FEEDING ECOLOGY OF THE HAWKS BILL TURTLE (ERETMOCHELYS IMBRICATA) :
SPONGIVORY AS A FEEDING NICHE IN THE CORAL REEF COMMUNITY
By
ANNE BARKAU t-lEYLAN
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF
THE UNIVERSirf OF FLORIDA
IN PARTL^L FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF DOCTOR OF PHILOSCPHY
UNIVERSITY OF FLORIDA
1984
ACKNOWLEDGEMENTS
I would like to thank the members of my supervisory committee, Dr.
Archie Carr (chairman), Dr. Carmine Lanciani, and Dr. John J. Ewel, for
their guidance and encouragement throughout the study. Discussions and
correspondence with K. Ruetzler, R. Garrone, S. Bloom, S. Pomponi, H.
Reiswig, and G. Schnahl were particularly helpful in developing my
interest and background in sponge biology. I would like to thank J.
Ottenwalder, C. Sanlley, S. Inchaus tegui , and N. Garcia for arranging
the collection of digestive tract samples in the Dominican Republic, and
Dr. M. Goodwin for those from Carriacou, Grenada. P. Jesse aided me
greatly in obtaining samples at Bocas del Toro, Panama. A. Ruiz, L.
Richardson, R. Witham, J. Fletemeyer, and N. Rouse also kindly provided
me with material for study. Field work in Panama was facilitated by the
logistic support provided by Y. Hidalgo, M. Panezo, and A. Ayala of
Recursos Naturales Renovables. I thank D. Galloway, A. Ruiz, and P.
Meylan for their assistance in the field in Panama. Specimens were
brought into the United States under U.S. Fish and Wildlife Service
Permit PRT 2-4481. 1 am grateful to Dr. S. Pomponi, B. Causey, and C.
Curtis for their help in collecting sponges in the Florida Keys. Col-
lections made in Looe Kay National Marine Sanctuary were carried out
under National Marine Sanctuary Permit KLNMS and LKNMS-04-83. I would
like to give special thanks to Dr. K. Ruetzler, of the U.S. National
Museum, for his assistance in the identification of sponges. I ac also
grateful to S. Blair, Dr.N. Eiseman, and P. Hall, of Harbor Branch
11
Foundation, for algae identifications; K. Auffenberg and D. Robinson,
Florida State Museun, for mollusk identifications; Dr. F. Maturo,
Department of Zoology, for identification of bryozoans and other inver-
tebrates; and G. Burgess, Florida State Museum, for identifying fish
eggs. Dr. E. Jacobsen provided technical assistance in histological
work. I thank Dr. J. Fiskell for the use of the IFAS Forest Soils
Laboratory to carry out nitrogen determinations. I thank M. McLeod for
teaching me the procedures. I am grateful to Dr. K. Bjorndal for many
helpful consultations concerning laboratory procedures for nutritional
analyses. Dr. J. Ewel, Dr. F. Putz, Dr. J. Anderson, Dr. F. Maturo, and
Dr. F. Nordlie kindly loaned me equipment. I thank Dr. L. Berner for
his advice on slide preparation and microphotography. I thank Dr. J.
Mortimer and Dr. W. Rainey for allowing me to use their unpublished data
on hawksbills. H. Kochman provided expert advice on formatting my data
for computer analyses. I thank my husband Peter for support of many
kinds throughout the study, including the preparation of the figures. I
am grateful to C. Barkau and G. Russell for their support and encourage-
ment. I thank Adele Koehler for typing the final version of the disser-
tation. Financial support for the project was provided primarily by the
World Wildlife Fund/International Union for the Conservation of Nature
and Natural Resources (Gland, Switzerland); supplemental funding was
provided by the U.S. National Marine Fisheries Service (Purchase Orders
03-78-D08-0025 and NA 80-GA-C-OOOl 1 , A. Carr, Principal Investigator)
and the Caribbean Conservation Corporation. I thank the Department of
Zoology for its generous support of many kinds.
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
ABSTRACT v
INTRODUCTION 1
METHODS 10
Diet Analyses 10
Laboratory Analyses of Fresh Sponges 21
RESULTS 25
Composition of the Diet 25
Structural Characteristics of Prey Sponges 46
Toxicity and Antibiotic Activity of Prey Sponges 58
Nutritional Characteristics of Prey Sponges 63
DISCUSSION 67
Composition of the Diet 67
Feeding Selectivity 77
Role of Feeding Deterrents 86
Nutritional Characteristics of Prey Sponges 100
Spongivory as a Feeding Niche 101
SU>aiARY 105
REFERENCES CITED 107
BIOGRAPHICAL SKETCH 118
IV
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy
FEEDING ECOLOGY OF THE HAWKSBILL TURTLE (ERETMOCHELYS IMBRICATA) :
SFONGIVORY AS A FEEDING NICUE IN THE CORAL REEF COMMUNITY
By
Anne Barkau Meylan
December, 1984
Chairman: Archie Carr
Major Department: Zoology
The feeding ecology of the reef-dwelling hawksbill turtle was
investigated in Caribbean Panama, the Dominican Republic and five
countries of the Lesser Antilles. The high percentage of sponges in
digestive tract contents (x = 94.2% of dry weight) and the high degree
of homogeneity among samples from turtles of different sexes, sizes
(over 23 cm carapace length), and geographic origins provide strong
evidence that the species is a strict spongivore. Widespread occurrence
of spongivory in Eretmochelys is proposed.
The presence of pelagic species of the alga Sargassum, pelagic fish
eggs, and other flotsam in digestive tract contents of hawksbills
smaller than 23 cm carapace length provides evidence linking post-
hatchlings to the pelagic Sargassum raft community.
Twenty-three species (14 genera) of demosponges, all representa-
tives of the tetractinomorph orders Hadromerida, Astrophorida and Spiro-
phortda, account for 98.8% (dry weight) of all identified sponges.
Comparison of the sample distribution with the composition of well-
studied Caribbean sponge faunas indicates that the diet is narrowly
restricted. Four major orders of sponges with reef-dwelling representa-
tives are poorly, if at all, represented. Ten species account for 87.4%
(dry weight) of all identified sponges.
Prey sponges are characterized in terras of structural and bio-
chemical properties. The effectiveness of assumed defensive mechanisms
of sponges is evaluated. Spongin fibers are absent in prey sponges,
providing circumstantial evidence that they serve as a feeding deter-
rent. Prey sponges are rich in collagen fibrils; carbohydrate-rich
compounds associated with the fibrils may impart nutritional value.
Silica content varies widely among prey sponges (0-51.6%), suggesting
that siliceous spicules do not deter predation by hawksbills. Astro-
phorid sponges are among the most highly silicified deraosponges. Samples
of intestinal contents consisted of up to 92% ash, which was largely
silica. Scanning electron micrographs of the intestinal epithelia show
numerous embedded spicules. Organic content, energy content, and
nitrogen content are determined for representative prey sponges.
INTRODUCTION
The hawksbill turtle (Eretmochelys Imbricata), one of seven species
of marine turtles, occurs in tropical and subtropical waters of the
Atlantic, Pacific and Indian oceans. It is widely distributed in the
Caribbean and western Atlantic, normally ranging from southern Florida
southward along the Central American mainland to Brazil, and throughout
the Bahamas and the Greater and Lesser Antilles. Two subspecies (E. i.
imbricata in the Atlantic Ocean and E^ i^ squama ta in the Indo-Pacif ic)
have been described (Carr, 1952), on the basis of differences in colora-
tion and carapace shape. The criteria have proven to be unreliable in
distinguishing the two forms, however, and subspecific designations are
rarely used.
The affinities of Eretmochelys with other sea turtle genera are not
x-^ell established. Osteological evidence (Carr, 1942) and serum protein
analysis (Frair, 1979) suggest closer affinities with the loggerhead
(Caretta) and ridley (Lepidochel ys) , than with the green turtle
(Che Ionia). On the basis of immunological distance, the genus.
Eretmochelys is estimated to have diverged from other turtles 29 million
years ago, in the Oligocene (Chen et al., 1980). Zangeri (1980) dates
the divergence time of the line leading to Eretmochelys as middle Mio-
cene, on the basis of morphological features.
The hawksbill is a small to medium-sized marine turtle; adult
females in the Caribbean range from 62.5-91.4 cm straight carapace
length. Nearly all published size data are for females, because of
-1-
-2-
limited access to males. The heaviest hawksbill ever recorded was a 127
kg individual caught at Grand Cayman, in the West Indies (Lewis, 1940).
Since 1970, the hawksbill has been listed as an endangered species
by the International Union for the Conservation of Nature and Natural
Resources (Honneger, 1970). International trade in tortoiseshel 1, the
translucent epidermal scutes of the carapace, is the single greatest
threat to the species (Grootnbridge , 1982). Throughout its circum-
tropical range, the hawksbill is also subject to intense exploitation
for meat and eggs. Immature animals are harvested in great numbers for
the taxidermy trade in the Far East. The diffuse distribution of the
species in both nesting and foraging habitats has impeded effective
conservation action.
Life history data on the hawksbill have been slow to accumulate,
partly because of the depleted status of populations throughout the
world, but also because of logistic difficulties inherent in the study
of highly mobile, marine animals. The tendency of hawksbills to nest
diffusely, rather than in large aggregations, has hindered the effec-
tiveness of land-based tagging programs, which, in the study of other
marine turtles, have been very useful. With few exceptions (Diamond,
1976; Hirth and Latif, 1980; Limpus, 1980; Limpus et al., 1983; Brooke
and Garnett, 1983) most data on the nesting biology of the hawksbill
have been collected incidental to investigations of other species.
Whether hawksbills undertake periodic migrations to distant nesting
beaches, as other sea turtles do, has not been determined. Tag re-
coveries indicate that some long-distance travel does occur (for review
see Meylan, 1982). Evidence to support the commonly held theory that
-3-
hawksbills nest on beaches adjacent to their feeding grounds is largely
inferential.
Coral reefs are widely recognized as the resident foraging habitat
of the hawksbill (Babcock, 1937; Carr et al., 1966; Carr and Stancyk,
1975; Alcala, 1980; Nietschmann, 1981; Carr et al., 1982). Homing
records (Nietschmann, 1981) and sightings of tagged individuals (Alcala,
1980; Boulon, 1983) suggest a relatively parochial existence on the
reef. Other habitats — such as rocky outcrops and, along the Pacific
coast of Central and South America, mangrove-bordered bays and estu-
aries— are occupied to a limited extent when coral reefs are absent.
Despite the association of the hawksbill with the well-studied
coral reef community, the species' ecological niche has never been
investigated. The present study of feeding ecology was initiated as an
approach to filling this gap in knowledge. The feeding biology of the
hawksbill has received little previous scientific study. A considerable
number of anecdotal accounts exist in the literature, reporting the
stomach contents of single individuals (for review see Witzell, 1983).
Although they provide useful information, their qualitative nature makes
it difficult, if not impossible, to construct a profile of the diet.
The authors seldom give any quantitative information on the relative
importance of the various food categories. The accounts suggest wide
variety in the hawKsbill's diet, and include such diverse food items as
mollusks, sponges, gorgonians, fish, seagrasses, crustaceans, sea
urchins, mangrove fruits and leaves, tunicates, jellyfish, algae and
cephalopods — to name only a few.
Current knowledge of the feeding habits of the hawksbill is based
largely on a study by Carr and Stancyk (1975). Theirs was one of the
-4-
few detailed studies of the hawksbill's diet and apparently the only
quantitative one. Stomach contents of 20 mature turtles caught off the
nesting beach at Tortuguero were examined. On the basis of frequency of
occurrence, sponges and tunicates were ranked as the most important
components of the diet. Small amounts of seagrass, algae, mollusks and
bottom material were also found. The authors concluded that "the
hawksbill is a relatively indiscriminate feeder whose food consists
mainly of benthic invertebrates" (p. 165).
Another study, which is useful because of its detail, was that of
Den Hartog (1980), who examined the contents of the entire digestive
tract of a single small hawksbill (36.2 cm carapace length) caught in
the Salvage Islands, eas tarn At lantic. He identified two species of
sponges, the actinian Anemonia sulcata, at least two species of pelagic
coe lenterates, fragments of marine algae, a spider crab, and some
gastropod mollusks. No attempt was made to quantify the various food
items and the total amount of food examined was not reported. Den
Hartog (1980) concluded from his analysis that the hawksbill was
essentially carnivorous but did not make any inferences about specific
food preferences.
The present study was influenced and, to a degree, channelized by
the discovery that the hawksbill feeds almost exclusively on sponges — at
least at 19 localities in the Caribbean where digestive tract samples
were obtained. This was an unexpected finding. Sponges were an impor-
tant component of diet samples examined by Carr and Stancyk (1975), but
they concluded that the hawksbill is an opportunistic oranivore, with a
preference for benthic invertebrates, and this view is widely accepted.
Spongivory is an unusual feeding niche, occupied by relatively few
animal groups Che world over. The list of animals that occasionally
feed on sponges includes diverse phyla — mollusks, echinodenns, annelids,
nematodes, crustaceans and vertebrates (for review see Sara and Vacelet,
1973). Relatively few species, however, subsist primarily on sponges.
Sponge-feeding is particularly rare among vertebrates. De Laubenfels
(1950b) commented on the extreme paucity of sponge-feeding records for
reptiles, birds, and mammals. Numerous surveys of the feeding habits of
marine fish, some involving over 200 species, have revealed very few
true spongivores (Dawson, Aleem, and Halstead, 1955; Hiatt and
Strasburg, 1960; Randall, 1967; unpub. references in Bakus, 1969;
Vivien, 1973; Hobsen, 1974; and Green, 1977). Angelfishes belonging to
the genera Holacanthus and Pomacanthus are among the few exceptions.
