Occasional papers of the California Academy of Sciences

\

OCCASIONAL PAPERS

OF THE

California Academy of Sciences

No. 56, 15 pages, 21 figures.

June 30, 1966

A NEW DEEP-WATER CHITON

FROM THE

NORTHEASTERN PACIFIC

By

Allyn G. Smith

California Academy of Sciences

and

Ian McT. Cowan

University of British Columbia

JUL 1 11966

SAN FRANCISCO

PUBLISHED BY THE ACADEMY
1966

OCCASIONAL PAPERS

OF THE

CALIFORNIA ACADEMY OF SCIENCES

No. 56, 15 pages, 21 figures. June 30, 1966

A NEW DEEP-WATER CHITON

FROM THE

NORTHEASTERN PACIFIC

By

Allyn G. Smith

California Academy of Sciences

and
Ian McT. Cowan

University of British Columbia

Abstract, lschnochiton abyssicola, a new species of deep-water chiton
from the northeastern Pacific, is described and illustrated in detail. Compari-
son is made between this and lschnochiton stearnsi Dall, 1902, figured for
the first time from the holotype specimen in the United States National Mu-
seum, and lschnochiton (Tripoplax) trifidus (Carpenter, 1864), which is also
figured.

A series of exploratory dredgings undertaken in 1964 in the northeast
Pacific by the Fisheries Research Board of Canada research vessel B. G. Reed
has produced several specimens of a large undescribed chiton. The first was
collected by I. McT. Cowan from among fishes brought up in an otter trawl op-
erating off the Semidi Islands, Alaska, in 140-118 fathoms. Others were col-
lected subsequently from Queen Charlotte Sound, British Columbia, and off
Cape Flattery, Washington, in the 500-fathom range. Another was dredged off
the coast of Oregon in over 1000 fathoms by the Oregon State University De-

CALIFORNIA ACADEMY OF SCIENCES

(Occ. Papers

Figures 1-3. Ischnochiton abyssicola Smith and Cowan, new species. Holo-
type (CAS Invertebrate Zoology Type Series no. 287) from 470-480 fathoms, Queen
Charlotte Sound, British Columbia, Canada. (1) dorsal; (2) side; and (3) ventral
views. Length, 46 mm.

No. 56)

SMITH & COWAN: NEW DEEP-WATER CHITON

partment of Oceanography. A further specimen was obtained by the United
States Bureau of Commercial Fisheries in 291-300 fathoms off the mouth of
the Columbia River. It would appear, therefore, that this chiton has a rather
wide distribution along the northwest coast of North America from the Aleu-
tian Islands as far south as Oregon, and that it may turn out to be a reason-
ably common species in depths ranging from over 100 to more than 1000 fath-
oms with its bathymetric mid-point in the vicinity of 500 fathoms.

This new species belongs in the family Ischnochitonidae, genus Isch-
nochiton Gray, 1847, the genotype being the South African /. texilis (Gray,
1828). Ischnochiton, sensu lato, is widespread, containing many described
species. Numerous attempts have been made to divide the group into smaller
units of more or less closely related forms and thus far no less than sixteen
valid names have been used to designate sections, subgenera, or genera, the
systematic positions chosen depending on the current view of taxonomists who
have endeavored to solve the complex problem (Smith, 1960, pp. 55-56)- Of all
the names available for use as possible subgenera of Ischnochiton. none ap-
pears to encompass both the general characters and the bathymetric range of
specimens at hand. In spite of this we hesitate to add a new subgeneric tax-
on to cover this new species until further critical work has been done on spe-
cies assigned to the genus Ischnochiton in order to define species and group
relationships more clearly and place them in a proper systematic order.

Figures 4-5. Ischnochiton abyssicola Smith and Cowan, new species. Para-
type (University of British Columbia Museum of Zoology Collection) from 149-118
fathoms south of the Semidi Islands, Alaska. (4) dorsal, and (5) ventral views.
Length (assembled shells only), 89.8 mm.

