33/53 187-198 (54)
(72) (45)
68/75 32/48 185-198 (17)
(70) (41)
70/76 38/46 190-200 (52)
(72) (42)
69/77 35/52 182-189 (38)
(73) (40)
69/76 34/43 200-210 (35)
(73) (40)
69/75 36/49 182 (25)
(72) (41)

an extreme example (see Table 2). So much so that we have
made a particular study of this species and we propose to
present our results in more detail in a following publication.

The most closely related (but distinct) species to Y. jeffreysi
is Yoldiella lata. Jeffreys (1876) failed to distinguish between
the two species in his samples from depths where their
distributions overlap (Y. /ata is confined to slope depths (see
p. 32). In general Y. jeffreysi is more inflated than Y. Jata, has
more hinge teeth and the posterior adductor muscle (usually
visible through the shell in live specimens) is smaller and
more elongate.

INTERNAL MORPHOLOGY (Fig. 106-107). The exhalent and
inhalent siphons are combined the latter being open ventrally
(Fig. 106). There is a moderately well-developed feeding
aperture immediately ventral to the inhalent siphon. A large
siphonal tentacle, more frequently on the left side originates
close to the base of the siphon. Antero-ventrally there is a
well-developed mantle sense organ. The adductor muscles
are unequal in size. The anterior is between two and three
times larger than the posterior. The posterior is oval in shape
while anterior is ‘bean’-shaped; the ‘quick’ and ‘catch’ parts
are clearly distinct.

The gills are relatively well-developed with 12-19 alternat-
ing gill plates, the number depending on the size of the
animal. The most posterior plate lies close to the junction
between the inhalent and exhalent siphons and to which the
gill axes join. The labial palps are relatively small with long
and slender palp proboscides. In their contracted state they
extend across one quarter to one third of the body. The palps
have been 11-14 palp ridges, again the number depending on
the size of the animal. The foot is large and in some
specimens it is preserved in a very long, anteriorly extended
fashion. It has a deep papillate sole. A large byssal gland is
present and in many specimens in the region of its aperture at
the posterior margin of the foot there is a considerable
amount of mucous material present.

The species has a very large stomach, the dorsal wall of
which lies close to the hinge plate, the stomach lies off centre
slightly to the left. A large style sac penetrates deep into the
foot. The gastric shield extends close to the opening of the
oesophagus, the latter slightly to the right on the antero-
dorsal wall. Right and left digestive diverticula are anterior
within the body and material similar to that found in the
stomach was observed in sections of both right and left
diverticula. The hind gut forms a clockwise loop to the right

66

Shell length (mm)
iw)

1 5 9
No. of teeth

Fig. 103 Yoldiella jeffreysi. The relationship of the number of
hinge-teeth to shell length of a subsample from Sta. 316, West
European Basin. The left and right limits of each bar indicate the
number of anterior and posterior hinge-teeth respectively.

anterior of the body, thereafter describing an ‘S’-shaped bend
before continuing as an anticlockwise loop which partially
overlies the first. Thereafter it runs parallel to and immedi-

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 102 Yoldiella jeffreysi. a, lateral view of a
right and left valve of a specimen from the
Jeffreys collection labelled Leda lata (USNM
No. 199695, Valorous Expedition, Sta. 9). b, a
dorsal and lateral view from the right side of a
specimen from Sta. INCAL DS 06, West
European Basin. (Scale = 1.0 mm).

ately below the dorsal margin to the anus (see Fig. 107). A
typhlosole is present along its entire length. The configura-
tion of the hind gut on the right side of the body, despite
being somewhat variable is characteristic of this species (Fig.
108). It can be very simply visualized as a doubled length of
rope making one and a half turns (Fig. 107). In this species,
more than any other we have examined, there is a consider- }
able variation in the course of the hind gut which can easily be |
resolved by reference to the rope analogy (Fig. 107). It would |
appear that this manner of accommodation of a long hind gut
is particularly susceptible to distortion during development, }
possibly because of displacement due to the large size of the
stomach and the anterior arrangement of the digestive diver-
ticula or possibly because the loop tends to impede its own
development in this particular configuration (Fig. 105).

The nervous system is well-developed with moderately
large ‘club’-shaped cerebral ganglia, slightly smaller elongate |
visceral ganglia and large, oval, pedal ganglia. Dorsal to the
latter are large, round, statocysts, filled with refractile gran-
ules. The visceral ganglia lie some distance anterior to the
posterior adductor.

The populations from the different basins have similar })
internal morphologies. |

Sexes are separate and the gonads overlie other internal
organs. Gonad development was followed in two Incalj}\,
samples (DS07, DS09; West European) and one Discovery)
sample (8521 No. 1, Cape Verde). In West European speci-j
mens the number of ova ranged from 160 (108 um maximum) *
diameter) in a female 2.1 mm long to 350 (144 wm max.)
diameter) in a female 3.1 mm long. The females from the}
Cape Verde Basin (2.2, 2.5 & 2.7 mm total length) contained
fewer eggs 41-103 but these had a slightly larger size range} “
130-156 pm. |

In July/August samples, maturing gonads had become}

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

ig. 104 Yoldiella jeffreysi. A comparison of selected specimens
from a number of deep-sea Basins showing the range in shell
shape. The shell outlines of each specimen are drawn in dorsal
and right lateral view. a & b, Sta. 197, Angola Basin; c, Sta. 288,
Guyana Basin; d, Sta. 334, North America Basin; e & f, Sta. 259,
Argentine Basin; g, Sta. 316, West European Basin; h, Sta. 85,
North America Basin. (Scale = 1.0 mm).

bvious (Fig. 109) and by October the gonads had largely
lled the body.

oldiella enata (new species)

PE LOCALITY. R.V. Knorr, Sta. 301, Guyana Basin,
55°50.2'W, Epibenthic Dredge,
487-2500 m.

PE SPECIMEN. Holotype: BM(NH) 1992033. Paratypes: in
ollection held by J.A. Allen.

Sta Depth No Lat Gear Date

(m)

Long

UYANA BASIN

orr 25 301 2487—- 175 8°12.4’N 55°50.2";W ES 29. 1.72
2500

303 2842— 23 8°28.8'N 56°04.5'W ES 1. 3.72
2853

67

60

W\L ee

= = (ome. °
40 7 Ps
80
s . APU or Gat

H\L ° se fF e ere a

60

60
WL :
40 mf! ae es aR
80
hare! iy pees CNN -
60
60

Length(mm)

Fig. 105 Yoldiella jeffreysi. Variation in ratios of height H/L, width
W/L and postero-umbonal length PL/TL to length against length
of a, samples from Sta. 126, North America Basin (large closed
circles) and Sta. 259, Argentine Basin (points); and of b, samples
from Sta. INCAL DS 07, West European Basin (points) and Sta.
6704, Canary Basin (large closed circles).

NORTH AMERICA BASIN
Atlantis] 118 1135- 20 32°99.4’'N 64°34.9.W ES 18. 8.66
1153 — 32°19.0'N 64°34.8’W

Several specimens of this species are included in a mixture of

species contained with U.S. Natl. Mus. No. 108197, labelled
Yoldiella pygmaea Munst. None of these correspond to Leda

68

Fig. 106 Yoldiella jeffreysi. Siphons as seen from the ventral side
of a preserved specimen form Sta. 85, North America Basin.
(Scale = 0.1 mm).

Fig. 107 Yoldiella jeffreysi. Lateral views from the right and left
sides of the internal morphology of a specimen from Sta. 85,
North America Basin. (Scale = 1.0 mm). For identification of
parts see Fig. 34.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 108 Yoldiella jeffreysi. Diagrammatic views of the course of
the hind gut as seen from the right side to show variations in the
configuration.

pygmaea Munst. as exemplified by U.S. Natl. Mus. No.
197285.

Distributed in the western Atlantic from southern limit of
the North America Basin to Guyana, from mid to lower slope
depths. Depth range: 1135-2853 m.

SHELL DESCRIPTION (Figs. 110 & 111). Shell ovate, not
particularly inflated, moderately elongate, slightly inequilat-
eral, faint concentric striae particularly close to ventral mar-
gin; umbo moderately large; postero-dorsal and antero-
dorsal margins slightly convex and slope gently from umbo, |
anterior margin rounded, anterior limit dorsal to mid hori- |
zontal plane, dorsal and ventral margins without angulation,
antero-ventral margin smooth curve, postero-ventral margin
very slightly sinuous posterior margin not angulated, broadly

blunt, posterior limit in mid horizontal plane; hinge moder- |
ately broad distally, narrows centrally, chevron-shaped teeth
robust, close set with ventral arm twice length of dorsal,
anterior and posterior plates with up to 7 teeth; ligament |
amphidetic, moderately large, extends ventral to hinge plate.

Prodissoconch length: — 287 w»m. Maximum recorded shell
length: 3.65 mm.

This species is similar to Yoldiella ella, however it is less 4
round in outline, shallower dorso-ventrally and less inflated.
The ligament is also similar to that of Y. ella as is the hinge |
plate, however, the latter in Y. ella is more broad and bears
one or two more teeth than that in Yoldiella enata. |

INTERNAL MORPHOLOGY (Fig. 112). This is similar to that of |
Yoldiella ella so much so that it is extremely difficult to

distinguish between the two species. Such distinction as there } |
is is a matter of slight difference in proportion. Thus, the}

anterior adductor muscle is slightly larger, the diameter of the |} }
hind gut is slight less and the exhalent siphon is somewhat}
larger than in Y. ella. Sections of the tightly coiled hindgut} »:

DEEP-SEA PROTOBRANCHIA (BIVALVIA) 69

12 1
2
10 3
4
5 Ya Way
6 x
|e
eas. NM Ills
et a flan a PHA Hvala
| 5 fae | hagiy. LL AG WneocZos rates -
x Y Hiab “ x) 4
|: 7 ae
Z 4 CRS |
6 Hass
DM Ed
I] x
wel
10 OT
il
A A A
1 2 3

Length (mm)

Fig. 109 Yoldiella jeffreysi. Length frequency histogram superimposed by a gonadial index. 1, immature, no gonadial development seen in
the intact specimen; 2, first sign of gonadial development at ventral edge of visceral mass; 3, gonad surrounds the periphery of the visceral
mass; 4, gonad covers half the visceral mass; 5, visceral mass wholly covered by gonad. Top, females; bottom, males.

specimen a few eggs were present in the mantle cavity. This is
not taken as evidence of brooding.