They have been identified as spongivores at numerous localities, in-
cluding the West Indies (Randall, 1967; Randall and Hartman, 1968),
Guyana (Lowe, 1962), Veracruz, Mexico (Green, 1977), Hawaii (Hobsen,
1974), and Madagascar (Vivien, 1973). Other sponge-feeding fish include
certain species of filefishes (Monacanthidae), trunkfishes (Ostracion-
tidae), puffers (Tetraodontidae), and the moorish idol (Zanclidae).
Among invertebrates spongivory is somewhat more common — although by
no means widespread. Certain species of dorid nudibranchs are appar-
ently obligate spongivores. A number of sponge associates — e.g., poly-
chaetes, isopods, shrimp, etc. — consume sponge, but the extent to which
sponges contribute to their diet has not been determined. Asteroid
echinoderms are major predators of sponges at McMurdo Sound, Antarctica
(Dayton et al., 1974). Sponge predation by sea urchins is reviewed by
Lawrence (1975). The food chains in which the majority of sponge
-6-
predators are involved tend Co be side chains, which do not lead to
higher trophic levels (Vacelet, 1979).
Spongivores tend to be highly specialized morphologically and, in
some cases, behaviorally. The highly evolved relationships of dorid
nudibranchs and their sponge prey are well known. Many nudibranchs form
species-specific feeding relationships with sponges. Some incorporate
secondary metabolites (including pigments) and spicules from their prey
and use them for their own defense. Spongivorous angelfishes (Chaeto-
dontidae), filefishes (Monacanthidae) and trunkfishes (Ostraciontidae)
are among the most advanced forms of modern teleosts (Randall and
Hartman, 1968).
The low level of predation on sponges is particularly remarkable
when one considers their great abundance and wide distribution. Sponges
are a quantitatively important component of hard-substrate marine com-
munities. On coral reefs, the contribution of sponges to reef biomass
frequently exceeds that of herraatyplc corals (Ruetzler, 1978). In the
spur and groove zones and on the outer fore reef at Carrie Bow Cay,
Belize, the standing crop of siliceous sponges may be as high as 2 kg
wet weight per m'^ suitable habitat (Ruetzler and Macintyre, 1978).
Sponge biomass on the solid exposed reef of the fore-reef slope platform
at Discovery Bay, Jamaica, attains an estimated volume density of 3 1
per m^ , and exceeds the coral-zooxanthe 1 lae tissue biomass (Reiswig,
1973). De Laubenfels (1950b) listed 115 species of shallow-water
sponges in the West Indian region, excluding Bermuda. If utilizable,
sponges clearly represent an extensive food resource.
The relative immunity of sponges to predation has been attributed
by many authors to the defensive protection provided by siliceous
-7-
spicules, tough organic fibers, and toxic or noxious chemicals (Hyman,
1940; Bakus, 1964, 1969, 1981; Randall, 1967; Randall and Hartman,
1968; Sara and Vacelet, 1973; Levi, 1973; Jackson, 1977; Bergquist,
1978; Vacelet, 1979; Bakus and Thun, 1979; and Hartman, 1981). Spicules
and fibers are considered to serve as mechanical deterrents to ingestion
and/or digestion, whereas chemical compounds, which may be emitted into
the surrounding sea water, presumably repel predators from a distance.
Not all authors agree on the utility of these mechanisms. The
defensive role of spicules and spongin is perhaps the most debated, some
authors (Bergquist, 1978) arguing that the functions of these elements
are strictly structural. Defensive utility is nevertheless suggested by
some field data. Pawlik (1933) reported that the sponge-feeding
polychaete Branchiosyl lis oculata consumes only the soft parts of its
siliceous prey. Long, protruding spicules of Cinachyra antarctica may
serve to prevent nudibranch and asteroid predators from reaching the
sponge surface (Dayton et al., 1974). Other evidence of a defensive
utility of spicules is the presence of morphological adaptations in
predators, such as spicule-compacting organs in sponge-feeding dorid
nudibranchs (Forrest, 1953; Bloom, 1976, 1981); and by physiological
adaptations such as copious mucus production by the digestive tract of
spongivorous nudibranchs (Forrest, 1953; Fournier, 1969) and fish
(Randall, 1963).
The defensive utility of secondary metabolites in sponges is almost
universally accepted. Certain classes of compounds found in sponges,
particularly terpenoids, are widely recognized as predator deterrents in
other contexts in both marine and terrestrial systems (Harborne, 1977;
Norris and Fenical, 1982).
Despite considerable discussion of the role of chemical and
mechanical feeding deterrents in sponges in the literature, evidence
from field data is relatively limited. This can be attributed mainly to
the fact that few investigators other than sponge taxonomists have
undertaken field studies of sponges, because of the difficulty of iden-
tifying them. Randall and Hartman (1968) examined the diets of 11 West
Indian fish for which sponges constituted 6% or more of stomach con-
tents. In an effort to discern patterns, prey sponges were described in
terms of ash content, fiber content, color, and growth form. Dayton et
al. (1974) studied the effects of asteroid and nudibranch predators on
sponges at McMurdo Sound, Antarctica. Although the latter study was
primarily concerned with the ecologic effects of sponge predation,
useful descriptive information was obtained on the diets of the
asteroids and nudibranchs, and on the physical characteristics of prey
sponges. Green (1977), Bakus and Thun (1979), and Bakus (1981) investi-
gated the toxicity of marine sponges to fish.
My study of the feeding ecology of the hawksbill revealed both
heavy dependence on sponges and unexpected selectivity in the sponges
eaten. Because the literature so strongly implicated structural and
chemical deterrents in limiting spongivory, I decided to test whether
patterns in the hawksbill's diet could be explained on this basis. The
feeding deterrents that have been proposed for sponges are not uniformly
represented among the various taxa. Thus, my hypothesis was that
effective deterrents would be revealed by avoidance or limited
consumption in the dietary patterns, or by physiological or
morphological adaptation. My study of spongivory in the hawksbill had
two goals: I) to try to explain how the species has been able to take
-9-
advantage of this rarely used, but potentially vast, feeding oppor-
tunity; and 2) to gain an understanding of spongivory as a feeding niche
in the coral reef community.
It seems probable that one of the reasons spongivory has received
little previous attention is the difficulty involved in identifying
sponges from the digestive tracts of the few known spongivores. Dorid
nudibranchs rasp their prey with radulae, and investigators are forced
to identify prey sponges from dissociated spicules found in fecal
pellets. Sponges are extremely difficult to identify even when whole,
and it is not surprising that quantitative analysis of the sponge diets
of nudibranchs has been limited. The small size of fragments is also a
problem in the case of sponge-feeding fish.
The hawksbill presents several advantages for a study of spong-
ivory. Bite-size is large, compared to that of other spongivores, and
food is not masticated. Like most turtles, hawksbills shear and gulp
their food, so relatively large, intact pieces of sponge are found in
the stomach and intestinal tract. The large amount of food in tho
digestive tract provides a good sample for quantitative analysis. In
the present study, it was a further advantage that relatively few taxa
of sponges (22 genera) were represented, and the sponges were nearly all
siliceous species, generic identification of which is based solely on
spicule complement. For sponges in which spicule placement within the
tissue or overall morphology are necessary for diagnosis, study of
sponge-feeding patterns would be far more difficult.
METHODS
Diet Analyses
Collection of Samples
Food samples were obtained from 68 hawksbills. The origins of the
turtles are given in Table 1 (see also Fig. 1). Sixty-one were captured
in Caribbean waters by subsistence fishermen using nets, spearguns or
harpoons, or were taken on nesting beaches. Three food samples (one
fecal pellet, two buccal cavity samples) were from live, wild turtles.
Four small turtles (14.0-21.3 cm straight carapace length) were obtained
through a government stranding network, after they had washed up dead or
moribund on Florida beaches. Data for these four are reported
separately because of the possibility that food items in the digestive
tract were not representative of the normal diet. Further justification
for considering these turtles separately is the likelihood that they
represent a distinct ontogenetic life history stage, with pelagic,
rather than benthic, feeding habits.
Samples included in quantitative analyses consisted of the
following: stomach and intestinal contents (37 turtles); stomach
contents only (17 turtles); stomach and partial intestinal contents (2
turtles); partial intestinal contents (4 turtles); and unknown site of
origin (1 turtle). Only one stomach was found to be empty; it was
included in calculations of percentage occurrence and average percentage
-10-
-11-
Table 1. Geographic origin of hawksbill turtles (Eretmochelys imbrlcata)
included in the feeding study.
Country
No. of Localities No. of Hawksbills
Anguilla
Antigua /Barbuda
Dominican Republic
Grenada
Montserrat
Netherlands Antilles
(St. Martin)
Panama
Turks/Caicos Islands
United States
(Florida)
Total
2
2
5
4
1
1
4
1
4
24
5
3
7
8
3
2
33
1
6
68
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-14-
contribution. All intestines contained some food. Esophagus contents
were not included in the study because only rarely were they fully
recovered. No attempt was made to quantify the digestive tract contents
of the four stranded turtles because of the small amounts of food
present and the high percentage of unidentified material. The two buccal
cavity samples and the fecal pellet were likewise not quantified.
Because of regional differences in fishing techniques and customs,
a well-balanced size series was not obtained for each geographic area.
Large turtles are the principal target of the net fishery in Bocas del
Toro, Panama, the origin of the largest group of samples. Small turtles
captured there are usually released unharmed. In the West Indies, small
turtles are the usual quarry, traditional net fishing having been
replaced at most localities by fishing with spear guns.
Figure 2 shows the size distribution of the turtles included in the
study. Sizes are reported as straight carapace lengths. When only
curved carapace measurements were taken, they were converted to straight
lengths using a regression equation. Missing size data were calculated
for three turtles from a regression of head width against carapace
length, and for six turtles from a regression of intestinal tract length
against carapace length. Although no size measurements are available
for 18 turtles, all but two could be assigned to either adult or non-
adult age categories. The size at which hawksbills attain sexual
maturity is not firmly established. Nietschmann (1981) recorded an
adult female only 62.5 cm in carapace length from the Caribbean coast of
Nicaragua. At Tortuguero, Costa Rica, the smallest female that has been
observed on the nesting beach was 72.4 cm in carapace length (Carr,
unpubl. data). In the present study, turtles of both sexes over 70 cm
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-17-
carapace length were considered adults. A male 66.7 cm in length and a
female 68 cm were deleted from age-specific analyses.
Samples that have been quantitatively analyzed include 17 males and
22 females. The remainder are unsexed. Sex was determined by gross or
histological examination of the gonads. Tail dimorphism was not a
useful indicator of sex except in large turtles. The smallest male
turtle in which elongation and thickening of the tail was noted was 74
cm; a male 52 cm in carapace length could not be sexed by external
characters. Data on the reproductive condition of females were gathered
whenever possible.
Sample Treatment
Digestive tract contents were initially preserved in 1 part 37%
formaldehyde: 19 parts sea water. Sponges and other invertebrates were
subsequently transferred to 70% ethanol. Stomach and intestinal
contents were kept separate (except in two cases). Prior to sorting,
digesta were placed in a strainer and flushed with water to separate
food items.
Food was initially sorted with the unaided eye. The degree of
sorting of stomach and intestinal samples was equal, except as regards-
sponges. Sponges in the stomach were identified as fully as possible,
with an effort being made to assign all fragments. Because of the gross
similarities of many sponges, especially within the family Stel lettidae,
initial sorting had to be routinely verified by examination of spicule
preparations (see below). Sponges in the intestine were closely
examined, but, because of the progressive state of their digestion along
the intestine, quantification of species representation was not
-18-
attempted. All sponge material contained in the intestinal tract was
therefore categorized as unidentified sponge, in spite of the fact that
much was readily identifiable. Sponges contained in partial intestinal
samples, in combined stomach and intestinal samples, and in the sample
of unknown anatomical origin were treated similarly. Because sponges
constituted over 95% of the total dry weight of all samples, and because
siliceous spicules and spongin fibers are resistant to digestion, a high
percentage of intestinal contents could be identified to phylum.
Identification of sponges was made by comparison to a reference
series of specimens and spicule preparations that had been developed
with the assistance of a sponge specialist. In order to make permanent
spicule preparations, fragments of sponge were digested in 5.25% sodium
hypochlorite, and the spicules collected by centrifugation and careful
decanting. They were washed twice in water, and once each in 70%, 95%
and 100% alcohol solutions for dehydration. The spicules were collected
after each wash by centrifugation and decanting, and finally transferred
to microscope slides with a small amount of 100% alcohol. The alcohol
residue was removed by combustion. The spicules were permanently
mounted in Canada Balsam. Temporary spicule preparations to aid the
sorting of sponges were made by dissolving fragments of sponge in a few
drops of sodium hypochlorite directly on a microscope slide. Spicules
could then be examined immediately.
For a few species in which spicule placement or overall sponge
architecture was important for identification, whole mounts of sponge
tissue were prepared for microscopic examination. Thin sections were
hand-cut with a scalpel, then stained with 1% basic fuchsin dissolved in
95% ethyl alcohol. The sections were transferred with forceps through a
-19-
series of alcohol solutions (30%, 50%, 75%, 95% and 100%) for
dehydration (15 min. each). The sections were cleared in xylene, and
mounted on microscope slides with very viscous Permount.
Sponge classification follows Levi (1973) except where otherwise
indicated. Species names could not be assigned in many cases because of
lack of diagnostic characters in the material or problems in the taxon-
omy of the group. One of the most important families represented in the
samples, the Stel lettidae, is badly in need of taxonomic revision.