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Figures 6-7- Same specimen as figures 4 and 5- Enlarged dorsal views of
head valve (figure 6) and tail valve (figure 7). Width of head valve, 25-5 mm.; of
tail valve, 22-9 mm.

Ischnochiton abyssicola Smith and Cowan, new species.
(Figures 1 - 19.)

General description. Adult animals large for the genus, ranging up to
nearly four inches in total length, elongate with a length somewhat more than

No. 56)

SMITH & COWAN: NEW DEEP-WATER CHITON

Figure 8. Same specimen as figures 4-5- Enlarged dorsal view, left side of
intermediate valve iv. Maximum distance, jugum to anterior margin, 20.6 mm.

twice the width, and with straight sides. Dorsal sculpture on the head valve,
lateral areas of intermediate valves, and posterior area of tail valve consist-
ing of numerous, closely spaced, diagonal, beaded riblets or of more widely
spaced, granulated ribs, the lateral areas well defined. Insertion plates on
the head valve cut by many deeply incised slits, of intermediate valves by
one to three slits, and of tail valve by twelve or more slits. Teeth between
the slits short and blunt, roughly laciniated at their summits and on the out-
side, especially on the head and tail valves, but not pectinated as in the Chi-
tonidae. Eaves narrow, solid. Girdle armature of erect, small, closely packed,
inwardly curving, blunt spinules, with rounded tops giving the girdle a finely
divided, pebbly surface. Gills ambient.

Holotype. A medium-sized adult, its length 2-3 times its width. Head
valve nearly semicircular, the side-slope straight, the apex slightly everted.
Intermediate valves with straight posterior margins and slightly convex side-
slopes, making an angle of about 100°with an acute jugum; lateral areas prom-
inently offset from the lateropleural areas, narrowly triangular, slightly round-
ed at the side margins. Tail valve with posterior margin broadly ovate, the
posterior area set off from the anterior by a rib of a wideV-shape that is most

CALIFORNIA ACADEMY OF SCIENCES

(Occ. Papers

No. 56) SMITH & COWAN: NEW DEEP-WATER CHITON 7

prominent at the margin and gradually becomes obsolete toward the low, cen-
trally placed mucro.

Dorsal sculpture of the head valve, lateral areas of the intermediate
valves, and the posterior area of the tail valve consisting of closely spaced,
diagonally arranged, somewhat anastomosing, finely beaded riblets, the beads
occurring generally in curving rows paralleling the lines of growth. Counted
at the valve margins there are about 85 riblets on the head valve, 11-13 on the
lateral areas of the intermediate valves, and 65 on the posterior area of the
tail valve. The lateropleural areas of intermediate valves and the anterior
area of the tail valve are sculptured with a more or less parallel, somewhat
sinuous network of very fine longitudinal riblets crossed by less prominent
lines of growth, forming a finely decussated pattern not raised into beads as
on the other shell areas. Thejugal areas are not prominently outlined and have
much the same sculptural pattern, the fine riblets sweeping down from the ju-
gum anteriorly in curving, V-shaped arrangement. No enlarged, scattered pus-
tules occur on the end valves or on the lateral areas of the intermediate val-
ves as in some species of the related genus Lepidozona.

The girdle, about 4 mm. wide, is relatively narrow for the size of the
animal, the dorsal side being thickly set with smooth, stout, inwardly curved,
scale-like spinules, somewhat resembling diminutive surfboards in general
form. They are erect, crowded but not imbricate, and are in close contact with
each other being randomly placed in their short-axis orientation. They vary
in size and shape somewhat, also in a random manner. A narrow band around
the outside margin of the girdle has the stout spinules replaced by a fringe of
slender white spines. At the junction of the two spinous areas there is a con-
tinuous row of fine setae, the longest 1 mm. in length. The ventral side of
the girdle has a surface composed of closely appressed, parallel fibers direc-
ted crossways. These are of random length and have their tips free from the
surface so that the entire undersurface of the girdle is minutely roughened.
They vary in color from white to brown. At the girdle margin they extend free,
forming the band of delicate fringing spines.