It is clear that Yoldiella enata and Yoldiella ella are very
closely related, however, we have no difficulty in distinguis-
ing them from their shell features. It should also be noted that
the depth distribution of the two species is very different,
Yoldiella enata is found on the lower slope while Yoldiella ella
is truly abyssal.

Yoldiella ella (new species)

TYPE LOCALITY. R.V. Chain Sta. 334, North America Basin,
30.9.1972, 40°42.6’N, 46°13.8’W — 40°44.0’N, 46°14.6’W,
Epibenthic Trawl, 4400 m.

TYPE SPECIMEN. Holotype: BM(NH) 1992034. Paratypes in
collection held by J.A. Allen.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
ig. 110 Yoldiella enata. Lateral view of a shell from the right side NORTH AMERICA BASIN
and detail of the hinge-plate of a left valve. Specimen from Sta. AtlantisIT 72 2864.1 38°16.0'N_ 71°47.0'W ET 24. 8.64
301, Guyana Basin. (Scale = 1.0 mm). 12
Chain 50 76 2862 1 39°38.3'N 67°57.8'W ET 29. 6.65
how the same number of coils. In a specimen 3.1 mm the USeg 33825 38 /08.0 Ni iG2 18-7, Wile Ey eO ae)
alps, which are moderately small, have 14 internal ridges Be a OBEN Ag Cola a Ty OF 202 W ET SOUS
a ‘ : y ¥ : : c= AtlantisII 126 3806 4 39°37.0'N 66°47.0'W ET 24. 8.66
e palp proboscides are large and broad. The kidney is large 4 _ 39°37.5'N 66°44.0'W
nd extends anteriorly lateral to the stomach. The largest Chain 106 334 4400 44 40°42.6’N 46°13.8’W ES 30. 8.72
pecimens are mature and in the case of one sectioned — 40°44.0'N 46°14.6’W

70 J.A. ALLEN, H.L. SANDERS AND F. HANNAH

3779 — 9°49.0'S 10°33.0’E

WEST EUROPEAN BASIN

Chain 106 323 =3356- 9 50°08.0’'N 13°53.77;W ES 21. 8.72
3338 13°50.9'W

326 3859 12 50°04.9'N 14°23.8’'W ES 22. 8.72
— 50°05.3’N 14°24.8'W
328 4426 9 50°04.7'N 15°44.8'W ES 23. 8.72
4435
330 4632 137 50°43.5'N 17°51.7'W ES 24. 8.72
— c50°43.4'N 17°52.9'W
Jean Charcot
(Polygas) DS20 4226 25 47°33.0'N 9°36.7'W DS _ 24.10.72
DS21 4190 8 47°31.5'N 9°40.7'W DS _ 24.10.72
DS22 4144 14 47°34.1'N 9°38.4’'W DS_ 25.10.72
DS23 4734 2 46°32.8'N 10°21.0'W DS_ 26.10.72
(Biogas III) DS41 3548 1 = 47°28.3'N 9°07.2’W_ DS 26. 8.73
DS42 4104 1 47°32.1'N 9°35.6’W DS 27. 8.73
(Biogas TV) DS55 4125 80 47°34.9'N 9°40.9'W DS 22. 2.74
3
6

DS56 4050 47°32.7'N —9°28.2".W_ DS 23. 2.74
DS60 3742 47°26.8'N 9°07.2"W DS 24. 2.74 |
Cyros
(Biogas V) DS66 3480 21 47°28.2'N 9°00.0'W DS 16. 6.74

: ; : : : DS67 4150. 4 = 47°31.0’N_—_-9°35.0'W_ DS’ 17. 6.74
Fig. 111 Yoldiella enata. Outlines of shells from the right side to DS68 4550 3 46°26.7'N 10°23.9'.W DS 19. 6.74

show variation in shape with growth of specimens from Sta. 301, Jean Charcot

Guyana Basin. (Scale = 1.0 mm). (Biogas VI) DS75 3150 4 47°28.1’N 9°07.8'W DS 22.10.74
DS76 4228 243 47°34.8'N 9°33.3'W DS 23.10.74
DS77 4240 55 47°31.8'N 9°34.6'W DS _ 24.10.74
DS78 4706 3 46°31.2'N 10°23.8’W DS _ 25.10.74
DS79 4715 9 46°30.4’N 10°27.1'W DS_ 26.10.74
DS80 4720 3 46°29.5’N 10°29.5'W DS _ 27.10.74
CP13 4134 2 47°34.4’'N 9°38.0'W DS _ 23.10.74
Jean Charcot 0S02 4829 8 48°19.2’N 15°15.7'W OS 2. 8.76
(Incal) OSOS 4248- 9 47°31.3’N 9°34.6'W OS _ 7. 8.76

4296 — 47°32.2'N —9°34.7'W
0S66 4316- 43 46°27.3’N 9°36.2'W OS _ 9. 8.76
4307 — 47°27.9'W 9°36.0'W.

0SO7 4249 63 47°31.8'N 9°34.3'W OS 10. 8.76
OSO8 4327 42 47°29.8'N 9°39.2'W OS 11. 8.76
— 47°29.S'N 9°38.8'W
WS03 4829 7+2v 48°19.2'N 15°23.3'W WS 1. 8.76
— 48°19.1'N = 15°22.5'W
WS07 4281— 30 47°30.6'N 9°37.1'W WS _ 7. 8.76

4274 — 47°31.2'N —9°35.7'W
WS08 4287— 36 47°30.5'N 9°33.7'W WS 9. 8.76
4301 — 47°29.3'N —9°34.1"W
WS09 427711 47°28.8'N 9°34.0'W WS 10. 8.76
— 47°27.9'N
WS10 4354 37 47°27.3'N _ 9°39.9'W__ WS 14. 8.76
— 47°28.3'N
Fig. 112 Yoldiella enata. Lateral view from the right side of the DS11 4823 3 48°19.2'N 15°23.3’,W DS 1. 8.76
internal morphology of a specimen from Sta. 301, Guyana Basin. — 15°22.5’'W
(Scale = 1.0 mm). For identification of the parts see Fig. 34. DS14 4248- 20 47°32.6'N 9°35.7"N DS_ 7. 8.76
4254 — 47°32.9'N —9°35.1'W
47°33.6' °39.1'W D . 8.76
335 3882— 26 40°25.3'N 46°30.0'W ES 31. 8.72 ge ae ‘e a i sae anil Se
3919 ‘
47°29.8' °33.4' . 8.76
Knorr 35 340 3264- 16 38°14.4'N 70°20.3,W ES 24.11.73 sna, al * i bea ain aca ae
Sb age eo NZS CP11 4823 1 48°20.4’N 15°14.6’W CP 1. 8.76
SIERRA LEONE BASIN — 48°21.1'N 15°13.7°W
AtlantisII 148 3814 1 10°37.0’N 18°14.0'W ES _ 7. 2.67
31 3818
149 3861 25 10°30.0’'N 18°18.0’'W ES 7. 2.67 Widely distributed throughout much of the Atlantic in North
155 3730- 7 00°03.0'S 27°48.0'W ES 13. 2.67 America, West European, Sierra Leone and Angola Basins
3783 at abyssal depths. With only two exceptions, all records are
156 3459 4 00°46.0’S 29°28.0'W ES 14. 2.67
— 00°46.5’S 29°24.0'W greater than 3200 m. Depth range: 2862-4829 m.
ANGOLA BASIN S ;
HELL DESCRIPTION (Figs. 113-116). Shell ovate, moderately |}
Atl le ; a ! : : é ‘3 z peak
aa hss 197 pees IOS NONE BS SHE eS) inflated, inequilateral, shell with very fine concentric lines, |}
198 4559- 4 10°24.0'S 9°09.0'R ES 21. 5.68 ventral lines usually more conspicuous, few fine radial lines |} |
4566 from umbo to ventral edge, periostracum light straw colour;

199 3764- 2 9°47.0'S 10°29.0'E ES 22. 5.68 umbo large, rounded, orthogyrate, anterior to midline, great-

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

Fig. 113 Yoldiella ella. Lateral view of a shell from the right side
and a left valve to show detail of the hinge-plate. Specimens from
Sta. INCAL 0S 06. (Scale = 1.0 mm).

est shell height posterior to umbo; dorsal margin slightly
convex, antero-dorsal anterior and ventral margins in a
smooth continuous curve, postero-ventral margin rounded
posteriorly, posterior margin in small specimens may have
very slight, blunt, angulation, posterior limit slightly dorsal to
horizontal midline, postero-dorsal margin curves gently from
umbo to distal edge to hinge plate then slopes more acutely to
posterior margin; hinge plate, long, characteristically angular
below umbo, moderately broad proximally, narrow ventral to
umbo, anterior and posterior ventral margins of hinge plate
more or less straight, teeth strong, well-developed, equal
number in anterior and posterior plates, up to 9 in specimen
4.2 mm, ligament amphidetic, moderate in size, rectangular
or slightly ‘goblet’-shaped, short, wide, posterior external
extension and long slender anterior external extension of
fused periostracum.

Prodissoconch length: 166 »m. Maximum recorded shell
length: 4.2 mm.

There is little change in the width/length or height/length
Tatios with increasing size, however, posterior umbonal
length increases slightly and the posterior margin becomes
‘}more smoothly curved.