Algae and the shells of mollusks showed little evidence of
digestion along the tract and could be recognized and sorted from
all regions. Algae and seagrasses were identified with the assistance
of an expert phycologist. Mollusks, fish eggs, and bryozoans were
identified independently by appropriate specialists. Most other inver-
tebrates were identified by me.
Food items were sorted according to 165 categories: 32 for
sponges, 55 for algae, 43 for mollusks, 19 for other invertebrates, and
16 for miscellaneous items. Individual food items were dried to a
constant weight at 105°C, cooled in a desiccator and weighed to the
nearest 0.01 g. The presence of items weighing less than 0.01 g was
also recorded. The use of dry weights to quantify digestive tract
contents introduces a bias because of differences in the ash weights of
food items. Sponges with high levels of silica are overrepresented, for
example, whereas sponges with little or no silica, such as Chondril la or
Chondrosia, are underrepresented. Biases exist across groups as well;
that is, algae are underrepresented as compared to sponges and mollusks,
and soft-bodied organisms such as coelenterates are more poorly repre-
sented than any other group. In spite of these problems, dry weight was
-20-
chosen as the measurement criterion because it was judged to be more
accurate than wet weight or volumetric measurements. In the case of
sponges these introduce unique problems (Ruetzler, 1978).
An inherent bias in diet studies based on digestive tract contents
is introduced by differential rates of digestion. Less digestible items
in the diet are overrepresented, particularly when intestinal contents
are included in analyses. This type of bias is difficult to correct
for, without detailed knowledge of the digestive physiology of the
animal .
A total of 12.4 kg (dry weight) of gut contents was examined from
61 turtles. More than 95% of the dry weight was made up of sponges,
which have an estimated dry:wet ratio of 1:5 (Ruetzler, 1978). An
approximation of the total wet weight of material examined is therefore
in excess of 62 kg.
Food samples obtained from the stomach averaged 13.4 g dry weight
(range 0-65.7 + 14.5,N = 54); intestinal samples weighed an average of
257. 6g (range 0.1-1096.0 + 327.4, N = 35). One partial intestinal
sample exceeded this maximum value, weighing 1378.9 g. The entire
digestive tract contents of 37 turtles averaged 281.7 g (range 0.59-
1113.7 + 330.38).
Data Analysis
In order to make comparisons between food samples of different
amounts (i.e., from small vs. large animals, or empty vs. full digestive
tracts), dry weights of individual food items were converted to per-
centages for each turtle. The average percent dry weight of a
particular food item in all turtles was then calculated. The chief
-21-
advantage of mathematically weighting data in this way is that equal
weight is given to each individual in the sample (Swanson et al., 1974).
Analyses were also calculated on the basis of percentage of total dry
weight. The percent dry weight contribution of an individual food item
or category to the total dry weight of all food items consumed by all
turtles was calculated. Although the implications of this method are
perhaps more intuitively clear, this treatment has several disadvantages
(Swanson et al., 1974). A few individuals consuming large amounts of
rare food items can distort the data. Data can also be biased towards
large individuals because of their larger contribution to the total dry
weight of all food items.
Importance ranks were calculated as the product of the average
percentage contribution and the frequency of occurrence of the item in
all turtles. This ranking method was adapted from Hobsen (1974), with
dry weight percentages substituted for volumetric percentages.
Laboratory Analyses of Fresh Sponges
Collection and handling of sponges. Live sponges were collected in
the Florida Keys at Key Largo, Tavernier, and Big Pine Cay, and
transported on ice to the laboratory in Gainesville. Some were
then temporarily frozen for storage; others were processed immediately.
Sediment adhering to the surface of the sponge, or present in the
aquiferous system, was removed as thoroughly as possible with running
water and a soft brush. All visible epibionts were removed with
forceps. Large sponges were cut in blocks to facilitate drying. The
samples were dried to a constant weight at 60°C in an oven with strong
-22-
circulation, and stored in plastic bags until used. For analyses of
nitrogen content, ash content, and energy content, dried sponges were
ground in a Wiley mill (//20 screen). Several fragments taken from
representative parts (mesohyl, pinacoderm) of each individual sponge
were pooled. Because of the small size of some of the specimens of
Chondrilla nucula, one of the samples is a composite of three
individuals. Maximum storage time of all samples was five months.
Spicule content. Several fragments taken from representative parts
of each individual sponge were pooled. The fragments were dried to a
constant weight (total 0.3-1.3 g) at 105°C, and transferred to flasks
containing glass beads. Concentrated nitric acid was added, and the
flasks were gently boiled until no further reaction (foaming) occurred
and the solution became clear. Spicules were collected under vacuum on
Whatman glass fiber filters (934AH Reeve Angel) and thoroughly rinsed
with distilled water to remove acid solids. Spicules were flushed with
95% ethanol into dry, weighed aluminum pans, and dried to a constant
weight at 105 °C. High (up to 10%) experimental error was observed using
this method and can be attributed to sampling difficulties imposed by
differential spicule distribution. This method has been used in order
to make results comparable to those of other workers.
Ash content. One-gram samples of ground sponge were dried to a
constant weight at 105°C and ashed in a muffle furnace for 3 hr at 500°C
(Allen, 1974). Each analysis was carried out in replicate; values for
replicates were accepted within 2% error. Ash values were corrected for
water of hydration of the silica in the spicules, based on the findings
of Vinogradov (1953) and Paine (1964). The correction factor was
calculated from the weight loss observed upon ashing dry (105 °C) cleaned
-23-
spicules of Geodla neptunl for 3 hrs at 500°C. The spicules had been
isolated with boiling nitric acid according to the method described
above. The average weight loss observed for three samples was 3.95% (+
0.16; N = 3). Ash content was also determined for samples of intestinal
contents of three turtles. The digesta had been originally preserved in
formaldehyde, transferred to alcohol, and dried at 105°C. The same
procedure for ashing was followed as outlined above.
Scanning electron microscopy. Standard procedures were followed in
preparing sections of the intestinal epithelia for examination with the
electron microscope. The intestines had originally been fixed in
formaldehyde (1 part 37% formaldehyde: 19 parts sea water) and then
transferred to 40% isopropyl. Digestive tracts were preserved and
transported with their contents in situ. Microscopic examination of the
intestinal epithelia had not been anticipated. The extent to which this
treatment affected the embedding of spicules in the epithelia is not
known. Given the delicate nature of the epithelia of the large
intestine and the abrasive characteristics of the digesta, I have little
doubt that embedding is a natural phenomenon. Nevertheless, handling
procedures may have caused additional embedding. Embedded spicules were
found in small numbers in the one intestine in which food was not
transported. The specimen was a reproductive female that had very
little food in its digestive tract when captured. The phenomenon of
spicule embeddment deserves additional study, using more appropriate
handling and preservation techniques.
Nitrogen determinations. Total nitrogen content was determined
using a semimicro version of the Kjeldahl method, with the salicylic
acid modification described by Nelson and Sommers (1972). The amount of
-24-
NH3 ii^ 10 ml aliquots of the digests was determined by steam
distillation and hand titration. Replicates were accepted within 3%
error, except in the case of one specimen of Geodia neptuni (3.6%) and
one Spheciospongia vesparium (4.8%). Values were corrected for
percentage dry matter and percentage ash (corrected for water of
hydration) based on results of separate analyses using portions of the
same powdered sample. Dry matter replicates were accepted within 1%
error; ash replicates were within 2% error.
Energy content. Energy content of sponges was determined by
combustion of ground samples in a Parr oxygen bomb calorimeter
(isothermal jacket). Procedure and calculations were carried out
according to the Parr manual (Parr Instrument Co., 1960). Corrections
for percentage dry matter and percentage ash were obtained by separate
analyses carried out on portions of the same samples. Replicate values
were within 3% error, except for Geodia neptuni (4.1%).
RESULTS
Composition of the Diet
Overall Composition
An overall summary of the diet is presented in Table 2. Several
broad categories of food items are ranked according to their percentage
contribution to the total dry weight of all food items examined. All
turtles are considered in the first analysis, including those for which
only partial digestive tract contents were available. Because of dif-
ferences in sample amounts and composition, gravid females have been
excluded from the second analysis. The sample is further restricted to
turtles for which the entire contents of both the stomach and intestine
were available, in order to remove any bias introduced by different
degrees of digestion of partial samples. The percentage composition is
very similar in both cases, and equivalent ranks result.
A second, perhaps more quantitatively accurate, approach to sum-
marizing the overall diet is presented in Table 3. This analysis, which.
uses the restricted data set as specified above, reports the mean per-
centage of the dry weight contributed by each category. Categories are
then ranked by the product of this mean and the percentage occurrence of
items in the category in all turtles (Hobsen, 1974). This method of
summarizing the overall diet produces results almost equivalent to those
shown in Table 2. Sponges remain clearly dominant; the ranks of three
minor categories are rearranged.
-26-
Table 2. Overall composition of digestive tract contents of
hawksbill turtles (Eretmochelys imbricata). Values represent per-
cent dry weight contribution of items in each food category to total
dry weight of all food items consumed by all turtles.
% Composition % Composition
12.4 kg (dry wt) 10.3 kg (dry wt)
Food Category Rank N = 61^ N = 28^
95.33 96.21
2.06 1.91
2.20 1.65
0.17 0.13
0.16 0.07
0.06 0.03
^Includes partial and complete contents.
Includes complete contents only; gravid females excluded.
Sponges
1
Algae
2
Substrate Material
3
Other
Invertebrates
4
Unidentified
5
Mollusks
6
-27-
Table 3. Overall composition of digestive tract contents of 28 hawksbills
(Er etmochelys imbricata). Sample consisted of 10.3 kg (dry weight) of
digesta. Gravid females are excluded from the analysis. Rank is calculated
as the product of the average percent dry weight contribution and the
percent occurrence in all turtles.
X % % Turtles Ranking
Food Category Rank Dry Wt . Range with Item Index
Sponges 1 94.2 + 12.0 41.9-99.9 100.0 94.2
Substrate
Material
2
2.1
+
3.2
0-16.6
96.4
2.0
Other
Invertebrates
3
2.1
+
8.9
0-47.0
78.6
1.6
Algae
4
1.1
+
4.7
0-25.1
82.1
0.9
Unidentified
5
0.4
+
1.8
0-9.7
82.1
0.4
Mollusks
6
0.1
+
0.1
0-0.6
53.6
0.03
-28-
Several categories of food items were usually represented in each
turtle, as indicated by the values for percent occurrence in Table 3.
However, sponges were clearly the dominant food category. The
cumulative contribution of all non-sponge food items in all analyses is
less than 6%. It should be pointed out that a sizable portion of this
6% was not ingested purposefully. Substrate material, algae, gastropod
mollusks, ophiuroids, hydroids, polychaetes, shrimp and scyphozoan
scyphistomae were found attached to, or inside of, sponges taken from
the digestive tracts.
Amounts of food present in the digestive tracts of 34 hawksbills
are plotted against carapace length in Figure 3. Tracts were sampled at
varying degrees of fullness, which explains the large variation in
values observed for large turtles. Female turtles that were gravid, as
evidenced by the presence of shelled eggs, or their being captured on a
nesting beach, had little or no food in their digestive tracts (stars in
Fig. 3). The average amount of food in all nine gravid females avail-
able for study was 15.4 g (+ 12.5, range 0.6-38.2) compared to an
average of 616.8 g (+275.6, range 230.4-1113.7) in 13 nongravid adult
females and adult males. The two samples spanned roughly equivalent
size ranges, as shown in Fig. 3. There was no overlap in values between
the two categories. The two nongravid adult females included in the
study contained large amounts of food (847.7 and 592.4 g).
The digestive tracts of gravid females showed conspicuous differ-
ences in appearance upon examination in the field. The tracts were
contracted, with small lumens, and contained appreciable amounts of
blackish-green fluid, presumably bile. In several of these females, the
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-31-
food present in the digestive tract was totally unrecognizable,
suggesting that it had remained there for a long time.
The maximum amount of food in a turtle of a given size appears to
be described by an exponential function (Figure 3). This is to be
expected, inasmuch as volume increases as the cube of a linear measure-
ment. Too few values are available for turtles between 50 and 70 cm
carapace length to allow plotting of the line. A maximum value of
1379 g was observed for a partial sample from a male hawksbill 72.9 cm
in carapace length.
Sponges
Sponges were present in all but one of the 61 hawksbills included
in quantitative analyses and in 63 of the 68 available for study. Four
of the five without sponges belong to a size class that is believed to
occupy a pelagic habitat (see section on lost-year turtles). Two food
samples removed from the mouths of hawksbills encountered on reefs off
Palm Beach, Florida, and a fecal pellet from a 33.6 cm turtle caught off
Pine Cay, Caicos Islands, consisted entirely of sponge. For the purpose
of examining patterns in the percentage sponge composition associated
with size, sex, reproductive condition, and geographic origin, 37.
hawksbills for which entire digestive tract contents were available were
considered. In some cases, missing values for size, sex, and
reproductive condition dictated further restriction of sample sizes.
Gravid females showed considerable variation in the percentage of
sponges in the digestive tract (Figure 4) and as a group had a smaller
mean value (x = 54.9% + 28.3, range 13.0-88.6, N = 9) than males and
nongravid females (x = 94.2% ±. 12.0, range 41.9-99.9, N = 28; Mann
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-34-
Whitney U Test, p = 0.0001). For seven of nine gravid females, however,
sponges were still the predominant food item. The wide variation in
percentage of sponges in gravid females can be attributed in part to the
small size of the food samples on which the percentages are based
(average sample = 15.4 g). When gravid females are excluded from the
data set, male and female turtles showed no significant difference in
the mean percentage of sponges in the digestive tract contents (females:
X = 95.2% + 7.5, range 78.3-99.2, N = 7; males: x = 96.4% + 7.7, range
72.3-99.9, N = 12; Mann Whitney U Test, p = 0.2067).