The foot terminates in a horseshoe-shaped oral plate, about as broad
as long. Gills are ambient, the anteriormost branchiae being immediately be-
neath or just in front of the posterior lappet of the oral plate. There are 41
branchiae in the left branchial groove and 42 on the right. At the level of the
posteriormost branchiae, the inside pedal margins of the girdle project inward
in the form of a pronounced lappet. The anal opening is well behind the foot.

Figures 9-11. Same specimen as figures 4-5- Enlarged views of ventral side.
(9) Head valve in normal position overlapping the sutural laminae of intermediate
valve ii showing slits and teeth in the insertion plate. (10) Looking down on the
insertion-plate margins of intermediate valves ii, iii, and iv showing multiple slits
and teeth. (11) Tail valve in normal position showing configuration of slits and
teeth.

CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

* 7AV jf 4 1'

■ * , ':.;'- . • *V If ' * -A1} f.v

Figure 12. Same specimen as figures 4-5- Portion of girdle, much enlarged,
showing girdle scales.

Dorsal color of the valves is a dark red-brown, especially on the lateral
areas of the intermediate valves, becoming darker toward the jugum. To the
unaided eye the girdle appears to be a uniform blackish brown, but under mag-
nification the rounded tips of the spinules are a random mixture of black,
brown, and whitish colors.

Measurements are: length, 46 mm.; width, 19.5mm.; height, 9 mm.

Holotype and paratypes. Holotype, a slightly curled perfect specimen,
preserved in alcohol, dredged in 470-480 fathoms (860-878 meters) WNW. of
Triangle Island, Queen Charlotte Sound, British Columbia, Canada (lat. 31°
09' W.) by M/V G. B. Reed, Fisheries Research Board Station no. 863, haul
no. 63-214, D. B. Quayle and I. McT. Cowan, collectors, September 11, 1964.
Deposited in the California Academy of Sciences Invertebrate Zoology type
series no. 287. The holotype is of medium size for the species and is some-
what contracted from preservation. Other specimens, here designated as para-
types, are as follows:

(1) Three specimens collected with the holotype, preserved in alcohol:
(a) A damaged adult measuring approximately 60 mm. in length; (b) A perfect
adult measuring approximately 48 mm. in length, 23 mm. in width, and 10 mm.
in height; (c) A fairly perfect juvenile measuring in length, 27-5 mm., width,
13.5mm., and height, 4-8 mm. Numbers of slits in this partially disarticulated
third specimen are 14+ in the incomplete head valve and 15 in the tail valve.

(2) A single very large adult specimen from 140-118 fathoms (256-216
meters) south of the Semidi Islands, Aleutian Chain, Alaska (lat. 55° 15'N.;
long. 188° W.), gravel bottom, I. McT. Cowan, collector; August, 1964, Uni-
versity of British Columbia Museum of Zoology, no. 5299 (Cowan collection).
Animal preserved in alcohol. The somewhat damaged valves have been repair-
ed and assembled separately. Measurements of valves only: length (assem-
bled), 89.8 mm.; maximum width, 293 mm.; height, 15-2 mm.; angle of diver-
gence, 102°.

No. 56)

SMITH & COWAN: NEW DEEP-WATER CHITON

(3) Two badly damaged adult or subadult specimens from 534 fathoms
(975 meters) off Swiftsure Bank, Washington (lat. 55° 15'N.; long. 156° 18'
W.), gravel bottom, Station 835, I. McT. Cowan and D. B. Quayle, collectors,
September 6, 1964, University of British Columbia Museum of Zoology, no.
5369 (Cowan collection). The shells of one specimen were complete enough
to assemble apart from the preserved animal. This measures: length, 52 mm.;
width, 21.5 mm.; height, 8-5 mm., the slit formula being 22-2/3-14. All val-
ves of the second specimen were too shattered to assemble, but it appears to
be about the same size as the other. There are 16 slits in the nearly complete
tail valve of this latter specimen. This lot also contains four juvenile speci-
mens ranging in length from 6.5 - 14.1 mm.