“\INTERNAL MORPHOLOGY (Fig. 117). The anterior sense
"organ is well-developed. Posterior mantle fusion is minimal,
limited to a fine bridge of tissue between the opposing inner
mantle lobes forming a short exhalent siphon. The gill axis is
| jattached laterally on either side of the bridge tissue. Ventral
_jto it the inhalent siphon is reduced to a pair of unfused
hickened pads of tissue. The feeding aperture is poorly
eveloped with a few gland cells present. Immediately ante-
_ rior to the feeding aperture, the inner mantle fold is enlarged

—S~

SS

Fig. 114 Yoldiella ella. Outlines of shells from the right side to
show variation in shape with growth of specimens from Sta. BG
VI DS 76, West European Basin. (Scale = 1.0 mm).

slightly and more obviously ciliated and may possibly indicate
a temporary point of adhesion in the living specimen. The
siphonal tentacle is usually to the left ventral side of the
shallow mantle embayment. The adductor muscles are rela-
tively small, approximately equal in size and more or less oval
in shape. There are up to 16 gill plates and the gills are
attached far posterior on the body wall. The gill axis is
well-supplied with muscle fibres. The labial palps, like the
gills, are relatively small with up to 14 ridges on their inner
faces with the result that in preserved specimens, there is a
marked separation of gill and palp which is unlikely to be true
in life. The palp proboscides are relatively long and broad.
The foot is of moderate size with a relatively wide neck and
an elongate, deeply divided sole. There is a large byssal gland
in the heel with a concentration of cilia around its opening.
The heel has a terminal papilla. The cerebral and visceral
ganglia are relatively small and ‘club’-shaped in lateral view.
The pedal ganglia is larger and slightly elongate.

The oesophagus has a wide opening into a large stomach.
The combined style sac and mid gut extend into the dorsal
half of the foot. The digestive duct from the right diverticu-
lum skirts dorsal to the hind gut loops to open into the
anterior wall of the stomach. On the left and close to the
oesophageal aperture is a relatively wide digestive duct from

YZ
60
WI\L bo Sn es oe ‘hae me
40 :
80 us se co J :
Gee
H\L

Length(mm)

Fig. 115 Yoldiella ella. Variation in ratios of height H/L. width
W/L and postero-umbonal length PL/TL to length against length
of a sample from Sta. BG VI DS 76, West European Basin.

Numbers

2 3
Length(mm)

Fig. 116 Yoldiella ella. Length frequency histogram of a sample
from Sta. BG VI DS 76, West European Basin.

the left anterior diverticulum. This latter opens mid-laterally
into the stomach. A second duct from the posterior digestive
diverticulum of the left side opens into the left wall of the
stomach below the gastric tooth. The course of the hind gut is
first anterior and then antero-ventral to the pedal ganglia
before turning back on itself to pass dorsally posterior to
the stomach. Thereafter it forms a series of complicated
coils to the right side of the body the design of which can be
derived from a doubled strand coiled clockwise as seen

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

from the right side (Fig. 117). The hind gut then passes to
the mid postero-dorsal margin and thence dorsal to the
posterior adductor muscle to the anus. There is a relatively
shallow typhlosole along the length of the hind gut. Faecal
rods were seen cradled by the gill axes between anus and
the siphon.

The kidney and heart are well-developed. The sexes are
separate. Animals smaller than 2.15 mm total length were all
immature with no obvious gonad present. The number of ova
varies from 16 in an individual 2.57 mm (maximum diameter
of ova 87 mm) to 90 in an individual 3.2 mm long (maximum
diameter of ova 132 ym). In samples collected in February,
June, August and September in specimens of 2.15 mm and
above, all have maturing ova.

Y. ella has all the characteristics of a shallow burrowing or
semi-submerged species i.e. a rounded, broad form, with
short siphons in a shallow siphonal embayment. Many speci-
mens bear hydroids attached to antero-ventral and ventral
margins.

Two intact specimens of a species of Yoldiella in which the
external shell features appear to be close to Y. ella (or
possibly Y. enata) were taken at Sta. 199 from the Angola
Basin at a depth of 3771 metres (Fig. 118). We have hesitated
to examine the internal anatomy of these. Unfortunately
because of the opaqueness of the shell all that can be seen of
the hind gut is part of one or possibly two coils to the right of
the body, close to the anterior adductor. These specimens
could either prove to be a new species or showing variation of
shell form.

Yoldiella fabula (new species)

TYPE LOCALITY. R.V. Chain Cruise 50, Sta, 85, North
America Basin, 5.7.1965, 37°59.2’N, 69°26.2'W, Epibenthic
Trawl, 3834 m.

TYPE SPECIMEN. Holotype: BM (NH) 1992037. Paratypes: in
collection held by J.A. Allen.

Leda sericea Jeffreys 1879, p. 579 (in part). Two specimens
were found in the Jeffreys collection (U.S. Natl. Mus. No.
199590 (Fig. 119a) and No. 199589 (Fig. 119b)). Although
these are labelled L. sericea and must have been so
identified by Jeffreys they clearly differ from that species.
A specimen stored with and labelled Yoldiella expansa
(U.S. Natl. Mus. No. 697343 (Fig. 119c)) is also this
species. All these specimens were collected off West Ire-
land in 2500-2670 metres.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
NORTH AMERICA BASIN
AtlantisII 62 2496 4 39°26.0'N 70°33.0'W ET 21. 8.64
12 64 2886 1 38°46.0'N 70°06.0'W ET 21. 8.64

70 4680 1 36°23.0'N 67°58.0'W ET 23. 8.64
72 2864 2 38°16.0'N 71°47.0'W ET 24. 8.64

Chain 50 77 ~=3806 = 1— 38°07.0'N__ 69°16.0'W_ ET 30. 6.65
78 3828 6 38°08.0'N 69°18.7'W ET 30. 6.65
84 4749 4 36°24.0'N 67°56.0'W ET 4. 7.65
85 3834 c13.-37°59.2'N 69°26.2’W_ ET 5. 7.65

AtlantisII 92 4694 1 36°59.2’N 69°26.2'W ET 5. 7.65

17

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

: | Jean Charcot DS21 4190

(Polygas)

DS23 4734

mm).
AtlantisII 125 4825 1
24
Knorr35 340 3264- 1
3356
ARGENTINE BASIN
AtlantisII 242 4382- 8
60 4402
243 3815- 10
3822
247 5208- 16
5223
256 3906- 3
3917
| WEST EUROPEAN BASIN
Chainl06 323 33536- 2
3338
330 4632 1

1
2

37°24.0'N
— 37°16.0'N
38°14.4'N
— 38°17.6'N

38°16.9'S

37°36.9'S

43°33.0'S

37°40.9'S

50°08.0'N
— 50°08.3'N
50°43.5'N
— 50°43.4'N
47°31.5'N
46°32.8'N

65°54.0'W
65°50.0'W
70°20.3'W
70°22.8'W

51°56.1"W

52°23.6"W

48°58.1'W

52°10.3'W

1375357) Wi
13°50.9'W
17°51.7'W
17°52.9'W

9°40.7'W
10°21.0'W

BD

ES

ES

ES

ES

ES

ES

ES

DS
DS

23. 8.66

24.11.73

IS SETA

14. 3.71

1G Sh 7fil

24. 3.71

21. 8.72

24. 8.72

24.10.72
26.10.72

(Biogas II)
(Biogas IIT)

(Biogas IV)

(Biogas V)
(Biogas VI)

(Incal)

DS28 4413
CV13 4252
DS30 4160

DS41 3548
DSS50 2124
DSS55 4125
DS56 4050
DSS7 2906
DS66 3480
DS67 4150
DS76 4228
DS84 4466
WS07 4281

WS08 4287
QS06 4316

QS07 4249

i)
RR RS SR NR WR eS

2)

2

2

44°23.8'N
47°31.8'N
47°38.3'N

47°18.3'N
44°08.9'N
47°34.9'N
47°32.7'N
47°31.7'N
47°28.2'N
47°32.0'N
47°34.8'N
44°25.4'N
47°30.6'N
— 37°31.2'N
47°30.5'N
— 47°29.3'N
46°27.3'N
— 47°27.9'N
47°31.3'N
— 47°31.3'N

4°47.5'W
9°34.2'W
9°33.9'W

9°07.2'W
4°15.9'W
9°40.9"W
9°28.2'W
9°06.2"W
9°

9°35.0'W
9°33.3'W
4°52.8'W
O37 law,
9°35.7'W
9°33.7'W
9°34.1'W
9°36.2"W
9°36.0'W
9°34.3'W
9°34.3'W

DS
CV
DS

DS
DS
DS
DS
DS
DS
DS
DS
DS
WS

WS

OS

OS

13

Fig. 117 Yoldiella ella. Details of the internal morphology. a & b, whole mounts from the right and left sides respectively; c, ventral view of
contracted siphon; d, ventral view of the sole of the foot and hindgut; e, course of the hindgut as seen from the right side. (Scales = 1.0

2 AGE
25.10.72

18.

26.

i
Des
23.
24.
16.

17

4.73

8.73
9.73
2.74
2.74
2.74
6.74
6.74

23.10.74
29.10.74

ie

3).

2.

19.