The percentage of sponges in the samples did not vary with size
(Figure 4). The mean value in samples from 28 males and females was
94.2% (+ 12.0, range 41.9-99.9). Gravid females were excluded from the
analysis. Other than these, there are only three outliers on the graph.
The most aberrant sample, with only 41.8% sponge, is from a 23 cm
hawksbill caught in the Dominican Republic. It is the smallest turtle
included in quantitative analyses. There is evidence that a major
ontogenetic change in habitat, and consequently diet, occurs at approx-
imately this size, and this would perhaps explain some of the unusual
aspects of the sample. The sample consisted of 47% invertebrates other
than sponges (largely goose barnacles and false corals). This was the
highest value observed for this food category for 61 turtles (see Table
3). It also contained vertebrae and fragments of the chondrocranium of
a fish. Fish remains were found in no other sample. The presence of
substrate material in the sample is an indication that the turtle was
feeding, at least in part, on the benthos.
Age classes (adult and nonadult) were also compared in order to
test for differences in percent sponge composition associated with size.
-35-
No significant difference was found between the means of these two
categories (adults: x = 96.2% + 7.6, range 72.3-99.9, N = 12; nonadults:
X = 92.4% + 14.9, range 41.9-99.6, N = 15; Mann Whitney U Test, p =
0.2074).
Geographic differences in the percentage of sponges in the samples
were also examined. Samples were grouped according to three regions of
geographic origin: Panama, the Dominican Republic, and the Lesser
Antilles (which includes the Leeward and Windward islands). Gravid
females were excluded from the analysis. No significant differences
were found in the mean values in samples from these three regions when
the aberrant sample from the 23 cm hawksbill from the Dominican Republic
(see above) was excluded from the analysis (Panama: x = 96.3% + 8.0,
range 72.3-99.9, N = 11; Dominican Republic: x = 95.8% + 2.2, range
93.4-97.9, N = 4; Lesser Antilles: x = 96.2% + 5.8, range 78.3-99.6, N =
12; Kruskal-Wallis Test, p = 0.1089).
A total of 584.0 g of sponges was examined from the stomach con-
tents of 54 turtles. Of this, 529.6 g (90.7%) could be identified. An
average of 91.1% (+ 15.62) of the sponges in individual samples was
Identified. Stomachs contained an average of 10.8 g of sponges (+
13.64, range 0-65.2, N = 54). As many as 10 species were present in the
stomach of a single individual (x = 3.4).
Thirty-one species of sponges were identified, all belonging to the
Class Demospongiae (Table 4). No calcareous, sclerosponge or hexac-
tinellid sponges were found. Seven orders were represented in the
samples (Figure 5). The orders As trophorida , Spirophorida and Hadro-
merida accounted for 98.8% of the total dry weight of all identified
sponges. These orders are members of the subclass Tetrac tinomorpha,
-36-
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-40-
which is distinguished from the subclass Ceractinomorpha by oviparous,
rather than viviparous, reproduction.
Table 4 lists the average percent dry weight that each sponge
species represented in the stomach contents of all 54 turtles. The
sponges are ranked according to the product of this value and the per-
centage occurrence of the species in all 54 turtles. The ten species of
highest rank are listed in order in Table 5. Also listed in this table
are the 10 most important species as calculated by percentage contribu-
tion to the total dry weight of all sponges. The 10 species shown in
each of these two columns represent, respectively, 79.1% and 87.4% of
all identified sponge. All are either astrophorids or hadromerids.
Chondrosia and Chondril la are considered to be incertae sedis in Levi's
(1973) classification, although he presents them in sequence with astro-
phorids and comments on their affinity with either this order or the
Hadromerida. The affinities of these two related genera and either the
Astrophorida or Hadromerida are widely recognized (Wiedenmayer , 1977;
Bergquist, 1978).
Rank indices based on the product of average percent dry weight
contribution and the frequency of occurrence (first method above) were
also calculated by genus. For this analysis, values within a genus
(i.e., for all Ancorina, all Myriastra and all Tethya) were combined.
The resulting rank indices are illustrated in Figure 6.
Other Elements of the Diet
Substrate material, defined as stones or gravel of calcareous
origin, was found in the digesta of all but seven of 61 turtles. Much
of it was attached to sponges and was probably ingested incidentally.
-41-
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-44-
The average percentage of this item in the gut was higher for gravid
females (x = 27.1% + 31.9, range 0-80.6, N = 9) than for all other
turtles (x = 2.1% + 3.2, range 0-16.6, N = 28). The digestive tracts of
two gravid females contained little other than substrate material (80.6%
and 77.3% of dry weight).
Over 50 species of algae were found in the digestive tracts of the
61 hawksbills included in the quantitative analyses. The 15 species
most frequently represented are listed in Table 6. Although algae were
present in most samples, they contributed an average of only 1.1% of the
dry weight in the 28 nongravid turtles for which entire digestive tract
contents were available (Table 3). In only six of these turtles did
algae contribute a larger percentage, the maximum being 25.1%. Several
species were found attached to sponges and were probably ingested inci-
dentally.
Codium isthmocladum and Lobophora variegata were found in
sufficiently large pieces and quantities to suggest purposeful inges-
tion. One adult male hawksbill had eaten 158 g of Codium isthmocladum,
in addition to 457 g of sponges.
Seagrasses were present in very small quantities (maximum of 0.25
g) in 16 of 61 turtles. Thalassia testudinum, Syringodium f iliforme and
Halodule wrightii were identified.
The 61 turtles included in quantitative analyses had remarkably
little man-made litter in their digestive tracts. In five individuals
small fragments of plastic, paper or string were found, the largest item
being a 0.13 g piece of plastic. Man-made litter was much more preva-
lent in the digesta of the four small hawksbills that stranded on
Florida beaches.
-45-
Table 6. Algae most frequently represented in the digestive
tracts of hawksbill turtles (Ere tmochelys imbr icata).
N = 61.
Species
Dictyopterls delicatula
Dictyota sp.
Lobophora variegata
Microdlctyon boergesenii
Halimeda sp.
Bryothamnion seaforthii
Codium isthmocladum
Kallymenia
linnninghii
Anadyomene
stellata
Gelidiopsis
planicaulis
Pterocladia
. bartlettii
Caulerpa microphysa
Galaxaura sp.
Caulerpa vickersiae
Gelidiella sanctarum
No. of
% Turtles
Occurrences
with Item
22
36.1
19
31.1
17
27.9
16
26.2
15
24.6
15
24.6
14
23.0
13
21.3
13
21.3
12
19.7
11
18.0
11
18.0
9
14.8
7
11.5
7
11.5
-46-
Lost-Year Turtles
There appear to be no data in the literature on the diet of wild
hawksbills of the size range represented by the four specimens that
stranded on Florida beaches (Witzell, 1983). Because of considerable
interest within the scientific community in marine turtles of this size
class — particularly as regards their habitat occupation — the results of
analyses of the digestive tract contents of these specimens are reported
in detail in Table 7.
Structural Characteristics of Prey Sponges
Inorganic Constituents
Table 8 presents data on the spicule content of astrophorid,
spirophorid, and hadromerid sponges that were identified in the stomach
contents of Eretmochelys imbricata or were represented in the samples at
the generic level. Sponges of these three orders accounted for 98.8% of
the total dry weight of all identified sponges. Because identification
to species was not possible for many of the sponges that had been aaten
by turtles, values in the literature for all Caribbean species of the
genera represented have been included. Data from Bergraann (1949) and-
Ruetzler and Macintyre (1978), used to supplement those obtained in the
present study, were derived by the sane isolation technique.
Spicule content of the 31 sponge species found in the stomach
contents of hawksbills (Table 4) varies widely. Chondrosia, Halisarca,
and Verongia contain no spicules at all. Chondrilla nucula, the second
most frequently represented sponge in the samples, has very few, and all
are microscleres. Geodia, which was Identified from 26 turtles, has one
-47-
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-49-
Table 8. Spicule content of astrophorid, spirophorid and hadromerid
sponges. Included are genera or species (denoted with an asterisk)
represented in stomach contents of hawksbills turtles. N = 1-2 indi-
viduals for this study.
Sponge
Spicules
(% of dry wt.)
Source
Astrophorida
Geodia neptuni
Geodia gibberosa
*Myriastra kallitetilla
*Chondrilla nucula
51.6
67.1
63
21.0
This study
Ruetzler and
Macintyre, 1978
Bergraann, 1949
This study
This study
Bergmann, 1949
Spirophorida
Cinachyra kuekenthali
Cinachyra cavernosa
25.4
43
This study
Bergmann, 1949
Hadromerida
Tethya aurantia
Aaptos sp.
Suberites compacta
*Spheciospongia vesparium
Spheciospongia sp.
33
17
75
48.7
58.7
22
Bergmann, 1949
Bergmann, 1949
Bergmann, 1949
This study
Ruetzler and
Macintyre, 1971
Bergmann, 1949
-50-
of the highest spicule contents of all siliceous demosponges; values are
given in the table for the two species that are common in the Caribbean.
High silica contents are characteristic of the Astrophorida and
Spirophorida, in general. Ancorina sp. 1, Ecionemia sp., and several of
the Myriastra species identified in the samples were very spiculate. 1
find no data in the literature on the spicule contents of these sponges,
or of the hadromerid Placospongia, and my attempts to collect them in
the Florida Keys were unsuccessful.
The total amount of ash in sponges is also of relevance to preda-
tors. Ash content is a measure of total mineral content, and in the
case of sponges can be considered an indicator of mechanical strength or
fortification. It can be seen in Table 9 that there is considerable
variation in ash content among prey sponges. Comparison of Table 8 and
9 shows that for some species ash content greatly exceeds spicule con-
tent, e.g., for C inachyra kuekentha li, Myriastra kallitetilla,
Spheciospongia vesparium, and Chondrilla nucula. It should be noted
that the same individual sponges were used in both analyses. In the
case of Chondrilla nucula, the difference between the two values is
largely due to adhering calcareous sediment. One habit of this species
is encrusting, and specimens frequently contain embedded sediment.
The highest ash content was found in Spheciospongia vesparium
(64.5%), the loggerhead sponge. This sponge species ranked sixth in
terms of contribution to the total dry weight of all identified sponges.
Geodia neptuni also has a notably high ash content.
The sponge Chondrosia (not analyzed in the present study) has one
of the lowest ash contents of the sponges represented in the diet. This
species lacks siliceous spicules and specimens are usually free of
-51-
Table 9. Ash content of a representative series of Caribbean
demosponges. Values are means (N = 1-3) + S.D. when N = 3. Species
identified in stomach contents of hawksbill turtles (Eretmochelys
imbricata) are denoted with an asterisk; "+" denotes genera that were
represented in the samples.
Ash
Sponge (% of dry wt . )
Astrophorida
-HSeodia neptuni 58.5
*Myriastra kallitetilla 36.6
*Chondrilla nucula 25.1+3.2
Spirophorida
-HCinachyra kuekenthali 52.1 + 3.9
Hadroraerida
*Spheciospongia vesparium 64 . 5
Poecilosclerida
*Iotrochota birotulata 41.6 + 4.3
+Agelas conif era 31.5
Haplosclerida
Haliclona compressa 39.1
Dictyoceratida
Ircinia strobilina 37.2
Spongia tubulifera 31.0 + 2.8
-52-
adhering foreign calcareous material- Randall and Hartman (1968) deter-
mined a value of only 1.5% for Chondrosia collectrix, the most common
Caribbean species. Chondrosia was represented in 13 turtles in the
present study.
No ash content data are available for several sponge genera that
were important in stomach contents, e.g., Ancor ina , Ec ionemia ,
Placospongia, and Suberites. Ash content is certain to be high for the
first three genera, because of their high silica content. It is notable
that the ash contents of Ircinia strobilina and Spongia tubulif era, both
of which lack siliceous spicules, are still of the order of 30-40%.
Ircinia is known to Incorporate foreign calcareous particles within its
spongin skeleton, which may account for the high value. Spongia does
not incorporate particles but may contain iron in its spongin fibers.
Ash values of intestinal contents were determined for three
turtles. Samples that appeared to have high ash contents were purposely
selected, in order to establish a maximum value. Ash contents of 92.0%,
76.6%, and 74.3% were measured. Because of species composition, the ash
can be considered to be mostly silica. Figure 7 shows the glass-like
appearance of dried intestinal contents. The first sample was taken
randomly from 490 g of intestinal contents. Sediment (1.56 g), algae
(0.5 g), and gastropod mollusks (0.21 g) had been previously removed.
The latter two samples were taken from unsorted digesta contained in the
terminal part of the digestive tract, just anterior to the junction with
the cloaca.
Spicules in ascrophorid, hadromerid, and spirophorid sponges are
not associated with spongin, and upon digestion are liberated in the gut
of the hawksbill. As a result, the large intestine contains
-53-
extraordinar i ly large numbers of sharp, free spicules. Scanning
electron micrographs of the intestinal epithelia revealed numerous
embedded spicules (Figure 8). The extent of penetration in the gut wall
was not histologically determined because of sectioning difficulties
caused by the large number of spicules.