14

Figures 13-14- I schno chiton aby ssicola Smith and Cowan, new species. En-
larged views of juvenile specimens. (13)Paratype (CAS Invertebrate Type Series no.
288) from 2000 meters (1093 fathoms) off the coast of Oregon. Length, about 20 mm.
(14) Paratype from 534 fathoms off Swiftsure Bank, Washington. Length, 14.1 mm.

10 CALIFORNIA ACADEMY OF SCIENCES (Occ. Papers

(4) A juvenile with tail valve missing from 1093 fathoms (2000 meters)
off the coast of Oregon (lat. 44° 33-5' N.; long. 125° 14.6' W.), collected July
6, 1962 with an anchor dredge by R/V< 4 cona, cruise no. 6207, Oregon State Uni-
versity accession no. 06-M-06 (Department of Oceanography). The length of
this specimen is approximately 20 mm., the slit formula being 16-2/3-?. De-
posited in the California Academy of Sciences, Department of Invertebrate Zo-
ology, type series no. 288-

Another specimen was taken in an otter-board trawl in 291-300 fath-
oms SW. of the mouth of the Columbia River in lat. 45° 59' N.; long 124° 49'
W. by the R/V Commando of the U- S. Bureau of Commercial Fisheries at Sta-
tion A-42, collected September 13, 1961- It is a large adult specimen in the
Department of Geology, Stanford University.

Paratypes from the type locality have been deposited in the Division of
Mollusks, U.S. National Museum; and in the National Museum of Canada, Ot-
tawa.

Extended Description and Discussion. Because the holotype is a per-
fect specimen we decided to leave it intact and rely on paratype specimens
for a description of the ventral surfaces of the valves and other features. The
large paratype of Ischnochiton abyssicola from 140-118 fathoms off the Semidi
Islands was used for this purpose, reference being made also to the assem-
bled valves of the paratype from 534 fathoms off Swiftsure Bank, Washington,
and to the two juvenile specimens.

Dorsally the numbers of riblets on the Semidi Island paratype are great-
er than on the holotype, being 125 on the head valve, 11-13 on the lateral ar-
eas of the intermediate valves, and about 80 on the posterior area of the tail
valve. On the youngest, 27-5 mm. specimen collected with the holotype, the
corresponding riblet count is 36+, 6-7, and 35, respectively. On the Oregon
specimen, which is somewhat smaller, the number of riblets is 46 on the head
valve, and 9-11 on the lateral areas of the intermediate valves. Thus it seems
obvious that this sculptural feature varies with the size and probably with
the age of the animal.

Another dorsal feature, seen only on disarticulated valves ii through
viii, is a small rounded extension of the posterior margin of the tegmentum at
the sutural sinuses to form well developed false beaks.

Ventrally, the tegmentum of valves i through vii on the large Semidi
Island paratype forms a narrow apical area 1.8-2.3 mm. wide, at the apex,
becoming much reduced in width toward the valve margins. Articulamentum
of the head valve terminates in a narrow insertion plate cut into many blunt
teeth by deeply incised slits. The first five teeth on each side are rather
broad, about equally spaced, and are marked further by three to five shallow
grooves at their summits that extend over the outside surfaces. Toward the

No. 56) SMITH & COWAN: NEW DEEP-WATER CHITON 11

center of this valve the teeth are much broken up by many closely spaced,
deeply incised slits, resulting in a total of about 30 in all. On the intermed-
iate valves the insertion plates are faintly grooved on the outside and are
cut by two shallow slits on each side, with indications of a third subobsolete
one just behind the posterior valve margin. Insertion plate of tail valve cut
by about 12 irregularly placed slits into a series of blunt teeth, strongly groov-
ed on the outside, with a tendency toward the center of the valve to be broken
up by 6 or 7 shallow subsidiary slits. The sutural laminae are smooth, broad,
rounded, sharp-edged, and continue the gentle curvature of the valve margins
to the rather narrow, generally semicircular-shaped sutural sinus. The latter
is crossed by a narrow sutural plate set off from the ends of the sutural lam-
inae by shallow notches. On the tail valve the sutural laminae are a bit trun-
cated at the anterior ends and the sutural sinus is squared off by a much more
prominent sutural plate that is delicately grooved on the outside. The muscle
scars on the articulamentum of both head and tail valves form a rough, irreg-
ular pattern. On the intermediate valves they consist of a low, broad V-shaped
ridge, widest at the jugum and tapering toward the valve margins. All valves
of this large specimen have narrow solid eaves that do not project beyond the
edges of the insertion plates. No slit rays occur.