8.76

8.76

8.76

8.76

74

DS05 2503 1 56°28.1’N_ 11°11.7’;W_ DS 18. 7.76
— 56°27.6'N 11°12.0’W
DS09 2897 7 55°07.0'N 12°52.6'W DS 20. 7.76
— 55°08.1'N 12°53.2’W
DS14 4254- 1 47°32.6'N 9°35.7'W DS _ 8. 8.76
4548 — 47°32.9'N —_9°35.5'W
DS16 4268 2 47°29.8’N 9°33.4;W DS _ 9. 8.76
— 47°30.3'N —-9°33.4"W
GUINEA BASIN
DS20 2514 2 2°32.0’'S 8°18.1'W DS
SIERRA LEONE BASIN
AtlantisII 149 3861 1 10°30.0’N 18°18.0’W ES_ 7. 2.67
31
ANGOLA BASIN
AtlantisII 195 3707 7 14°49.0'N 9°56.0'W ES 19. 5.68
42 — 14°40.0'N = 9°54.0’W
196 4612—- 2 10°29.0’'N 9°03.0'W ES 21. 5.68
4630 — 10°29.5'W =: 9°04.0"W
197 3865- 5 10°29.0’N 9°04.0'W ES 21. 5.68
4595 — 10°29.0'N 9°04.0'N
198 4559- 10 10°24.0'N 9°09.0'W ES 21. 5.68
4566 — 9°47.0'N 10°29.0'W
199 3764- 2 9°49.0'N 10°33.0'W ES 22. 5.68
3779 — 9°41.0'N 10°55.0’W
GUYANA BASIN
Knorr 25 291 3859- 1 10°06.1’N 55°14.0'W ES 26. 2.72
3868
303. 2842- 2 8°28.8’N 56°04.0’'W ES 1. 3.72
2853

Y. fabula is wide-spread at abyssal depths throughout the
Atlantic in small but persistent numbers in most of the
Atlantic Basins. Low population density and low sampling
density probably explains the lack of records in the Guyana &
Canaries Basins. Depth range: 2503-5223 m.

SHELL DESCRIPTION (Figs. 119-121). Shell small, moderately
swollen, ‘bean’-shaped, inequilateral, post umbonal length

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 118 Yoldiella sp. Lateral views of two shells taken from Sta.
119, Angola Basin. (Scale = 1.0 mm).

40-46% of total length, sculpture regular with conspicuous
ridges, periostracum straw coloured, often discoloured and
brown at valve margins; umbo relatively large, posterior in
position and inflated, directed posteriorly; proximal dorsal
margins slightly convex either side of umbo, proximally
evenly curved to rounded anterior and posterior margins,
ventral margin long, smooth curve; hinge plate moderately

Fig. 119 Yoldiella fabula.
Lateral views of two shells
from the right and left sides
respectively and a left valve
to show details of the
hinge-plate. a, Jeffreys
collection USNM No. 199590
labelled Leda sericea Jeffr.
St. 21, 1476 fm. N.W.
Ireland, Porc. Ex. 1870; b,
USNM No. 199589 labelled
Leda sericea Jeff. St. 19a,
1366 fm, W. of Ireland, Porc.
Ex. 1869; c, USNM No.
697343 labelled Yoldiella
expansa Jeffreys 49°37'N,
13°34'W. S.W. of Ireland.
(Scale = 1.0 mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

strong, extends almost to outer limit of both adductor
muscles, anterior hinge plate with 1-2 more teeth than
posterior, total number of teeth up to 17 depending on size,
distal teeth small and difficult to identify; ligament internal,
slightly opisthodetic, ‘goblet’-shaped with posterior extension
ventral to hinge plate.

Prodissoconch length: 229 4»m. Maximum recorded shell
length: 3.1 mm.

The shell outline in this species is somewhat variable (Figs.
120 & 121). It is the only yoldiellid species indeed species in
our collections, with this highly characteristic ‘bean’-shape.
For a time we misidentified Yoldiella fabula as Y. dissimilis
Verrill & Bush 1898. On closer examination we note that Y.
dissimilis unlike Y. fabula is nearly equilateral, that the
anterior hinge is oblique that the number of hinge teeth is
greater and that an oblong prominent tooth-like process at
the proximal end of the posterior hinge series is not present.
Y. fabula, as all other yoldiellids, has only very small external
anterior and posterior componants visible which are derived
from secondary fused periostracum.

The range in length of the specimens in the collection
varies from 1.72 mm to 3.08 mm. The following are the
overall proportions:- H/L ratio 0.63-0.72; W/L ratio
0.41-0.50; and PL/TL ratio 0.40-0.46. Although these ratios
vary the overall range is related to growth and to some
variation in the populations.

INTERNAL MORPHOLOGY (Figs. 122 & 123). Ventral to the
anterior adductor muscle the middle lobe of the mantle is
modified to form a well-developed anterior sense organ. The
combined siphon has a single lumen which is open ventrally.
The gill axes join laterally indicating an inhalent as well as an
exhalent component. A small, single, tentacle is present close
to the postero-ventral margin of siphonal embayment.
Numerous groups of glandular epithelial mantle cells are
found peripheral to the inner siphonal aperture and the
feeding aperture and anterior to the latter.The adductor
muscles are large and approximately equal in size. The
posterior adductor is oval in cross section while the anterior is
‘crescent’-shaped with ‘catch’ and ‘quick’ portions clearly
marked.

The gills are parallel to the dorsal posterior shell margin

| with up to 17 well-developed plates. The distal gill filaments

lie close to the siphon. Moderately large labial palps extend
approximately halfway across the body and have up to 20
ridges on their inner surface. The foot is well-developed with
a narrow neck and a deeply divided sole. At the heel there is
a conspicuous median papilla directly posterior to the open-
ing of the byssal gland. The byssal gland is well-developed.
The pedal musculature is similar to Y. /ata. Both cerebral and
visceral ganglia are cylindrical and moderately well-
developed with a stout connecting commissure. The pedal
ganglia are circular and not particularly large.

The oesophagus, stomach, and style sac are basically

‘| similar to those of Y. Jata. A long duct from the right
_ digestive diverticula passes dorsally over the hind gut to enter

7

-
7

the stomach close to the oesophageal aperture. The duct from
one left diverticulum opens slightly more posteriorly on left

| ventral wall, while the duct from the second left diverticulum

opens ventral to the gastric tooth on the left side. The hind
gut has a typhlosole and forms a single loop to the right side
of the body.

Initially the gonads develop ventral to the digestive diver-

75

ticula and the hind gut loop, but gradully they spread
posteriorly and dorsally to the stomach. A female 2.5 mm
long, from a July sample had approximately 74 large ova
(maximum diameter 180 ym) while a second female of similar
size (2.3 mm) had 62 ova (maximum diameter 160 .m). The
gonadial aperture is close to that of the kidney, and anterior
to the posterior pedal retractor muscle. The large kidney
extends from the postero-dorsal margin to the foot and
anteriorly over the viscera to a point just anterior to the
lateral pedal retractor muscle. A large pericardial cavity is
present.

Yoldiella veletta (new species)

TYPE LOCALITY. R.V. Jean Charcot, Biogas VI, Sta. DS87,
Bay of Biscay, 31.10.1974, 44°05.2’N, 4°15.7’W, Epibenthic
Trawl, 1913 m.

TYPE SPECIMEN. Holotype: Museum National d’Histoire
Naturelle, Paris.

MATERIAL.
Cruise Sta Depth No Lat Long Gear Date
(m)
CANARIES BASIN
Discovery 6701 1934 1 27°45.2’N 14°13.0'W ES 16. 3.68
CAPE VERDE BASIN
145 2185 1 10°36.0'N 17°49.0'W ES _ 6. 2.67
WEST EUROPEAN BASIN
Jean Charcot
Biogas VI DS87 1913 1 44°05.2'N 4°15.7'W ES 31.10.74

Distributed on the lower slope in the eastern Atlantic from
the Bay of Biscay to the Cape Verde Islands. Depth range:
1913-2105 m.

SHELL DESCRIPTION (Fig. 124). Shell moderately swollen,

Fig. 120 Yoldiella fabula. Lateral view of a shell from the right
side and detail of the hinge-plate of a left valve. Specimens from
Sta. BG VI DS 84, West European Basin. (Scale = 1.0 mm).

76

6
W\L 4 °
40 wart itenatss
80
H\L e e e ‘ e
5 4 ore
60

Length(mm)

Fig. 121 Yoldiella fabula. Variation in ratios of height H/L. width
W/L and postero-umbonal length PL/TL to length against length
of specimens taken by the Biogas Expeditions as listed in the
table of material.

ovate, inequilateral, sculpture, fine concentric lines and
growth rings, no lunule, no escutcheon; umbo large, poste-
rior to midline, directed to posterior; antero-dorsal margin
merges with anterior margin in smooth curve, limit of ante-
rior margin close to midline, ventral margin long, shallow
curve, limit of posterior margin ventral to midline, proximal
postero-dorsal margin slopes more steeply than antero-dorsal
margin and results in narrowing of post umbonal shell in
lateral view; hinge plate moderately strong, except ventral to
umbo where it is very narrow, hinge teeth strong, somewhat
elongate, 1 or 2 more teeth in anterior series; ligament
amphidetic, largely ventral to hinge margin; no chondro-
phore.

Prodissoconch length: 165 »m. Maximum recorded shell
length: 4.56 mm.

INTERNAL MORPHOLOGY (Fig. 125). The combined siphon
has a single lumen which is open ventrally, as in Y. fabula.
The siphonal embayment is deep, a fine tentacle is present on
the right side. The adductor muscles are particularly large but
approximately equal in size, the anterior being fractionally
the larger. The anterior sense organ is not particularly
well-developed. The gill is elongate and attenuated with up to
27 plates. The palps are large each with up to 13 internal
ridges. The foot is also large and with papillate margins
anteriorly directed. The stomach is moderately large and the
hind gut forms a single loop to the right side of the body. The
hind gut has a wide diameter and a single typhlosole is
present along its entire length.

This species has clear affinities with species of Portlandia.
Nevertheless, it is a fragile shell without any trace of lunule or
escutcheon. On the basis of only three specimens, the shell
morphology seems most similar to Y. fabula.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 122 Yoldiella fabula. Lateral view from the right side of the
internal morphology of a specimen from Sta. 85, North America
Basin. (Scale = 1.0 mm). For identification of parts see Fig. 34.

Fig. 123 Yoldiella fabula. External views of intact stomach an style
sac in a, left; b, right and c, antero-frontal views. Dissected from
a specimen from Sta. 85, North America Basin. (Scale = 1.0
mm).