The principle megascleres of astrophorids and spirophorids are
tetraxonid (4 axes) and are among the largest (up to 5.3 mm in one
species of Myrlas tra in the samples) siliceous spicules found in
shallow-water demosponges. Geodia, Myriastra, Cinachyra, Ancorina, and
Ecionemia contain trienes with sharp, and in some cases recurved, hooks.
Each clad is bifurcated in Ancorina sp. 1, so that one spicule actually
bears seven sharp points. The cladomes — bearing the hooks — are usually
directed outward, toward the surface of the sponges. Needle-like
monaxonid spicules of the hadromerid, Suberi tes , project from the
surface to form a hispid coat. The megascleres of Jaspis are robust,
double-pointed monaxons.
The principle megascleres of the orders of siliceous sponges that
are not consumed by hawksbills are simple (1-axis) oxeas. Megascleres
of non-prey sponge orders tend to be smaller than those of prey sponges.
Spicules are noticeably concentrated in the periphery of several
prey sponges. Millions of sterrasters are tightly packed to form a
thick (up to 4 mm), stony cortex in Geodia. It has been described as a
"sterraster armour" (de Laubenfels, 1950a). Placospongia also has a
stony cortex, formed by irregular polygonal plates of small sterrasters.
Cortices are not characteristic of the siliceous ceractinomorph orders,
Poecilosclerida, Haplosclerida, and Halichondrida .
Figure 7. Dried intestinal contents of a hawksbill turtle
(Eretmochelys imbricata) . Glass-like needles are siliceous
spicules. Ash content ca. 92% of dry weight.
Figure 8. Scanning electron micrograph of intestinal epi-
thelia of a hawksbill turtle, showing embedded siliceous
spicules .
-55-
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-56-
Organic Constituents
There are two distinct components of the organic skeleton of demo-
sponges: spongin and collagen fibrils. Both are made of the fibrous
protein, collagen. The sponges identified from the stomach contents of
hawksbills show distinct properties with respect to both of these con-
stituents .
The sponges that were predominant in the samples apparently contain
no spongin in the form of fibers (spiculated spongin fibers or horny
fibers), and little, if any, spongin in other forms. As Table 10 indi-
cates, the Astrophorida, Spirophorida, and Hadromerida are three of six
orders that lack spongin fibers. With the exception of the small and
primitive group Homosclerophorida, these are the only orders of sponges
that lack spongin fibers and are possible food sources, the Desmophorida
and Tabulospongida being unsuitable because of their stony consistency.
The types of sponges that were identified in the stomach contents
of hawksbills are rich in collagen fibrils. Sponges of the subclass
Tetractinomorpha tend to have a higher density of collagen fibrils in
the intercellular matrix than do those of the subclass Ceractinoinorpha
(Garrone, 1978). By contrast, loose-textured sponges are characterized
by extracellular spaces poor in fibrillar components. The tetractinel-
lid tetractinomorphs (which include Astrophorida and Spirophorida) are
particularly rich in collagen fibrils (Levi, 1973).
There is considerable documentation in the literature of a high
collagen fibril content in several genera that are consumed by
hawksbills. Tethya and Chondrosia are singled out by Garrone (1978) as
examples of dense-textured sponges. In the latter, fibrils constitute
the only skeletal framework of the sponge (Garrone et al., 1975). A
-57-
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-58-
high collagen fibril content has been observed in Jaspis stellifera
(Wilkinson, 1979) and in Stelletta grubii (Simpson, pers. comm.)- The
latter is a member of the Stel lettidae, which includes the prey genera
Myriastra, Ancorina, and Ecionemia. Fibrillar bundles, formed by the
association of several hundred collagen fibrils, have been observed in
Ghondrosia, Tethya, and Suberites (Garrone, 1978).
The amount of collagen fibrils present in the digestive tract
contents is high, not only because of the particular species of sponges
present, but also because large amounts of fibril-rich ectosome or
cortex had been eaten. Densely packed collagen fibrils form the cortex
of Chondros ia, Chondril la , and Te thya and the thickened ectosome of
Jaspis stel lifera and Suberites massa (Garrone, 1978; Wilkinson, 1979).
Collagen fibril content is also high in the external asexual buds that
occur in some sponges, such as Tethya lyncurium (Connes, 1967). A large
number of buds of Tethya cf . actinia were present in the digesta.
Toxicity and Antibiotic Activity of Prey Sponges
A considerable body of data on the secondary metabolites of sponges
is accumulating as a result of natural products chemistry research. In
only a few instances has the relevance of specific chemical constituents
been developed in the context of predator-prey interactions. Data on
the toxicity and antibiotic activity of these chemical constituents are
far more available. Toxicity is usually tested by immersing fish in
water containing sponge extracts. Evidence from the literature bearing
on the toxicity to fish of sponges eaten by Eretmochelys is presented in
Table 11. All data available for genera that were represented in the
stomach contents of turtles are included. As is evident in the table.
-59-
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-61-
intrageneric and intraspecif ic differences in toxicity of sponges have
been observed. The bioassay techniques used by different authors vary
in detail, but are generally similar. Test fish used in the bioassays
are as follows: Green (1977), goldfish (Carassius auratus); Bakus and
Abbott (1980), mosquito fish (Gambusia aff inis); Bakus and Thun (1979),
sargeant majors (Abudefduf saxatilis); Bakus (1981), goldfish. Criteria
for toxicity ratings also vary from study to study, but in all cases are
based on fish responses, i.e., loss of equilibrium, convulsions, paraly-
sis, and death.
Several sponge genera and species that were determined to be toxic
to fish in these tests were important components of the stomach contents
of Eretmochelys, including Geodia, Chondril la nucula, Tethya actinia,
and Spheciospongia vesparium. The toxicity of different species of
Geodia appears to vary, ranging from nontoxic for G. neptuni to mildly
toxic for G. gibberosa. Both are Caribbean species. Chondrilla nucula,
one of the most common sponges in the stomach contents, was found in all
tests to be toxic to some degree. Wrasses (Hal ichoeres bivittatus)
force-fed Chondrilla nucula from Caribbean Mexico showed "paralysis-like
signs" within 7 rain and "convulsive- like signs" within 8 min (Green,
1977). Goldfish placed in water containing extracts of this species
died in only 34 min (Green, 1977). Specimens of Chondril la nucula
collected in Puerto Rico have been reported to cause contact dermatitis
in humans (M.B. Mathews, pers. comm.). This malady is commonly asso-
ciated with the sponges Tedania ignis and Neofibularia nolitangere; I am
unaware of any previous reports attributed to Chondril la nucula.
Tethya actinia obtained from Veracruz, Mexico, was rated as
moderately toxic by Green (1977). Tethya was a particularly common
-62-
genus in the stomach contents. One digestive tract was completely
filled with a sponge that was very similar, if not identical, to this
species. Another sponge that was considered mildly toxic in the above
tests, Spheciospongia vesparium, has been shown to be toxic when in-
jected intraperitoneal ly in mice (Halstead, 1965). Suberites ficus was
found to be nontoxic to fish. Extracts of another species of this
genus, Suberites domunculus, found in European waters, caused hemor-
rhaging and death in a wide variety of lab animals (Richet, 1906a, b).
Representatives of three sponge genera of minor importance in
stomach contents were also determined to be toxic in these bioassays.
Both Lissodendoryx aff. kyma and Hymeniacidon ? amphilecta were highly
toxic to fish; lotrochota biro tula ta, present in small amounts in 6
turtles, was found to be nontoxic in tests by Green (1977) and mildly
toxic in those of Bakus and Thun (1979). Green (1977) reported that
fish avoid the colored, strong-smelling exudate of this species.
Another area of sponge chemistry of possible relevance to predator-
prey interactions is that of antibiosis. The current, broad interpreta-
tion of this terra, elucidated by Burkholder (1973), is that of "a
phenomenon in which special products of certain organisms severely limit
the life activities of other organisms" (p. 118). Marine demosponges
exhibit a high incidence of antibiotic activity. The usual test
organisms used in screening for this activity are bacteria and yeast,
although tumors and viruses are also tested. Bergquist (1979) points
out that "antibiotic activity demonstrated in the laboratory is a mani-
festation of something which in nature could also be toxic, bad tasting
or active in quite another way" (p. 390). Antibiotic activity is often
used to screen potential sources of secondary metabolites. According to
-63-
the literature, several sponges consumed by the hawksbill turtle have
been demonstrated to exhibit antibiotic activity (Table 12).
Nutritional Characteristics of Prey Sponges
Little has been written about the nutritional characteristics of
sponges. These animals are of no importance as a food source to people
and figure only slightly in the diets of most other animals. A thorough
study of the nutritional characteristics of sponges is obviously beyond
the scope of the present study. I have instead gathered data on a few
basic nutritional parameters for those sponges eaten by hawksbills and
for a few representatives of major non-prey orders. Although the diges-
tive physiology of the hawksbill remains unstudied, nutritional data on
its food are useful background in a discussion of feeding patterns.
Organic matter, energy, and nitrogen content of several sponge
species and genera represented in stomach contents of hawksbills are
given in Table 13, along with data for common reef-dwelling representa-
tives of major non-food orders. Sponges eaten by hawksbills vary widely
with respect to all of these parameters. The highest percentage of
organic matter was observed for Chondril la nucula, a species that was
well represented in stomach contents. Geodia neptuni, Cinachyra
kuekenthali, and Spheciospongia vesparium are low in organic matter, and
this is reflected in their total dry weight energy and nitrogen con-
tents. This pattern can also be expected to hold true for the other
heavily silicified astrophorids in the diet, e.g., Ancorina, Myriastra,
and Ecionemia, and for the hadromerid P lacospongia. Total dry weight
values, which include ash content, are perhaps of greatest relevance
from the standpoint of predators. When high-ash food items are
-64-
Table 12. Antibiotic activity of sponge species (denoted with an
asterisk) or genera that were represented in stomach contents of
Eretmochelys imbricata.
Sponge
Antibiotic
activity
Reference
Cinachyra cavernosa Antitumor
*Spheciospongia vesparium Antitumor
Burkholder, 1968
Geodia cydonium
*Chondrilla nucula
Tethya aurantium
Suberites domuncula
Placospongia decorticans
Antibacterial
No activity
detected
No activity
detected
No activity
detected
Antimicrobial
Burkholder and Ruetzler,
1969
Cinachyra cavernosa
*Spheciospongia vesparium
Hymeniacidon sp.
Antimicrobial
Antimicrobial
Antimicrobial
Burkholder, 1973
Ancorina alata
Cinachyra n. sp.
Tethya aurantia
Hymeniacidon perleve
Antibacterial
Antibacterial
Antibacterial
Antibacterial
Bergquist and Bedford,
1978
Chondrosia collectrix
Antibacterial
Stierle and Faulkner,
1979
-65-
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-66-
consumed, larger amounts must be eaten to obtain the same nutritional
value.
Energy and nitrogen contents of the various sponges are more
homogeneous when put on an ash-free basis. Ash-free energy values of
prey species and genera are relatively low when compared to other animal
tissues (Paine, 1964). A very approximate estimation of crude protein
content of the sponges can be obtained by multiplying nitrogen content
by 6.25.
DISCUSSION
Composition of the Diet
Sponges
In the digestive tract samples from hawksbills over 23 cm in
carapace length sponges were clearly the dominant food item. No dif-
ferences in the percentage of sponges were found for turtles of dif-
ferent sexes (except gravid females), sizes (over 23 cm), or geo-
graphic origins. Sponges were also the dominant food item in samples
from gravid females, although they contributed a smaller percentage to
the total digestive tract contents. The difference was made up
largely by substrate material.
The high percentage of sponges in the diet and the high degree of
homogeneity among samples from turtles of different sizes, sexes and
origins provide strong evidence that the hawksbill is a strict spongi-
vore. No other food category contributed significantly to the
samples; much of the non-sponge material was apparently ingested acci-
dentally along with the sponges. The only vertebrates known to have
comparable diets in terms of percent sponge are the gray angelfish
(Holacanthus arcuatus, 98.3% sponge, N = 6, Hobson, 1974), the queen
angelfish (Holacanthus ciliaris, 96.8% sponge, N = 24, Randall and
Hartman, 1968) and the rocky beauty (Holacanthus tricolor , 97.1%
sponge, N = 24, Randall and Hartman, 1968).
-67-
-68-
The fact that sponges were dominant in samples of such wide geo-
graphic origin (7 countries, 19 localities) suggests that spongivory
in hawksbills is not a parochial tendency but a widespread feeding
habit. Spongivory is such a peculiarly specialized feeding habit that
it seems unlikely that it would occur in only a portion of any given
population.
Table 14 lists all records of sponge-feeding by Eretmochelys that
have been reported in the literature, received by me through personal
communications or compiled in the present study. The table documents
the fact that sponges are eaten by hawksbills, at least to some
degree, throughout the range of the species. Without more quantita-
tive data, one cannot say that the hawksbill feeds primarily on
sponges throughout its range. This will probably prove to be the case,
however, when adequate samples are available.
Other Elements of the Diet
The presence of substrate material in the samples can in most
cases be attributed to incidental ingestion. The percentage of this
item in the samples varied little (standard deviation 3.2) except in
gravid females, and this is consistent with the hypothesis that sub-
strate material enters the diet incidentally.
The high levels of substrate material observed in several of the
gravid females that had not been feeding are more difficult to
explain. They might be a consequence of retention in inactive
digestive tracts or of purposeful ingestion. Several other reptiles,
including other turtles, crocodiles, and lizards, are known to ingest
sediment purposely (Sckol, 1971). The purported adaptive aims of
-69-
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-71-
lithophagy or geophagy, as it Is called, are varied. In crocodiles,
stones in the digestive tract have been proposed to have gastrolithic
and hydrostatic functions (Cott, 1961; Webb et al., 1982). In the
present case a gastrolithic function would not appear to be applicable
because the animals that had the largest amount of substrate material
in their digestive tracts were not actively feeding.