The articulamentum of younger specimens with thinner shells is por-
cellaneous and less chalky than in older ones; the slits are more regularly
placed, the teeth not being divided by subsidiary slits although the tendency
toward subsidiary slitting begins to show on the head valves. Well developed
slit rays are present in the valves of these younger specimens. Their color is
lighter brown dorsally, with the valve margins whitish.

Radula. The radulae of two animals, one from the large Semidi Island
specimen and another from a somewhat smaller specimen from off Triangle Is-
land, were extracted and studied by Mr. Spencer R. Thorpe, Jr., of El Cerrito,
California, who also has contributed the accompanying drawings (figures 15-
19), which speak for themselves. He is of the opinion that the radula of /. ab-
yssicola is clearly of the Lepidozona type, taking into consideration the con-
figuration of all the separate parts and radula size in relation to the size of
the animal. Within these general limits Thorpe says that the subgenus Tripo-
plax Berry, 1919, represented by the single species Ischnochiton trifidus (Car-
penter, 1864) from Alaskan waters has the same bidentate cusps on the major
laterals as /. abyssicola. However, he points out that the broad and massive
character of the spatulate uncinals, the markedly bilobed configuration of the
alate process, and the general ruggedness of the radular parts distinguish
/. abyssicola from /. trifidus, although the two species appear to be quite
closely related on the basis of the radula only.

12

CALIFORNIA ACADEMY OF SCIENCES

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-Ky^

17

19

0.20 mm

Figures 15-19. I schno chiton abyssicola Smith and Cowan, new species. Rad-
ula details. (15) Major lateral, large paratype from the Semidi Islands, viewed from
the anterior. The outline of the cusp where it joins the shaft may not be completely
correct as it is not clearly defined because of the thickness and translucence of
the shaft. (16) Cusp of major lateral, smaller paratype from off Triangle Island,

No. 56) SMITH & COWAN: NEW DEEP-WATER CHITON 13

Comparative relationships. Ischnochiton abyssicola is by far the larg-
est Ischnochiton ever taken from deep water of the Pacific Coast of North
America. In size and general shape it is much like the larger ischnochitonid
species in the genus Stenoplax, such as the Panamic S. magdalenensis (Hinds)
and the Californian S. conspicua (Dall), S. heathiana Berry, and S. fallax (Car-
penter). The configuration of the head and tail valves of /. abyssicola is much
the same as in these species of Stenoplax. However, the finely beaded dorsal
sculpture, the tendency toward multiple slitting of the end valves with age,
and the peculiar girdle decoration all are markedly different from Stenoplax.
Moreover the benthic range of /. abyssicola leaves a wide gap in habitat, as
all the larger species of Stenoplax live in a low intertidal or high subtidal
zone; so far as we are aware there are no dredging records for any of the lat-
ter. The geographical range of the new species, based on the material at hand,
extends well to the north of any known member of the genus Stenoplax, the
northernmost record being Vancouver Island for S. fallax. and Coos Bay, Ore-
gon, for S. heathiana.