Genus Portlandia Morch 1857

TYPE. By selection (Soot-Ryen, 1984: Opinion 769 ICZN)
Nucula artica J.E. Grey, 1824.

Shell moderately small, slightly inflated, moderately frag-
ile, oblong, posteriorly angular, more or less truncate, sub-
rostrate, not gaping, escutcheon present, usually defined by
weak or occasionally moderate carina, lunule may be present;
usually glossy, smooth, postero-ventral margin may be sinu-
ous, proximal postero-dorsal margin almost straight or
slightly concave; umbo prominent, anterior, chondrophore
variously developed; ligament amphidetic, largely internal
with small part external, hind gut single loop to right of body.

Portlandia lenticula (Moller 1842)
TYPE LOCALITY. Greenland.
TYPE SPECIMEN. Lectotype BM(NH) 1843.7.3.31, right-hand

DEEP-SEA PROTOBRANCHIA (BIVALVIA) 77

Fig. 126 Portlandia lenticula. Lateral view of a shell from the right
side from Sta. S44, West European Basin. An interval view of a
right valve with an enlarged detail of the hinge-plate and a dorsal
view of a shell. Specimens from North East Atlantic (det. K.
Ockelmann). (Scales = 1.0 mm).

Fig. 124 Yoldiella veletta. Lateral views of two shells from the right
side, detail of the hinge-plate of a right valve and a dorsal view of
a shell. Specimens form a, Sta. 145, Cape Verde Basin; b & c,
Sta. BG VI DS 87, West European Basin; d, Sta. 6701, Canaries
Basin. (Scales = 1.0 mm).

Fig. 127 Portlandia lenticula. Lateral view from the right side of
the internal morphology of a specimen from Sta. S44, West
European Basin. (Scale = 1.0 mm) for identification of the parts
see Fig. 34.

Yoldiella lenticula Dautzenberg & Fischer, 1912, 406; Scar-
Fig. 125 Yoldiella veletta. Lateral view from the right side of the lato 1981, 209. Fig. 113.

internal morphology of a specimen from Sta. 145, Cape Verde
Basin. (Scale = 1.0 mm). For identification of parts see Fig. 34.

MATERIAL.

specimen of 5 mounted specimens; Paralectotypes BM(NH)
1843.7.3.27-30 4 mounted specimens. Cruise Sta Depth No Lat Long Gear Date
(m)
Nucula lenticula Moller 1842, 17.
Portlandia lenticula Sars 1878, 39, tab. 4, Fig. 10a,b; Thiele = west EUROPEAN BASIN
1928, 617; Ockelmann 1958, Fig. 13, pl. 1, Fig. 12. Sarsia 44 1739 4 43°40.8'N 3°35.2,W_ ED 16. 7.67
Yoldia (Yoldiella) lenticula Richards 1962, pl. 1, Figs, 23, 24. Thalassa 2438 1400 3 48°33.7’N 10°15.0'W PBS 26.10.73

78

Jean Charcot DS37 2110 1 47°31.8’N 834.6’W DS 24. 8.73
(Biogas IIT)

(Biogas IV) CPO1 2245 2 47°34.6'N 8°38.8'W CP 25. 2.74
CANARY BASIN

Discovery 6704 2129 1 27°44.9'N 14°25.0'W ED 17.3.68

This is predominantly a northern Atlantic species occurring
mostly from 10-200 metres (Ockelmann, 1958). Neverthe-
less, there are sufficient past records to confirm the present
identifications that at its southern limits it is present at lower
slope depths. Perhaps indicative of a temperature/depth
relationship.

SHELL DESCRIPTION (Fig. 126). Shell moderately stout,
ovate, slightly inequilateral, fine concentric lines, with elon-
gate lunule and escutcheon, shell very wide dorsally and
medially (such that when it rests on a valve the dorsal part is
centred so that both umbos are characteristically visible in the
lateral view); umbo very large, raised, internally directed,
orthogyrate, immediately anterior to midline; antero-dorsal
margin joins anterior and antero-ventral margins in a smooth
curve, postero-dorsal margin also joins posterior and postero-
ventral margin in smooth curve but is more attenuate than
anterior margin; hinge plate moderately stout, not quite
reaching level with the outer margins of adductor muscles,
hinge plate narrows almost to margin below umbo, anterior
and posterior hinge teeth equal in number or with one
additional on the posterior plate; ligament amphidetic large,
globular, extending far ventral to the hinge plate, slight
secondary anterior and posterior external extension of fused
periostracum. (See Ockelmann 1958, for typical shell dimen-
sions).

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

INTERNAL MORPHOLOGY (Fig. 127). The combined siphons
are short within a relatively shallow siphonal embayment.
There is a slender sensory tentacle on the right side. The
anterior sense organ and the feeding aperture are not particu-
larly well-developed. The adductor muscles are very large,
more or less oval in shape the anterior being the larger in size.
The gills are slender with approximately 13 plates in a small
specimen 14 mm in length. The palps are large with 13 ridges.
The foot is small and in the one whole mount contracted to
the level of the ventral edge of the palps. There is a small
byssal gland. The stomach is large and the hind gut stout, the
latter describes a single loop on the right side of the body.

Portlandia fora (new species)

TYPE LOCALITY. R.V. Sarsia, Sta. 56, Bay of Biscay,
19.7.1967, 43°43.0'N, 3°47.8’W, Epibenthic Trawl, 641 m.

TYPE SPECIMEN. Holotype: BM (NH) 1992041. Paratypes: in
collection held by J.A. Allen.

MATERIAL.

Cruise Sta Depth No Lat Long Gear Date
(m)

WEST EUROPEAN BASIN

Sarsia 56 641) = 15) 43°43.0’N -33°47.8'W Ss ED 19.7.67

Only taken from the Bay of Biscay at one Station on the
upper slope. Depth 641 m.

SHELL DESCRIPTION (Fig. 128). Shell relatively stout,

Fig. 128 Portlandia fora. Lateral
view of a shell from the left side and
a dorsal view of the same shell, detail:
of the hinge-plate of a left valve and
lateral views of two small specimens
to show variation in shape with
growth. Specimens from Sta. S 56,
West European Basin. (Scales = 1.0
mm).

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

inflated, ovate, inequilateral; umbos inflated, orthogyrate,
anterior to midline; proximal dorsal margin slightly convex,
antero-dorsal margin convex, slightly angulate at limit of
hinge plate, then relatively straight section to dorsal limit of
anterior margin, antero-ventral margin and ventral margin
smoothly curved, posterior margin drawn out into broad
rounded medial tip, postero-dorsal margin slightly convex
sloping gradually towards tip, slight dip in outline at limit of
hinge plate, posterior margin slightly rostrate; no marked
rostral ridge but a small lunule and escutcheon present close
to umbos; hinge plate moderately broad, long strong teeth,
12 in posterior and 10 in anterior series in largest individual,
ligament internal, amphidetic, moderately large, ‘goblet’-
shaped, short anterior and posterior external extensions of
fused periostracal.
Maximum observed shell length: 3.86 mm.

INTERNAL MORPHOLOGY (Fig. 129). Well-developed com-

bined inhalent and exhalent siphons are present, the inhalent

siphon is somewhat the shorter than exhalent. The siphonal
tentacle is usually on the left side. The anterior sense organ is
well-developed. The adductor muscles are relatively small.

The anterior adductor is ‘cresent’-shaped and approximately
twice the size of the oval posterior adductor.

The gills have up to 14 alternating filaments. The labial
palps are small, extending over approximately 1/4 distance of
body and have up to 9 inner palp ridges. The palp probos-
cides are long and thin. The visceral ganglia are relatively
slender, the cerebral ganglia are larger and more oval in

shape and the pedal ganglia are large and round. The foot is
“moderate in size with a large byssal gland. There is a
| relatively large stomach with the style sac ventral and slightly
posterior to it. The hind gut penetrates deep into the foot
‘ventral and anterior to the pedal ganglia before turning
dorsally to umbonal region where it passes to the right side of
| the body and forms a single loop. The hind gut has a
typhlosole along its length.
This species is similar in shell shape and internal morphol-
ogy to Portlandia minuta but differs from the latter in that P.
fora has a less angulate shell margin, slightly larger internal

‘ig. 129 Portlandia fora. Lateral view from the right side of the
| internal morphology of a specimen from Sta. S 56, West
| | European Basin. (Scale = 1.0 mm). For identification of parts see
Fig. 34.

79

ligament, a more obvious external ligament, is less inflated
and has a greater number of hinge teeth, the hinge plate is
narrower, and the post-umbonal length shorter. Anatomi-
cally there are relatively few differences, the gill plates and
palp ridges are marginally fewer in specimens of a similar
size.

Portlandia minuta (new species)

TYPE LOCATION. R.V. Atlantis, Il Cruise 42, Sta. 203,
Angola Basin, 23.5.1968, 8°48’S, 12°52'E, Epibenthic Trawl,
527-542 m.

TYPE SPECIMEN. Holotype BM (NH) 1992040. Paratypes: in
collection held by J.A. Allen.

MATERIAL.

Cruise Sta. Depth No Lat Long Gear Date
(m)

CAPE BASIN

Atlantis II 188 619-622 33 23°00.0'S 12°58.0'E ES 16.5.68

ANGOLA BASIN

Atlantis 42 203 527-5S44352 8°48.0'S 12°52.0'E ES 23.5.68

Occurs off S.W. Africa at upper slope depths in Angola &
Cape Basins. Depth range: 527-622 m.