Marlow and Tollestrup (1982) reported that female desert
tortoises (Gopherus agassizi) actively seek out and eat calcium-rich
deposits of earth during the nesting season and suggested that this
activity served to replenish calcium reserves depleted by egg shell
production. Calcium requirements are undoubtedly high in
Eretmochelys. According to Pritchard (1979a) Atlantic hawksbills have
the largest average clutch size of any turtle (about 150 eggs). The
fact that the amount of sediment in two reproductively active females
was comparable to or higher than the maximum amount found in other
turtles, including those with full digestive tracts, offers support to
the hypothesis of purposeful ingestion. Perhaps more convincing
evidence are the observations of a number of turtle fishermen and
commercial divers in the Leeward Islands and Panama who reported to me
that hawksbills feed on coral rubble, gravel and even Mi 1 lepora coral.
Algae were a minor component of the samples and, in most cases,
can be considered to have been ingested accidentally. Codium
Isthmoc ladum and Lobophora variegata were the only species that
appeared to have been ingested purposefully. Codium was mentioned as
one of the two genera of algae found in the stomachs of two hawksbills
captured in the Central Visayas, Philippines (Alcala, 1980). Codium
is a major dietary component of Hawaiian green turtles (Balazs, 1980).
-72-
Although algae were of little importance in the samples examined
in the present study, their possible role in the diet of some
hawksbill populations cannot be dismissed. Algae are mentioned as
having been found in digestive tract contents of hawksbills at locali-
ties in the Atlantic (Carr and Stancyk, 1975; Den Hartog, 1980;
Bjorndal, in press), Pacific (Swinhoe, 1863; Pritchard, 1977, 1979b;
Limpus, 1979; Alcala, 1980) and Indian oceans (Fryer, 1911; Hornell,
1927; Deraniyagala, 1939; Hirth and Carr, 1970).
Few of the above authors reported the amount or relative impor-
tance of this food item in their samples. Hirth and Carr (1970) and
Den Hartog (1980) found only small amounts of algae in specimens they
examined. Swinhoe (1863), Hornell (1927), Carr (1952) and
Deraniyagala (1939) stated or implied that algae were important com-
ponents in samples examined by them. Hornell (1927) provided the most
detailed information, stating that the stomachs of adult hawksbills in
Seychelles waters were repeatedly found to be full of masses of
sargasso weed (Sargassum) in various stages of digestion.
Deraniyagala (1939) reported that the hawksbill frequently subsists on
an entirely vegetarian diet, although he cited data on only one
specimen. Swinhoe (1863), too, had examined only a single specimen.
The question whether hawksbills can digest algae has been raised
by Den Hartog (1980). He noted that algae found in the digestive
tract of a specimen examined by him seemed poorly digested. Observa-
tions made during the present study are consistent with those of Den
Hartog. Algae appeared relatively unaltered by digestive processes
all along the tract. In Hornell's observations in the Seychelles, he
mentions finding Sargassum in varying states of digestion. It is
-73-
significant that Sargassum is the genus that was found in the
digestive tracts of very small turtles of lost-year sizes.
The small amount of seagrasses in the samples, together with the
minor importance of algae, are clear evidence of different food
requirements of the hawksbill and green turtle at the various study
sites. These two species are very commonly found in close association
in coastal waters in the study area and elsewhere in the Caribbean.
Throughout the world the green turtle is known to be a rather strict
herbivore (Mortimer, 1982). In the Caribbean green turtles feed
primarily on the seagrass Thalassia testudinum (Bjorndal, 1980;
Mortimer, 1981). Their feeding habits in the Lesser Antilles have yet
to be studied, but the herbivorous feeding preference of the species
is widely established. Limited evidence gathered during the present
study suggests that immature green turtles at some localities in the
Lesser Antilles consume appreciable quantities of algae, as well as
seagrass. In neither case, however, do they appear to be in competi-
tion with hawksbills for food.
There are few records in the literature of hawksbills feeding on
seagrasses. Alcala (1980) mentions the presence of seagrass in the
stomachs of two specimens from the Central Visayas, Philippines.
Their abundance in the samples is not reported. Seagrasses were also
reported in the diet of hawksbills in the Eastern Caroline Islands,
Micronesia (Pritchard, 1977).
Lost-Year Turtles
A significant gap exists in knowledge of the life history of all
sea turtles from the time newly emerged hatchlings leave the nesting
-74-
beach to the time they appear in the foraging habitats characteristic
of subadults and adults. Marine turtles of all species are rarely
sighted during this period, and this has led biologists to call this
stage of the life history the lost year (Carr, 1967). The length of
the lost-year interlude and the sizes at which turtles of various
species enter coastal habitats have yet to be established. In the
Lesser Antilles, where much of the present study was carried out,
hawksbills less than 23 or 24 cm carapace length are rarely sighted.
Interviews with turtle fishermen and commercial divers during the
course of field work yielded information on only one or two specimens
of this size range.
There is considerable evidence that small turtles of at least
some species spend the lost year in the open sea (Carr, 1967; Carr and
Meylan, 1980). In the Atlantic Ocean, green turtles and loggerheads
have repeatedly been found drifting in association with rafts formed
by the floating alga Sargassum (Carr and Meylan, 1980; Carr, 1983).
There is little evidence, however, linking post-hatchling hawksbills
to this habitat. Only a few notes in the literature refer specifi-
cally to lost-year hawksbills. Hornell (1927) reported an observation
made by L. E. Lanier of hawksbills drifting in association with masses
of seaweed many miles from land. Vaughan (1981) reported that
hatchling-sized and slightly larger turtles are frequently found in
the deep sea associated with long skeins of rubbish and seaweed
downcurrent from a major hawksbill nesting beach in the Solomon
Islands. whether these were hawksbills could not be verified,
although this seems likely.
-75-
Data collected by Kajihara and Uchida (1974) on the carapace
lengths of 146 hawksbills caught for the taxidermy trade in Southeast
Asia offer some of the most convincing evidence ever presented for the
existence and length of the lost-year period for hawksbills. In spite
of intensive economic incentive for fishermen to supply the taxidermy
trade, no turtles under 15 cm carapace length and only a few in the
15-20 cm range were found in the factory. The authors suggested that
a change in habitat occupation takes place at approximately 16-18 cm
carapace length.
An alternative solution to the lost-year puzzle for hawksbills is
offered by Witzell and Banner (1980), who reported that at least some
pos t-hatchling hawksbills (> 4 cm) inhabit coral reefs in Western
Samoa .
The contents of the digestive tracts of four hawksbills reported
here provide corroboration of the theory that the lost year is spent
associated with Sargassum rafts, although caution must be taken in
interpreting data from stranded specimens. The possibility exists
that atypical foods were consumed subsequent to the injury or onset of
disease that resulted in death. The food sample from UF 54846 can
probably be considered free of this bias because death was almost
certainly due to asphyxiation by tar. Food present in the digestive
tract was therefore consumed beforehand, and can be assumed to be
characteristic of the normal diet.
Sargassum was present in all four specimens, although in only two
cases was the material identified as one of the pelagic species of the
genus that is known to form large floating mats. Fish eggs of the
suborder Exocoetoidei were attached to Sargassum in UF 54846. This
-76-
suborder includes flying fish, half-beaks and needlefish; most of Che
species within it are known to be pelagic. The presence of these eggs
in the digestive tract is evidence of surface feeding, in any case, as
is that of bouyant styrofoam particles and plastic beads.
The relative importance of plant and animal matter is difficult
to assess with the limited sample. Both were well represented.
Sargassum was present in sufficient quantity to suggest purposeful
ingestion. Norris and Fenical (1982) discuss the apparent avoidance
of Sargassum by many herbivores in the Caribbean and suggest that the
presence of tannin-like polyphenolic substances within members of the
family Sargassaceae may be responsible. In a wide survey of the
feeding habits of West Indian fish, Randall (1967) found that rela-
tively few fish feed on drifting Sargassum, sea chubs and the trigger-
fish Melichthys being notable exceptions.
The abundance and diversity of man-made debris in the digestive
tract contents reveal the vulnerability of marine turtles — at least at
this life history stage — to oceanic pollution. All four specimens
examined had plastic refuse in the digestive tract; some had several
different types. Of the many oceanic pollutants, petroleum products
undoubtedly represent the greatest threat to survival. Death of at
least one, and probably two, of the specimens can be attributed with
some confidence to this cause. Two were fouled externally, and three
had tar present in the digestive tract. The esophagus of UF 50027 was
heavily coated, and tar aggregates were present throughout the
digestive tract.
The presence of oceanic pollutants in the digestive tracts of the
turtles may be a result of their association with the Sargassum raft
-77-
community. Pollutants such as oil, styrofoam and other plastics are
well known components of the rafts. Their presence there has been
identified by Carr (1983) as a potential threat to marine turtles of
lost-year size.
Feeding Selectivity
The sponge diet of Eretmochelys, as indicated by the samples, is
restricted to a relatively few taxa. Sponges belonging to the orders
Astrophorida and Hadromerida represented 97.6% of the dry weight of
all identified sponges. The order Spirophorida, which represented an
additional 1.15%, is considered by Wiedenmayer (1977) to be a suborder
within the Astrophorida. These represent three of the five orders of
the subclass Tetractinomorpha; the two not represented in the samples
are the Desmophorida , a group with a stony composition, and the
Axinellida, which includes several reef-dwelling sponges. The
remaining sponges, all cerac t inoraorphs, represented 1.25% of the
sponges identified.
That the Astrophorida, Spirophorida, and Hadromerida make up a
relatively small part of the Caribbean sponge fauna is evidence of
strong selectivity by foraging hawksbills. Figure 9 shows the com-
position of the sponge faunas at four localities in the Caribbean.
The number of species within each order present at each locality is
indicated. Slightly different classification schemes are employed by
the various authors. The order Choristida, used in the figure, is
synonymous wi th As trophor ida in the classification system of Levi
(1973), which has been employed in the present study. For comparison,
orders that include astrophorid and hadromerid genera (as defined by
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-80-
Levi, 1973) that were represented in the stomach contents of
hawksbills are marked with stippling. An average of 22% (range 13-27)
of the total number of species represented at each locality is
included in the stippled columns. This is an overestimate of the
percentage of the fauna represented in the digestive tract samples of
hawksbills, because not all genera or species within prey orders were
consumed. No comparable data have been published on the composition
of sponge faunas in the western Caribbean.
Another measure of feeding selectivity can be obtained by com-
paring Figure 9 to Figure 5. The latter shows the ordinal composition
of the sponges found in stomach contents. Hadromerids and
astrophorids represent less than a quarter of the fauna, and yet they
constitute 97.6% of the total dry weight of all sponges identified in
stomach contents.
All sponges included in Figure 9 are considered shallow-water
sponges. In the studies of Hechtel (1965), Wiedenmayer (1977) and
Cambiaso (1981), sponges were collected by diving with snorkel or
SCUBA gear. De Laubenfels' (1936) survey additionally included
dredged specimens, but only those collected in water less than 50 m
deep have been used in the figure. Considering the diving capacity of
the hawksbi 1 1 (individuals have been sighted at 80 m, Frazier, 1971),
nearly all of these sponges would potentially be available as food.
The order Keratosa (= Dictyoceratida plus Dendroceratida, Levi,
1973) was not represented in stomach contents except for a few small
fragments of the dendroceratid Halisarca. A small number of fragments
of keratose sponges were also seen in intestinal contents. This is
a large group, and as shown in Figure 9, one that is very well
-81-
represented in Caribbean sponge faunas. Van Soest (1978) listed 52
well established species (21 genera) of Keratosa in the West Indies.
Of the 33 species described in his study, 18 (10 genera) preferred
reef habitats.
The order Haplosclerida is another large group that was nearly
absent from the samples. Van Soest (1980) listed 62 West Indian
haplosclerids. Sixteen species (7 genera) of the 36 included in his
study were described as preferring reef habitats. Fragments of
Cal lyspongia and Cribocha lina (see Table 4) were the only material
representing this large order.
The order Poeci lose lerida, which also includes reef-dwelling
species, constituted only 0.63% of all identified sponges in the
stomach contents. No axinellids were represented. In the survey of
De Laubenfels (1936), the order Axinellida is treated as part of the
Halichondrida (see Figure 9).
The sponge diet of the hawksbill as reflected by the samples is
also restricted in terms of the number of genera and species repre-
sented. Only 22 genera (31 species) were identified in the stomach
contents of all turtles from all localities. Ten species accounted
for 87.4% of the total dry weight of all identified sponges. The
cumulative total of shallow-water demosponges present at the col-
lecting localities is unknown, but is certain to be well over one
hundred. De Laubenfels (1950b) listed 115 species from the West
Indies (excluding Bermuda). Over a hundred species of sponges occur
on the fore reef slope at one locality in Jamaica (Reiswig, 1973).
Feeding selectivity is also indicated by the high degree of
similarity in the sponge composition of digestive tract samples from
-82-
the widely separate geographic localities (Table 15). Many genera
were represented in all regions by the same species. Myriastra,
however, was represented by different species (a total of 6) at each
of three localities: Panama (1); Carriacou (2); and the Leeward
Islands (3). Both of the buccal cavity samples from live hawksbills
at Palm Beach, Florida, were Geodia. The fecal pellet from the
juvenile hawksbill captured in the Caicos Islands consisted entirely
of Chondrilla nucula.