The relationship to the Alaskan Ischnochiton trijidus based on radular
characters only, has already been mentioned. Ischnochiton trijidus (figure
20) also has multiple-slitted insertion plates in the intermediate valves, but
the dorsal sculpture of the valves and girdle armature of solid, rather convex
scales are different from /. abyssicola. While the bathymetric range of /. tri-
jidus is not well known, it is generally found in moderate depths widely sep-
arated from the abyssal niche of /. abyssicola. A possible other near relative
is the little-known Ischnochiton stearnsi Dall, 1902, until recently based on
a single specimen inthe United States National Museum (holotype no. 109024)
collected by the U. S. S. Albatross in 391 fathoms off the Farallon Islands,
California. Another specimen of 1. stearnsi was dredged by the R/V Velero
IV. cruise no. 641, on February 13, 1965, in 255 fathoms, 7-2 miles 190° True
from Long Point, Santa Catalina Island, California (Los Angeles County Mu-
seum no. 10328). Comparison of this second specimen, which was made avail-
able by James H. McLean of the Los Angeles County Museum, with the holo-
type loaned through the courtesy of Dr. Harald A. Rehder of the U.S. National
Museum (figure 21) leaves no doubt as to its identity. Ischnochiton stearnsi
is a relatively small species, around 25 mm. in length, and has a more or less
typically granular Lepidozona-type dorsal sculpture with no enlarged scattered
pustules on the valve surfaces. Ventrally some of the intermediate valves are
double-slitted although there seems to be no tendency toward subsidiary slit-
Queen Charlotte Sound. Ventral Aspect. (17) Detail of central tooth, Semidi Island
paratype: (a) from directly above, (b) from the side. (18) Two views of a spatulate
uncinal, Semidi Island paratype. The smaller branching part at the bottom of the
bottom of the figure is directed anteriorly in the complete radula. (19) Detail of cen-
tral portion of radula, Semidi Island paratype: (a) central tooth, (b) minor laterals,
(c) bases of major laterals. Dorsal view.

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CALIFORNIA ACADEMY OF SCIENCES

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ting in the end valves. The general configuration of the teeth in the insertion
plates and the similarity of the girdle scales of both /. abyssicola and /. steam-
si seem to us to be sufficient evidence to warrant placing the two in the same

21

Figure 20- Ischnochiton trifidus (Carpenter). Dorsal view of adult specimen
from Auke Hay, Alaska. Jack R. Bailey, collector. Length, 37.6 mm.

Figure 21. Ischnochiton stearnsi Dall. Enlarged side view of the holotype
(United States National Museum no. 10^024) from 3Q1 fathoms off the Farallon Is-
lands, California, showing intermediate valves iii-vi, inclusive, and girdle scales.
Height, 6 mm.

No. 56) SMITH & COWAN: NEW DEEP-WATER CHITON 15

species-group. Although the sculptural pattern of /. stearnsi is quite different
from that of /. abyssicola this appears to be a specific rather than a subgener-
ic or generic difference in this instance.

In conclusion, the authors wish to express special thanks to Mr. Spen-
cer Thorpe for his study of the radula of the new species and for his coopera-
tion in providing excellent illustrations (figures 15-19) for inclusion in this
paper; to Mrs. G. Samuel Alspach of the Department of Oceanography, Oregon
State University, for supplying the Oregon specimen; and to Mr. Maurice Giles
and Dr. G Dallas Hanna of the California Academy of Sciences for assistance
with the production of the illustrations. Figures 1-14 and 20-21 are from Koda-
chrome slides by the senior author.

REFERENCES

Berry, S. Stillman

1946- A re-examination of the chiton, Stenop lax magdalenensis (Hinds), with
description of a new species. Proceedings of the Malacological So-
ciety of London, vol. 26, pt. 6, pp. 161-166, pis. 4-5, text figs. 1-6,
January 31.

Dall, William H.

1902- Illustrations and descriptions of new, unfigured, or imperfectly known
shells, chiefly American, in the U. S. National Museum. Proceedings
of the United States National Museum, vol. 24, no. 1264, pp- 499-
566, pis. 27-40. Washington. (Is chno chiton stearnsi Dall, pp. 557-588.)

Pilsbry, Henry A.

1892- Polyplacophora. Manual of Conchology, vol. 14, pp. i-xxxiv, 1-350, pis.
1893. 1-68- Philadelphia. (Genus Ischnochiton, pp. 53-148.)

Smith, Allyn G.

I960- Amphineura. Treatise on Invertebrate Paleontology (R. C- Moore, ed.),
pt. I, Mollusca 1, pp. 41-76, text figs. 31-45- Geological Society
of America. University of Kansas Press.

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