SHELL DESCRIPTION (Figs. 130 & 131). Shell small, inflated,
subovate, posteriorly narrow, robust, fine but somewhat
irregular concentric lines; slightly iridescent, pale yellow
periostracum; umbos inflated, anterior to midline, internally
directed; lunule and escutcheon barely visible; slightly ros-
trate, slight indication of rostral ridge in some specimens;
antero-dorsal margin convex, slopes rapidly and evenly to
anterior margin, postero-dorsal margin slightly convex,
slopes gradually to posterior margin, slight angle at limit of
posterior hinge plate, ventral margin smoothly curved, cen-
trally deep, posterior margin drawn out but moderately
rounded, in mid horizontal plane; hinge plate strong, fairly
narrow on either side of ligament, broadens out distally,
distal teeth prominent, 3-4 small proximal teeth, in total 9 in
anterior series and 11 in posterior series of largest specimen;
ligament amphidetic, rectangular in shape, small external
secondary extensions of fused periostracum on either side of
umbo.

Prodissoconch length: 166 1m. Maximum recorded shell
length: 2.28 mm.

INTERNAL MORPHOLOGY (Fig. 132). There is a combined
siphon with a single lumen, thus there is no separation
between inhalent and exhalent lumena, however, the gill axes
join mid-laterally. Gland cells are present at the junction of
axis and siphon. There is a well-developed elongate anterior
sense organ. The adductor muscles are unequal in size. The
smaller posterior muscle is oval in shape, the anterior is
almost twice the size and ‘cresent’-shaped. The gills are
well-developed with up to 14 relatively large plates, the most
posterior of the inner plates are clearly interlocked even in
the preserved specimens. The labial palps are relatively small
with up to 8 palp ridges, the most posterior of these being
much broader than the rest.

The foot is large but the pedal musculature is not as
well-developed as in some Yoldiella species. There is a large

80

60 °

40

80 ee
H\L ° e &, “”~ Pe

70

Length (mm)

Fig. 131 Portlandia fora. Variation in ratios of height H/L, width
W/L and postero-umbonal length PT/TL to length against length
of a subsample taken from Sta. 203, Cape Basin.

byssal gland in the heel with a relatively large blood space
surrounding it. The nervous system is well-developed with
large ‘club’-shaped visceral and cerebral ganglia and large,
round, pedal ganglia with associated large statocysts dorsal to
them. The stomach and style sac are large. From the style sac
the hind gut penetrates the foot for a short distance before
turning dorsally to parallel the posterior edge of the body.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 130 Portlandia
minuta. Lateral views of
shells from the left and
right sides, detail of the
hinge-plate of a left valve
and the dorsal view of a
shell. Specimens from Sta.
203, Cape Basin. (Scale =
1.0 mm).

Thereafter it makes a single loop to the right side of the body.
A small amount of food material was observed in parts of the
left digestive diverticulum. The kidneys are small. Gonads
are present in specimens larger than 1.6 mm. The testes occur
dorsally, ventrally and internally to the digestive diverticula
with posterior dorsal and lateral extensions.

In the course of growth, the valves become more inflated
and more rostrate but there is little change in the height/
length or posterior umbonal length/ total length ratios (Fig.
131). Hydroids were present on one individual and these
covered the dorsal and posterior shell margins.

Fig. 132 Portlandia fora. Lateral view from the right side of the
internal morphology of a specimen from Sta. 203, Cape Basin.
(Scale = 1.0 mm). For identification of parts see Fig. 34.

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

|

| Fig. 133. Portlandia abyssorum. Lateral views of three shells from
the right side to show variation in shape. Dorsal view of a shell
and detail of the hinge-plate of a left valve. Specimens taken from
a, Sta. 24, Galathea Expedition, Sierra Leone Basin (Type
specimen); b, c & d, Sta. 8528', Cape Verde Basin; e, Sta. 8521°,

| Cape Verde Basin. (Scales = 1.0 mm).

|

Portlandia abyssorum (Knudsen 1970)

| TYPE LOCALITY. R.V. Galathea, Sta. 24, E., Atlantic off W.
Africa, 1950, 3°54'N, 8°22'W, Trawl, 3196 m.

\

|| TYPE SPECIMEN. Holotype: Zoological Museum, University
of Copenhagen.

Yoldiella abyssorum Knudsen 1970, 47, Fig. 29, pl. 1, Fig. 17.

81

MATERIAL.
Cruise Sta DepthNo Lat Long Gear Date
(m)
CAPE VERDE BASIN
Discovery 8521° 3070-6+2v 20°47.9'N 18°53.4";W WS 26. 6.74
3064
8528! 3155— 72+2v 17°38.7'N 18°35.8'W WS 2. 7.74
3150 — 17°38.3'N 18°34.9'W
AtlantisII 148 31114 1 10°37.0’N 18°14.0'W ES 7. 2.67
31 3828
149 3861 3 10°30.0'N 18°18.0'W ES I PAO
ANGOLA BASIN
AtlantisII 195 3797 45 14°40.0'S 9°54.0'E ES 19: 5.68
42 196 4612- 1 10°29.0'S 9°54.0'E ES 21. 5.68
4630 — 10°29.0'S 9°04.0'E
CAPE BASIN
Jean Charcot DS05 4560 1 33°20.5'S 2°34.9'E DS 30.12.78
(Walvis) CP13 3550 «1 S221 Sl SeeS215-O/E) MCP s12401.79

Occurs at abyssal depths off the west coast of Africa, Cape
Verde, Angola & Cape Basins. Depth range: 3064-4630 mm.

SHELL DESCRIPTION (Figs. 133 & 134). An accurate descrip-
tion is given by Knudsen (1970). Populations of this species
vary somewhat in the shape of the shell outline from that of
the type specimen to specimens with a more straight or
slightly concave postero-dorsal margin and a more convex
postero-ventral margin with intermediates between these two
extremes.

INTERNAL MORPHOLOGY (Fig. 135). In contrast to the
description of Knudsen (1970) there is an inhalent as well as
an exhalent siphon. The short ventral inhalent siphon is not
particularly obvious being much shorter than the exhalent,
however, sections show a twin siphon, the inhalent being
open ventrally. A siphonal tentacle originates on the left of
the siphon and there is a fairly well-developed feeding
aperture below. The adductor muscles are large, the anterior
being approximately twice the size of the posterior. The gills
are well-developed with up to 24 gill filaments. The labial
palps are large with a large number of internal ridges (up to
30), the number depending on the size of the individual. The
foot is large and extends anterior and ventral to the anterior
adductor muscle. It also has a large byssal gland. The cerebral
and visceral ganglia are elongate with a moderately thick
commissure. The pedal ganglia are large, elongate with large
associated statocyts. The hind gut forms a single loop on the
right side of the body and has a typhlosole along its length.

Prodissoconch length: 187-198 wm. Maximum recorded
shell length: 4.76 mm.

This species at first sight might be confused with Yoldiella
biscayensis, however there are a number of differences.
These include: — the presence of a lunule and escutcheon; the
umbo is raised only slightly above the dorsal margin; the
posterior and postero-dorsal margin is faintly rostrate; the
prodissoconch is much narrower and is shorter in length; the
palps have many more ridges.

82

—~
W\L Asse 4 © cf rue to 0
a e

a

a a ° e
a a a a ee Aon 0%
a e
- a oa ae “ e ee ‘2 ee &
a a ° e e° ee Gea e
a ae ° °
e
a e
PL\TL .
a e e e
e e ee e
es a eo e
a aa es eee
a 4
a a a Aa e Se aod 3° OD 5
a aa e e
rm e e

40

30
80-H\L
2
60

50

a eS aan ae el a Se ee
2 4

Length (mm)

Fig. 134 Portlandia abyssorum. Variation in ratios of height H/L
width W/L and postero-umbonal length PL/TL to length against
length of specimens from Sta. 195, Angola Basin (closed
triangles) and Sta. 8528', Cape Verde Basin (closed circles).

Fig. 135 Portlandia abyssorum. Lateral view from the right side of
the internal morphology of a specimen from Sta. 8528', Cape
Verde Basin. (Scale = 1.0 mm). For identification of the parts see
Fig. 34.

DISCUSSION

Of all the protobranch bivalves of the Atlantic, the yoldiellids
are by far the most difficult nuculanid subgroup in which to
discern evolutionary pathways. Despite the large number of
species we believe that these are closely related within a
subfamily. With possibly one exception, of 28 species of
Yolidella described here, there is a fine gradation in morpho-
logical features that both combines them within a single genus
and distinguishes them as a separate group. It must be

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

emphasized that as in all deep-sea protobranch species, the
shell proportion changes with increasing size. In general the
post-umbonal length increases at a rate greater than the other
dimensions. In some cases this also applies to the height of
the shell. As a result the small shells of a species may be
mistaken as being of a different species. This also creates
difficulties in making comparisons between species.

To the experienced eye, species and subspecies can be
separated and defined. Of particular diagnostic importance is
the form and course taken by the hind gut. Although there
may be some variation within a species, the course and
diameter of the gut alone is usually diagnostic. Nevertheless,
it is often difficult to define and only becomes clear after close
comparison of a range of species. We can distinguish eight
broad configurations of the hindgut of which the simplest, a
single loop to the right of the viscera occurs in 17 of 32 species
and subspecies of Yoldiella (Table 3) (Allen, 1992). The most
complex configuration occurs in only one species (see below). |

The species of Portlandia form a compact group in which
the shells are relatively robust, elongate, with the posterior
margin approximately central to the horizontal midline. The
hind gut has a single loop to the right and has a large
diameter. The adductor muscles are large and oval and more
or less equal in size. The hinge plates are long and stout,
continuous with the amphidetic internal ligament. The lunule
and escutcheon are usually well-defined. Three of the four
species are from upper to mid-slope depths and one P.
abyssorum, is from abyssal depths. The latter, apart from
having larger palps, smaller adductor muscles and more
dorsal anterior and posterior limits to the shell outline than
the other species described, its morphology is basically the
same.

One rare species of Yoldiella, Y. veletta, has many of the
characteristics of Portlandia described above. However, it isa
fragile shell without lunule or escutcheon. Furthermore, the
hinge plates form in a narrow bridge below the umbo with the.
amphidetic ligament ventral to it. With only three specimens
at hand we defer categorical judgement, but hypothesize that
the primitive form of Yoldiella must have had similar charac-
teristics.