In assessing the actual biomass represented by prey species, both
frequency of occurrence and size must be considered. A few prey
sponges — e.g., Spheciospongia vesparium, Chondri 1 la nucula, and
Geodia — are considered common. Ruetzler and Macintyre (1978) listed
S_. vesparium and G^. nep tuni among the ten most common siliceous
sponges at Carrie Bow Cay, Belize. Spheciospongia is also abundantly
represented on Jamaican reefs (Reiswig, 1973). Both of these genera
are also very large. S^. vesparium was reported by De Laubenfels
(1936) to be the largest representative of the phylum Porifera,
although data by Dayton et al. (1974) suggest that this species may be
rivaled in size by some species of Antarctic hexact ine 1 lids.
Specimens of G^. neptuni a meter in diameter have been observed
(Wiedenmayer, 1977).
Other genera in the samp 1 es--e.g. , Ancorina, Ec ionemia ,
Myriastra, and Placospongia — are poorly represented in faunal lists of
Caribbean sponges (De Laubenfels, 1936, 1950a; Hechtel, 1965;
Wiedenmayer, 1977; Carabiaso, 1981) and are considered relatively
uncommon by some sponge biologists working in the Caribbean and on the
Florida reef tract (S. Pomponi, pers. comm.; G. Schmahl, pers. comm.).
-83-
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-84-
The reason for their poor representation, or at least low apparency,
in known faunas is not clear. The sponges may simply have been
overlooked in these surveys because of sparse distribution or incon-
spicuousness, in which case their abundance in the digestive tract
samples is due solely to the feeding selectivity of the hawksbill.
Another possible explanation is that there may be greater between-site
variability in sponge faunas than is currently recognized. If this is
the case, these sponges may prove to be more common when additional
faunas are studied. Their poor representation may also be due to
other sampling artifacts; they may occur in microhabita ts that are
difficult to sample, such as caves and ledges, or at depths beyond
those normally sampled. Some prey genera are definitely known to
occur in deep water. Placospongia has been dredged from a depth of 70
m in the Florida Keys (De Laubenfels, 1936). Based on West Indian
records De Laubenfels (1950b) listed Placospongia and three species of
other prey genera (Ancorina, Myriastra, and Cinachyra) as deep water
(> 50 m) species. The distribution and abundance of prey sponges
clearly deserve further study.
Narrowness and specificity of the diet of the hawksbill are sup-
ported by data from other investigators. A mature male hawksbill
examined by Carr et al. (1966) at Tortuguero, Costa Rica, contained
only large amounts of Geodia gibberosa. A second specimen also con-
tained this sponge, as well as other invertebrates. In a later study
at this same locality Carr and Stancyk (1975) reported that Geodia
gibberosa was one of the two most important components in stomach
contents of 20 hawksbills. The tunicate Stye la was the other. G^.
gibberosa was present in 90% of the turtles they examined. The only
-85-
other sponges represented in more than 5% of the turtles were uniden-
tified choristids (= astrophorids), which were present in 25% of the
animals. Chondrllla nucula was one of the species they identified.
When the data of Carr and Stancyk (1975, p. 164) are considered
according to Levi's (1973) classification, all prey sponges are
hadromerids or astrophorids, except for one poecilosclerid identified
from a single turtle. In reexamining the material on which Carr and
Stancyk's (1975) paper is based, I found large pieces of the same
species of Suberites as identified in the present study, as well as
fragments of Placospongia.
Additional data on the species of sponges eaten by hawksbills are
available for several of the reports listed in Table 14. The juvenile
captured at St. Thomas, U.S. Virgin Islands, had been feeding on
Chondrllla nucula (W. Rainey, pers. comm.). The digestive tract of
the 61 cm individual captured at Andros Island in the Bahamas was
filled with Chondril la nucula, Geodia neptuni, and Polymastia sp. (W.
Rainey, pers. comm.). Polymastia is a hadromerid. Chondril la nucula
was also identified from the juvenile captured at La Parguera, Puerto
Rico (Erdman, unpub. ms.). Hawksbills have been reported to feed on
clionid sponges at Carriacou, Grenada (M. Goodwin, pers. comm.).
Clionids are hadromerids. It is notable that so many reports have
identified the same orders, and in some cases the same species, as
those found in the present study. The reports encompass a wide
geographic range in the Caribbean — Costa Rica, the U.S. Virgin
Islands, the Bahamas, Puerto Rico, and Carriacou, Grenada.
Two accounts in the literature report feeding on sponges other
than hadromerids and astrophorids. Hawksbills at Ascension Island
-86-
were reported to eat the keratose sponge Ircinia. This identification
was apparently based on a description, rather than examination of
specimens, and deserves further study (A. Carr, pers. comm.)» Two
species of sponges found in the digestive tract of a hawksbill
captured in the eastern Atlantic were not identified, but based on the
descriptions given (Den Hartog, 1980) are clearly not hadromerids or
astrophorids. One was a keratose sponge. Species identifications are
not available for any of the other reports listed in Table 14.
Role of Feeding Deterrents
The selective feeding habits of the hawksbill indicate that not
all sponges are acceptable as food. One aspect of the present study
was to investigate whether patterns in the diet were correlated with
the presence or absence in prey sponges of feeding deterrents such as
siliceous spicules, tough organic fibers, or secondary metabolites.
Inorganic Constituents
The large amount of silica present in important prey sponges and
the wide variation in silica content among the various prey species
suggest that siliceous spicules do not influence feeding patterns of
hawksbills. This conclusion is supported by data on the geometry and
placement of spicules in prey sponges. The large size and hook-like
shapes of spicules, and their concentration in thick, stony cortices,
are characteristics that should confer maximum deterrent effects. The
fact that sponges with spicules having these attributes are major com-
ponents in the digestive tract samples is evidence that spicules are
ineffective in deterring predation by hawksbills. Non-prey orders of
-87-
demosponges , by contrast, tend to have lower spicule contents, and
smaller and geometrically more simple spicules. In non-prey orders
there is no equivalent of the stony cortices of Geodia and
Placospongia. Ash content, which is a measure of total mineral con-
tent, closely parallels silica content in prey sponges. Because it,
too, is a measure of mechanical strength in sponges, wide variation
and high values for this parameter support the same conclusions.
Despite widespread acceptance of a defensive role for spicules in
sponges, previous studies have also revealed little evidence that high
spicule content or ash content in sponges deters predators. Randall
and Hartmann (1968) noted that two of the sponges most frequently
consumed by West Indian fish had a low spicule content, but they found
no correlation between spicule content and frequency of occurrence in
the diet among the next 20 most common species. Nine species of
astrophorids, including Geodia gibberosa, were among the 70 sponges
they identified. A high ratio of ash to organic matter is character-
istic of hexactinel lid sponges, which are a regular dietary component
of asteroid and nudibranch predators at McMurdo Sound, Antarctica
(Dayton et al., 1974).
Ash contents of intestinal samples from hawksbills provide a
crude estimate of the percentage of silica in the digesta. Micro-
scopic examination of the material before ashing confirmed its
siliceous composition (see Figure 7). Ash constituted 92.0%, 76.6%,
and 74.3% of the dry weight of three samples. Using 50% as a con-
servative estimate of the percentage of silica in digesta throughout
the digestive tract, it can be calculated that as much as 557 g of
silica are present at one time in an actively feeding adult turtle.
With the exception of other strict spongivores, few animals have
a comparable diet in terms of silica. Silica is a prorainant
structural component in a few groups of algae (notably diatoms),
protozoans (sarcodines, radio larians) , and plants (grasses and
cereals — Poaceae, sedges — Cyperaceae, and scouring rushes — Equi-
setaceae). In few, if any, of these groups, however, is silica
content comparable to that in sponges. In scouring rushes and rice,
which are considered to be among the most heavily silicified plants,
silica accounts for only 20% of dry weight (Kaufman et al., 1981).
Silica in grasses is often contained in projecting hairs or trichomas.
It is considered to act as a feeding deterrent to herbivorous range
animals. Diatoms are notably high in silica. Silica content of
frustules of some species is as high as 72% of dry weight (Voicani,
1981). The percentage of silica on a whole weight basis was not
given. It would be interesting to determine the silica content of
digesta of fish or microcarnivores that feed on diatoms.
The abrasive quality of the digesta of hawksbills deserves
discussion. Gut contents could not be handled without gloves and
tools. Spicules easily pierce human skin and cause painful reactions.
It is not clear how material of this abrasive nature is passed through
the tract without causing mechanical damage to the intestinal
epithelia. Scanning electron micrographs reveal that the tips of
spicules do become embedded in the tissues (Figure 8).
The extent to which spicules cause mechanical damage in
spongivores has never been investigated. Forrest (1950) reported that
spicules often pierce the stomach wall of the nudibranch Archidoris
pseudoargus. Bloom (1976, 1981) correlated the presence of spicule-
compacting organs in some species of spongivorous nudibranchs with the
-89-
consuraption of "non-reticulate" sponges, i.e., sponges in which the
spicules are not bound by spongin. The sponges consumed by hawksbills
are of this type. Nudibranchs that feed on reticulate sponge prey
were found to lack spicule-compac ting organs, but showed other
morphological adaptations, such as large radular teeth and muscular
intestines (Bloom, 1976, 1981). These characteristics were judged to
facilitate the handling of sponges containing spongin. I found no
evidence in hawksbills of gross morphological adaptations for handling
spicules. Large numbers of spicules were free throughout the large
intestine .
Copious mucus production by nudibranchs has been proposed as a
physiological mechanism for handling abrasive sponges in the diet
(Forrest, 1953). The sponge food of some dorid nudibranchs is
liberally coated with mucus produced by glands of the digestive tract
(Forrest, 1953; Fournier, 1969). Randall (1963) observed a thick coat
of mucus on sponges in the stomachs of angelfishes and proposed a
similar function. Mucus production by hawksbills was not addressed in
this study. Mucus present in the digestive tracts would have been
likely to have been destroyed by preservatives before the digesta were
examined. In the digestive tracts of the few turtles that I examined
immediately after they had been killed by fishermen mucus was not
conspicuous. The turtles were all gravid females, however, and may
not be representative because of low feeding rates.
Organic Constituents
Spongin (the spongin B of Gross et al., 1956) is a type of
collagen unique to sponges. It forms the macroscopic organic skeleton
-90-
of many species and is a component of a number of specialized
structures. Spongin is the organic constituent of sponges most often
implicated as a feeding deterrent. It can constitute a large per-
centage of the volume and dry weight of a sponge (e.g., 48.2% of the
dry weight of Myca le acerata, Dayton et al., 1974). The spongin
content of some keratose sponges may be even higher.
One of the highest correlations found between patterns in the
diet of hawksbills and assumed feeding deterrents involved spongin.
With the exception of the small group Homosclerophorida, the orders
Astrophorida, Spirophorida, and Hadromerida are the only edible (non-
stony) demosponges that lack this skeletal constituent. Spongin
fibers are present in all other sponges, and in many, form extensive
skeletons, either alone or in combination with inorganic elements. In
the skeletons of axinellids, poeci lose ler ids, haplosc ler ids, and
halichondrids, spongin is usually associated with silica. Sponges of
the Dictyoceratida and Dendroceratida, the keratose sponges, contain
no spicules, but instead have highly developed fiber skeletons. The
fibers in these two orders are either homogeneous, cored with a
medullary substance, or impregnated with foreign bodies such as sand
grains, exochthonous sponge spicules, or even radiolarian and
f oraminif eran skeletons. These fibers, as well as the spongin
filaments of Ircinia, also contain iron deposits in the form of
lepidocrocite (Towe and Ruetzler, 1968). Iron can constitute as much
as 5.5% of the dry weight of the fiber (Junqua et al., 1974). The
functional significance of this mineralization is unknown, but it can
be speculated that iron adds structural rigidity to the fibers and
thus enhances their defensive utility.
-91-
Spongin, with its various reiaforcements, provides strength and
elasticity to a sponge (Levi, 1973), but the apparent avoidance of it
by hawksbills is difficult to explain on the basis of mechanical
deterrence. Hawksbills have very powerful jaws, as evidenced by their
ability to feed on heavily silicifed sponges such as Geodia and
Plascospongia, and on very rubbery, carti lagenous species like
Chondrosia. The jaws of hawksbills are certainly more powerful than
those of the various angelfishes known to feed on fibrous sponges,
such as Ca 1 lyspongia (Randall and Hartman, 1968). In any case, one
would expect some predation on sponges with weak spongin fiber
development, but this is not the case.
One of the unusual properties of spongin that may be relevant to
the present discussion is its resistance to enzymatic hydrolysis
(Gross et al., 1956; Junqua et al., 1974). Spongin fibers have been
found to be resistant to diverse bacterial collagenases and other
proteolytic enzymes, and to mild acid or alkaline hydrolysis (Garrone,
1978). The fact that spongin is affected by cuprammonium hydroxide — a
reagent that dissolves cellulose — has led to speculation that there
are molecular interactions in spongin that are comparable to those
binding polysaccharide chains in cellulose (Garrone, 1978). Whether
or not spongin is digestible by hawksbills is not known. Even if one
assumes that it is not, this would not satisfactorily explain why it
is not eaten. Several of the sponges consumed by hawksbills contain
high levels of silica, which is totally indigestible. There is
circumstantial evidence for the avoidance of spongin by other sponge
predators. Both asteroid echinodems (Dayton et al., 1974) and dorid
-92-
nudibranch mollusks (Garrone, pers. coram.) have been observed to eat
around the spongin fibers.
Feeding patterns of hawksbills also show correlation with the
collagen fibril content of sponges. The types of sponges that were
found in the digestive tract contents are rich in collagen fibrils.