On the premise that the simplest form of hind gut is likely
to reflect the primitive condition we derive an evolutionary
pattern that originates in species with this character but
among others, for it is unwise to base evolutionary conclu-
sions on one character alone. In passing, it should be said that
it is a sad fact that for many malacologists, it is the shells
rather than the viscera that are all-important.

It seems reasonable to assume that deep-water species if
the Atlantic originated either from shallow water, possibl}
tethyan and arctic seas, or by migration at depth from the
Southern Ocean. For reasons that we will describe elsewhere
(Allen and Sanders in m/s), we think it unlikely that the
major colonization of the deep Atlantic was from high }j,
southern latitudes. Yet there are only a limited number of
yoldiellids present in shelf sediments, but all of these species |,
have a relatively short hindgut with a simple single loop to the att
right side of the viscera. This character is also found in other }j;,
shallow-water nuculanids and in shallow-water neilonellids},
and tindariids, however, species of Yoldiella differ from these mn
latter three groups in not having heavy, concentrically orna Jy,
mented shells. When concentric ornamentation is present i114 }}\:,
yoldiellid it is always of a fine, delicate nature, and more })j,
often than not confined to the periphery of the shell.

We identify two species from shallow water in the North

(Ope

DEEP-SEA PROTOBRANCHIA (BIVALVIA)

and West Atlantic, Y. frigida and Y. robusta that have
characters which we believe a ‘stem’ group might possess.
These include, in addition to the single hind gut loop, a
symmetrical ovate shell outine, relatively short fragile sepa-
rate hinge plates and a moderately large central amphidetic
ligament (Fig. 148). In addition, they have large siphons with
combined lumena which are ventrally unfused, moderately
large, oval, subequal, adductor muscles, a few large fringing
papillae to the foot, a relatively elongate gill with a moderate
number of plates and broad palp ridges which are relatively
few in number.

From this basic form, we derive a number of evolutionary
pathways. The most simple derivation appears to be that
shells become somewhat higher in proportion to length and
the antero-dorsal and postero-dorsal margins become more
horizontal such that the anterior and posterior limits of the
shell are dorsal to the horizontal midline (Figs. 136-147).
(For details of the comparative overlay technique, see Fig.
136). In other respects, a characters are similar to those given
above. These latter species include Y. inconspicua, Y. extensa

_and Y. argentinensis, again from the North and West Atlantic
but further downslope than Y. frigida and Y. robusta. In
addition, Y. curta, a common and widespread species from
the base of the continental slope would also appear to belong
here, however, unlike the species mentioned so far, the
hindgut of Y. curta passes anterior to the mouth to form a
single loop on the left side of the body. This disposition of the
hind gut is a simple derivation from the primitive condition
}and can be explained simply in terms of elongation and
accommodation of the hindgut. In all other respects Y. curta
is similar to the species of the Y. frigida group.
_ In juxtaposition to this possible stem group is a group of
/ species centred upon Y. /ucida (shelf/upper slope), Y. obesa
)(mid/lower slope) and Y. similiris (upper/mid slope).
' Although having similar ovate shape to Y. frigida and Y.
| robusta, these species have much longer but still separated
| hinge plates, smaller adductor muscles of which the posterior
)is significantly smaller than the elongate anterior muscle. The
|hind gut loops are somewhat longer and more broadly looped
and the lumena have a wider diameter. Most other characters
are in common with the ‘stem group’. These include an
‘internal amphedetic ligament ventral to the hinge plate which
jis somewhat smaller than that of the stem group, a relatively
jelongate gill, small palps with broad ridges which are few in
number and papillae of the foot which are relatively large.
The differences between Y. lucida, Y. obesa and Y. similiris
sand the species of the ‘stem group’ relate to the strength of
ithe hinge plate. As the length and robustness of the hinge
jincreases there is less requirement for large adductors and a
large ligament to ensure the integrity of the two valves. In
laddition, as the maximum depth limit of the species
increases, the hind gut tends to enlarge either in length or
diameter or both.
| As in the case of the ‘stem group’, we believe evolution
rom the ‘/ucida’ group also involves an increase in the height
bf the shell and the antero- and postero-dorsal shell margins
becoming more horizontal such that the posterior limit of the
shell margin becomes characteristically sharply rounded. The
Dosterior adductor muscle is reduced in size, the hind gut
penetrates deep into the foot and the palps are small with
very few ridges. The siphonal lumena are separate. Species
with these characterstics include Y. bilanta, Y. hanna and Y.
fapensis, all of which are restricted to slope depths.
Y. artipica is intermediate in its characters to the two above

83

groups, however the hind gut is more sinuous in its course to
the right. This represents the initial stage in a trend that leads
to the coiling of the hind gut on the right side of the body.
Note the Y. artipica has a deeper distribution than those
species described above and is found on the abyssal rise.
Similarly, Y. sinuosa, Y. blanda and Y. biscayensis which
have similar characteristics to the ‘bilanta’ group, all have
sinuous hind guts. There are other internal differences shown
by these three species. Thus, while the adductor muscles are
dissimilar in size, the posterior muscle is not greatly reduced
in size, the palp ridges, although wide, are more numerous
and thus the palp is large and the siphon is particularly large
with combined lumena. As in the case of Y. artipica these are
species from the abyssal rise.

The logical sequence to the trend is seen in Y. /ata in which
the hind gut has a larger diameter, has lengthened such that
the ‘reverse-S’ course has progressed to a double-loop. Also
to be noted are that the adductors are moderately large and
are equal in size, and the hinge plates, although elongate, are
narrow and much less robust. Similarly the gill is relatively
small and the palp has numerous narrow ridges.

As a continuation of this trend, a specialized group of
species comprising Y. ella, Y. enata and Y. jeffreysi is arrived
at. A sequence of events can be envisaged. Starting from the
condition in Y. /ucida the hind gut lengthens, remains to the
right of the body and becomes increasingly coiled. (Up to
four times). The shell becomes characteristically rounded,
high with large umbos. The hinge plates become very strong
and the ligament small and rounded. The adductor muscles
are small, and may be subequal in size. The generating
outline curve of the shell is rotated somewhat to the right so
that the umbo is distinctly anterior and the maximum ventral
limit is posterior to the vertical midline. We can envisage a
sequence of increasing hind gut complexity from Y. lucida
through Y. blanda, Y. lata, Y. jeffreysi to Y. enata, however,
we have some reservations as to whether Y. ella is the
terminal species of this line. While Y. e/la has the most coiled
hind gut of all the yoldiellids in our collections, the more
evenly rounded shape, the extremely small subequal adduc-
tor muscles and the massive hinge plates might indicate a
separate derivation or at least a marked terminal divergence
in the series. These species have a depth distribution from the
abyssal rise to the greatest abyssal depth. There is little doubt
in our opinion that the differences in morphology exhibited in
both the latter group are related to the requirements of life at
abyssal depths.

We also derive an even more complex but distinct group
comprising Y. americana, Y. subcircularis, Y. biguttata and Y.
ovata from the Y. lucida stock. These four species also have
characteristically rounded, high shells which have large
umbos. The evolutionary sequence probably included an
intermediate stage with a form similar to that of Y. perplexa
in which shell characters are similar but in which there is a
relatively short single hind gut loop to the right, robust
subequal adductor muscles, small palps and gills, large pedal
papillae and long hinge plates. Y. americana differs relatively
little from this, except that the hind gut is considerably
lengthened and which takes a course to the left and right of
the stomach and the palp is a deep semicircular shape with a
large number of ridges (see p. 54). A similar morphology to
that of Y. americana is found in Y. subcircularis except that
the hindgut loops are doubled on either side of the body.
Similarly, Y. biguttata and Y. ovata also have two hind gut
loops to the right and the left and in Y. insculpta, the hind gut

84 J.A. ALLEN, H.L. SANDERS AND F. HANNAH

c

Fig. 136 Yoldiella frigida (f), Y. robusta (r), & (separately) Y.
curta(c).
Fig. 137 Yoldiella argentinensis (a), Y. extensa (e) & (separately)
Yoldiella inconspicua inconspicua (i), Y. i. profundorum (p) & Y.
i. africana (a).

Fig. 138 Yoldiella lucida (1), Y. similirus (s) & Y. obesa incala (0). 7

Fig. 139 Yoldiella bilanta (b), Y. hanna (h) & Y. capensis (c).

Figs 136-147 Outline comparisons of species by overlay technique. Each outline is centred with reference to the junction of the centre of th
mid-length axis with the centre of the mid-height axis. Outlines are drawn to a similar length. Each shell outline comprises the margin in
right lateral view, hindgut loop and anterior and posterior adductor muscles. Anterior, posterior ventral and dorsal limits are indicated
parallel to the relevant shell axis as too is the apex of the umbo. Each limit is identified by the first letters of the species name. The figures
are in the order of the discussion on the evolution of form. (See text pp. 82-86).

DEEP-SEA PROTOBRANCHIA (BIVALVIA) 85

ig. 140 Yoldiella artipica (a), Y. similis (s) & (separately) Y.
lata(p).

Fig. 143 Yoldiella ella (el).

*p

Fig. 144 Yoldiella americana (a), Y. subcircularis (s) & Y. perplexa
(p).

86 J.A. ALLEN, H.L. SANDERS AND F. HANNAH

Fig. 147 Portlandia abyssorum (a), P. minuta (m), P. fora (f),
(separately) Yoldiella velleta (v) & Portlandia lenticula (1).

Fig. 145 Yoldiella ovata (0), Y. biguttata (b) & (separately) Y.
insculpta (i).

f
€ ) | | i:
f

Fig. 146 Yoldiella fabula (f).

DEEP-SEA PROTOBRANCHIA (BIVALVIA) 87

robust shells with lunule & escutcheon
long broad continuous hinge
large adductors

P.minuta

P fora

P. lenticula

P abyssorum

fragile shells without lunule & escutcheon
7? separate hinge plates

one moderately large subequal adductors
Y.veletta

Y. frigida

Y.robusta

ee
Y.inconspicua
Y.extensa
Y.argentinea

hind gut thru’ :
to left

Weurtae se ee —¥. friaida | °|——=— {aah be. > ¥.fabula

short hinges

subrostrate = ¥.lucida

long hinges i
small unequal adductors Sa: Y.obes ak
/ £Y.similiris

‘
if
/?
Yoox
‘

yh
‘s’ shaped hind guts Y.artipica :

£°9
Y.bilanta —
Y.hanna
Y.capensis

x
x.
x,
‘
Ny

single
hind gut loop

x,
\,
y

Y. sinuosa
Y.blanda

Y. biscayensis Y.perplexa |

<--------------------- = +--+ ~~ ++ == == ------- = -----

Y.pseudolata Y.similis Y.americana
ovate Y. subcircularis
Y.enata coils loops Y. biguttata aie
Y.jeffreysi . Y. ovata hind gut loops
& coils
Y.ella Y.insculpta
increasing
shell height

&hind gut length

‘ig. 148 Diagram to show resemblances in shell form of the deep-sea species of the genus Yoldiella found in the Atlantic, arranged in
increasing length and complexity of the hindgut and each group of species arranged in increasing depth sequence.

J.A. ALLEN, H.L. SANDERS AND F. HANNAH

88

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DEEP-SEA PROTOBRANCHIA (BIVALVIA)

is further extended to three loops to the right and two to the

left with one loop on each side short and anterior to the

stomach. The latter is a configuration unique among proto-
_ branchs. In these relatively large rounded species the liga-
- ment tends to be relatively large and the hinge plates robust
and elongate. Most of these species extend from the abyssal
rise across the abyssal plain.

Comparison of this latter group with Y. similis is of
interest. While Y. similis species has a double hind gut loop
its anatomy is similar to Y. artipica. In particular it has small
palps with a few broad ridges.

One species of Yoldiella remains to be mentioned. Yoldi-
ella fabula differs markedly in its shell characters from all
other species of Yoldiella (and Portlandia) yet in its internal
morphology it is close to the basic stem group of the genus.
Thus, while it is elongate with the umbo set far posterior to
the mid-vertical line, it has relatively short separate hinge
plates with a small amphidetic ligament. Very large subequal
adductors are present and the hind gut is a simple, single loop
to the right. Although its shell outline is unique, these
| characters fit best with the yoldiellids and we are presently
_ persuaded to keep it within this subfamily.

In the foregoing discussion, stress has been placed on

changes in the shape of the shell and the disposition of the
gut. It is clear that for the most part species that occur at great
depths have longer guts than those in shallow water and that
this increased length has been accommodated within a small
body space. This in turn relates to the digestive requirement
to deal with sparse complex organics in deep-sea sediments.
Indeed, in general the body space of deep-sea bivalves as a
| percentage of shell volume is significantly smaller than that of
| shallow-water congeners. Similarly, in regard to shell shape,
species in shallower depths appear to be of an elongate-ovate
shape, whereas those from the deep have higher shells which
are either more rounded or have the greatest length measure-
ment dorsal to the mid-horizontal plane. This we believe is
_ related to the softness of the abyssal sediments and the ease
| of movement within them.
There are other evolutionary trends that may or may not be
| depth related. For instance, the size of the adductors clearly
relates inversely to the strength and length of the hinge plates
| and the size of the ligament. In contrast, the size of the palps
| appears to increase with increasing depth range, while gills
| tend to reduce in size. We believe this is for a different reason
' from that of the change in shape of gut and shell, and relates
| in part to a difference of energy demand at high pressures and
| in part to the lack of importance of the protobranch gill in the
| feeding process. Because of this latter there is a reduction in
|the size of the gill, however, the loss in ciliated tissue is
compensated by an increase in palp area which is required in
/ order to maintain ciliary flow within the mantle cavity. At the
| Same time it provides ciliary activity where is is most needed
| in the processing of fine abyssal sediments.

As might be expected the siphons also show modifications.
Reduction in the gill area results in lower inhalent siphonal
|flow rates. In contrast, larger palp surfaces result in higher
jinflow via the feeding aperture. The processing of large
| quantities of fine sediment must produce increased numbers
\of faecal pellets. The predicted result from these changes is
\realized in the increased importance and size of the feeding
japerture, the reduction and in some cases elimination of the
jinhalent siphon or the loss of division between exhalent and
inhalent siphonal lumena such that the combined siphon is
largely used for the passage of faecal material to the outside.

89

Finally, we speculate that not only that the small size of the
body in comparison with the shell volume in the deep-water
species is related to reduced food resources (as in reduced
numbers of ova) but it is also related to the reduction of
overall metabolic energy requirements at high pressures.

Like all protobranchs, the yoldiellids have large eggs and
larval development is almost certainly short-lasting, non-
feeding and takes place close to the sea floor. Although there
are subtle basinal differences in shell form, some of which
may be sufficiently distinct to establish subspecies (e.g. Y.
obesa and Y. inconspicua) there is little doubt that either
widespread gene flow occurs and/or that genetic change is
slow. Clearly, the yoldiellid form is one that is extremely
successful. The subfamily contains by far the most species of
all the protobranch families and subfamilies.

Much of the scientific discussion on the distribution of the
Yoldiellinae will be incorporated into a following and final
round-up paper on the diversity and zoogeography of the
deep-sea protobranchs of the Atlantic. Nevertheless, because
it is such a large group, the distribution of the 29 species of
the genus Yoldiella described here reflect many of the general
features of protobranch distribution. Thus, of the 29 species
only a limited number can be regarded as being widespread
(Table 3). Only six species are present in five or more of the
Atlantic abyssal basins and of these, five are abyssal and one
is lower slope/abyssal rise in its depth distribution. There are
16 endemic species, of these five species and one subspecies
are restricted to the Argentine Basin. A further four endemic
species are found in the Cape or Angola Basins. Thus, it is
clear that most endemic species are in the South Atlantic.
Furthermore, the South Atlantic has been much less sampled
than the North and one would suspect that more rare
endemic species will be reported in the future. This is clearly
of considerable importance in speculations on the origin of
the protobranch fauna of the Atlantic. Although endemic
species are not restricted to upper slope depths, most of the
species at upper slope depths are endemic and those few that
are not, are restricted to the North European and North
American Basins. Thus, in general, the deeper the species
occurs, the more widespread is likely to be its distribution.
This may simply reflect the fact that the abyssal plains contain
enormous areas of sediment of similar characteristics and that
distribution simply reflects the commonality of the environ-
ment.

REFERENCES

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Allen, J.A. & Hannah, F.J. 1986. A reclassification of the Recent genera of the
sublass Protobranchia (Mollusca: Bivalvia). Journal of Conchology, 32:
225-249,

Allen, J.A. & Hannah F.J. 1989. Studies on the deep-sea Protobranchia. The
subfamily Ledellinae (Nuculanidae). Bulletin of the British Museum (Natural
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Allen, J.A. & Sanders, H.L. 1973. Studies on the deep-sea Protobranchia, the
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Allen, J.A. & Sanders, H.L. 1982. H.L. Studies on the deep-sea Protobranchia.
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Jeffreys, J.G. 1879. On the Mollusca procured during the ‘Lightening’ and
‘Porcupine’ Expeditions 1868-1870. Part II. Proceedings of the Zoological
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Knudsen, J. 1970. The symstematics and biology of the abyssal and hadal
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Locard, A. 1898. Mollusques Testaces. Expeditions Scientifiques du Travailleur
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Rhind, P.M. & Allen, J.A. 1992 Studies on the deep-sea Protobranchia
(Bivalvia): the family Nuculidae. Bulletin of the British Museum (Natural
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Sars, G.O. 1878. Bidrag til kundskaben om Norges artiske Fauna 1, Mollusca
regionis articae Norvegiae. 466 p. Christiana.

Sanders, H.L. & Allen, J.A. 1973. Studies on the deep-sea Protobranchia
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Comparative Zoology Harvard, 145: 237-261.

Sanders, H.L. & Allen, J.A. 1977. Studies on the deep-sea Protobranchia
(Bivalvia): the family Tindariidae and the genus Pseudotindaria. Bulletin of
the Museum of Comparative Zoology Harvard, 148: 23-59.

Sanders, H.L. & Allen, J.A. 1985. Studies on the deep-sea Protobranchia
(Bivalvia): the family Malletiidae. Bulletin of the British Museum (Natural
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Shileiko, A.A. 1985. [The genus Yoldiella auct. as a combined group (Bivalvia,
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Stasek, C.R. 1965. Feeding and particle-sorting in Yoldia ensifera (Bivalvia:
Protobranchia), with notes on other nuculanids. Malacologia, 2: 349-366.
Verril, A.E. & Bush, K.J. 1898. Revision of the deep-water Mollusca of the
Atlantic coast of North America with descriptions of new genera and species.

Part 1. Bivalvia. Proceedings of the U.S. National Museum, 20: 775-901.

Warén, A. 1978. The taxonomy of some North Atlantic species referred to
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Warén, A. 1980. Marine Mollusca described by John Gwyn Jeffreys, with the
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Warén, A. 1989. Taxonomic comments on some protobranch bivalves from the
northeastern Atlantic. Sarsia: 223-259.

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CONTENTS

1 Arevised familial classification for certain cirrhitoid genera (Teleostei, Percoidei
Cirrhitoidea), with comments on the group’s monophyly and taxonomic ranking
P.H. Greenwood
11 Studies on the deep-sea Protobranchia (Bivalvia); the Subfamily Yoldiellinae
J.A. Allen, H.L. Sanders and F. Hannah

Bulletin of The Natural History Museum

a

ZOOLOGY SERIES
Vol. 61, No. 1, June 1995