Collagen fibrils (the spongin A of Gross et al., 1956) are a struc-
tural form of collagen visible only with the electron microscope. The
fibrils are similar, if not Identical, to those found in connective
tissue throughout the animal kingdom (Bairati, 1972). Although
universally present in the phylum Porifera, the fibrils vary in
density in the interstitial stroma of various species (Garrone, 1978;
Wilkinson, 1979).
A high collagen fibril content imparts a dense, rubbery con-
sistency to a sponge. This is particularly apparent in species that
contain little or no silica, such as Chondrll la or Chondrosla. This
consistency could conceivably serve as a mechanical feeding deterrent
to some predators, but does not appear to discourage predation by
hawksbills.
A high collagen fibril content in sponges may represent a posi-
tive attribute from a predator's standpoint because of the nutritional
value they impart. The fibrils have been found to be among the most
highly glycosylated in the animal kingdom (Garrone, 1978). Carbo-
hydrates were found to constitute 15% of the ash-free dry weight of
collagen fibrils of Spongia graminea (Gross et al., 1958) and 10% of
the weight of fibrils of Ircinia variabilis (Junqua et al., 1974).
Data on the amino acid composition, nitrogen content, and carbohydrate
content of fibrils of various sponge species are given by Gross et al.
-93-
(1956), Gross et al. (1958), Piez and Gross (1959), Junqua et al.
(1974), and Garrone et al. (1975).
The nutritional value of sponge fibrils is dependent, however, on
their being digestible. Although they are structurally and bio-
chemically indistinguishable from those found in the rest of the
animal kingdom, they have the unique property of being resistant to
enzymatic hydrolysis (Garrone, 1978). The fibrils are unaffected by
collagenases of various origins and other proteolytic enzymes
(Garrone, 1978). Whether hawksbills are capable of digesting this
form of collagen is not known.
Carbohydrate-rich compounds (glycoproteins and acid mucopolysac-
charides) that are associated with the fibrils (Thiney and Garrone,
1970) may represent a more substantial and accessible source of
nutrition than the fibrils themselves. Various studies of the inter-
cellular matrix have revealed the presence of uronic acid,
hexosamines, acid polysaccharides, glycoproteins, and several sugars,
such as glucose, galactose, mannose, xylose, fucose, and arabinose
(Garrone, 1978). Although these compounds have been isolated from
sponges of diverse taxonomic groups — not all of which can be con-
sidered rich in collagen fibrils — some are known to be intimately
linked to the fibrils, and thus would impart additional nutritional
value to fibril-rich sponges.
Lack of knowledge of the nutritional requirements and digestive
capabilities of hawksbills makes it difficult to speculate further on
the significance of the patterns observed in the collagen composition
of sponges in the diet.
-94-
Chemical Constituents
Sponges have long been known to produce irritating and odorous
chemicals. As a result of recent interest in marine natural products
chemistry, there has been a concerted effort to isolate and charac-
terize these compounds. Because sponges proved to be a rich source of
novel compounds — particularly ones with antibiotic activity — they have
become one of the best studied marine invertebrate phyla (for reviews
see Minale et al., 1976; Minale, 1973).
Several functions have been proposed for secondary metabolites in
sponges, including predator deterrence (Bakus and Green, 1974;
Bergquist, 1978; Fenical, 1981; Thompson et al., 1983); facilitation
of feeding by the sponge (Bergquist, 1978); inhibition of nonsymbiotic
bacteria (Thompson et al., 1983); and participation in al lelochemical
interactions with other sedentary reef organisms (Jackson and Buss,
1975). Secondary metabolites are present in large amounts in sponges
(up to 13% of dry weight in Verongia aerophoba, De Rosa et al.,
1973a), and are known to be released into the surrounding sea water by
some species (Thompson et al., 1983). These two observations are
consistent with the hypothesis that metabolites serve to deter preda-
tion, although other functions are likewise supported.
There is abundant evidence that sponges have inhibitory, noxious
and sometimes lethal effects on other organisms. Sponge extracts in-
jected into laboratory rabbits, dogs, mice and fish cause hemor-
rhaging, hypertension, paralysis and death (Richet, 1906a, b; Halstead,
1965; Baslow, 1969). Brominated metabolites isolated from the sponge
Ap lysina fistularis have been shown to inhibit feeding by fish
(Thompson et al., 1983). Fish that are force-fed sponges have been
-95-
observed Co experience convulsions, paralysis, and death (De
Laubenfels, 1950b; Green, 1977).
A considerable number of Caribbean sponges have been found to be
toxic in bioassays using fish. Green (1977) found 27 of 36 species
(75%) of sponges from Veracruz, Mexico (Caribbean), to be toxic.
Bakus and Thun (1979) rated 31 of 54 (57%) Caribbean sponges from
Belize and Mexico as toxic.
Several sponge genera and species that were important in the diet
of hawksbills have been rated as toxic in bioassays with fish and
other laboratory animals. These include Chondril la nucula, Geodia,
Spheciospongia vesparium, Tethya actinia, and Suber ites. These
results do not, of course, indicate toxicity to hawksbills, but they
do reveal the presence of potentially toxic compounds in these
species. Because of the endangered status of hawksbills, direct
toxicity tests will probably never be carried out, and rightly so.
The susceptibility of hawksbills to sponge toxins has, however, been
demonstrated. Alcala (1980) attributed the deaths of several captive
Philippine hawksbills to ingested sponges. The identity of the
sponges was not known (Alcala, per. comm.).
Extracts of many prey sponges show antibiotic activity. The
significance of these observations is somewhat indirect. Antibiotic
activity is highly correlated with the presence of secondary
metabolites (Minale et al., 1976; Bergquist, 1978), which are, in
turn, implicated as feeding deterrents. Although the primary function
of secondary compounds remains unknown, their role in determining
toxicity and palatability in plants is widely accepted (Harborne,
1977).
-96-
Terpenes and brominated compounds are two major classes of
metabolites in sponges that may have a role in deterring predation.
The phylum Porifera has been described as one of the richest sources
of bromine-containing metabolites (Minale et al., 1976). These com-
pounds, which are apparently all of marine origin, produce strong
odors (Fenical, 1981) and are known to be emitted into the surrounding
sea water by some sponges (Thompson et al., 1983). Evidence that they
deter predation is based largely on studies involving predators of
marine algae (Norris and Fenical, 1982), but recent data (Thompson et
al., 1983) suggest the possibility that they play a similar role in
marine sponges. As do other halogens, bromine acts to enhance the
toxicity of other compounds, such as terpenes (Fenical, 1981).
The distribution of brominated compounds within the class
Demospongiae is reviewed by Minale et al. (1976). According to these
authors, brominated compounds are produced by sponges of several
orders, including the Dictyoceratida, Verongida, Poecilosclerida, and
Axinellida. No brominated compounds are listed from sponges identi-
fied in the diet of the hawksbill. Cimino et al. (1975) reported
negative results in tests for dibromotyrosine-derived compounds and
bromo-pyrro le derivatives — the two major categories of brominated
compounds in Porifera — for several hadromerid and astrophorid species,
including Suberites domuncula, Tethya aurantium, Chondril la nucula ,
and Geodia cydonium.
Sponges are also a rich source of terpenoids, which are known to
impart bitter flavor and toxic properties to marine algae (Norris and
Fenical, 1982) and to terrestrial plants and insects (Harborne, 1977).
Over a hundred terpenoids have been isolated from sponges, primarily
from the order Dictyoceratida (Minale, 1978). It is interesting to
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noCe that this order of sponges was completely unrepresented in the
digestive tract samples from hawksbills. Extensive reviews of the
distribution of terpenoids in sponges are given by Cimino (1977) and
Minale et al. (1978). According to these authors, terpenoids have
also been isolated from poecilosclerid, halichondrid, and axinellid
sponges, but not from astrophorids, hadromerids, or spirophorids. A
more recent reference (Bergquist, 1978), however, reports the isola-
tion of a toxic terpenoid from Cinachyra, a spirophorid genus that was
identified in the stomach contents. This discovery would seem to
suggest that further studies are needed to elucidate the chemical
composition of prey sponges.
Sponges also contain other classes of metabolites. Stierle and
Faulkner (1979) reported the isolation of five different metabolites
from the Caribbean sponge Chondrosia co 1 lee tr ix, including two
peroxides with antibiotic activity. Sponges of this genus were found
in the digestive tracts of 13 hawksbills.
Sponges exhibit the widest diversity of sterols in the animal
kingdom (De Rosa et al., 1973b). It is not known whether this group
of metabolites is involved in predator deterrence in sponges.
Steroids synthesized from sterols are used as defensive secretions by
three families of coleopteran insects (Blum, 1981). Sterols occur in
all groups of sponges, including Suberites, Aaptos, Spheciospongia,
Tethya, Geodia, Cinachyra, and Chondril la nucula (for review see Goad,
1976). Many different sterols can be present in an individual
species; seven to ten distinct sterols are common.
Secondary metabolites of sponges have been shown to be trans-
ferred to predators. In some cases, metabolites are concentrated and
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used by predators for their own defense (Burreson et al., 1975;
Faulkner and Ireland, 1977; Castiello et al., 1979). The nudlbranch
Phyllidia varicosa gains protection by secreting an odoriferous, toxic
substance that it concentrates from its prey sponge (Burreson et al.,
1975). Pathak and Dey (1956) remarked on the large amounts of high
molecular weight unsaturated fatty acids in hawksbills and noted that
their abundance distinguished the fat of Eretmochelys from that of
other turtles. It would be interesting to investigate whether this is
a consequence of the sponge diet.
The transfer of secondary metabolites may be responsible for the
toxicity that is occasionally exhibited by hawksbill flesh. Carr and
Stancyk (1975) commented on the possible role of sponges in the
numerous cases of human poisoning associated with hawksbills. Witzell
(1983) listed 15 countries around the world where hawksbill meat is
avoided, or rarely consumed, because of its reputed toxicity. Reiswig
(pers. comm.) reported that students experienced a contact reaction
from the blood of a dead hawksbill that was being autopsied.
Kittredge et al. (1974) described the evolutionary steps by which
secondary metabolites, primarily used for defense, may become feeding
attractants to specialized predators. Castiello et al. (1979) showed
that the nudibranch Pel todoris atromaculata was attracted by extracts
of its prey sponge and speculated that secondary compounds could be
involved in food localization. In this particular case, the secondary
metabolites were a sterol and an acetylenic compound. Terpenoids,
however, are also common in sponges and, because they produce strong
odors, are likely to be involved in chemical communication. Olfactory
cues would seem to be the most likely mechanism by which hawksbills
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distinguish their prey sponges. Because of great similarity in gross
appearance among prey sponges, it is difficult to see how visual cues
could operate.
It is evident that sponges represent a chemically diverse and
potentially toxic food source. On the basis of available evidence,
however, one cannot draw any conclusions as to whether prey sponges
are toxic to hawksbills, or whether the narrowness of the diet is in
any way related to sponge chemistry. It might be mentioned in this
context that, despite the restriction of the diet to three orders of
sponges, many species within each order were exploited, and diversity
within individual stomach samples was relatively high. As many as 10
species of sponges were found in individual stomachs (x = 3.4). One
notable exception was a hawksbill whose entire digestive tract was
filled with Tethya cf. actinia.
Randall and Hartman (1968) noted great diversity in the sponge
diets of angelfishes of the genera Pomacanthus and Holacanthus and
interpreted this as a strategy for feeding on toxic sponges. Over
forty species of sponges, representing diverse taxonomic groups, were
found in stomach contents of 26 individual queen angelfishes
(Holacanthus ciliaris). By comparison, 31 species of sponges were
identified from the stomach contents of 54 hawksbills. Three other
species of angelfishes showed similar diversity in their diets, with
24 to 28 species of sponges represented in each (Randall and Hartman,
1968). These authors argued that the smorgasbord type of feeding
would eliminate the risk of ingesting too much of a toxic sponge.
Freeland and Janzen (1974) refuted this general concept with data from
mammals. They argued that the presence of high concentrations of
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toxins in plants, and Che low dosage required to cause serious harm,
make the smorgasbord strategy untenable. Data on the secondary
metabolites of sponges suggest an analogous situation. Sponge toxins
occur in large amounts and are effective at low dosages. If one
assumes that there are toxic sponges in the diet of the hawksbilL, the
feeding strategy of the hawksbill might be interpreted as an energetic
compromise. While avoiding strict stenophagy as seen in nudibranchs —
a strategy that would probably be impossible because of large food
requirements — the diet of hawksbills is sufficiently narrow to
minimize energetic costs associated with detoxification.
Nutritional Characteristics of Prey Sponges
Prey sponges, particularly astrophorids and spirophorids, are low
in organic matter as compared to most other sponges. This may account
for the low values of energy and nitrogen observed for Geodia neptuni
and Cinachyra kuekenthali. Values for these parameters are more
nearly equivalent for all species analyzed when results are put on an
ash-free basis. Chondrilla nucula, the sponge that was the second
most frequently encountered in stomach contents of hawksbills, has the
highest energy content on a total dry weight basis, and a high
nitrogen content on both a total dry weight and ash-free basis. One
could speculate that high values for these parameters are due in part
to the high collagen fibril content of this sponge.
The nutritional value of some sponges may be enhanced by the
presence of macrosymbionts such as polychaetes, ophiuroids, shrimp,
etc., and of large amounts of symbiotic bacteria. Large numbers of
-101-
bacteria are frequently encountered in astrophorid sponges (Vacelet,
1977). In the present case, macrosymbionts were poorly represented in
the digestive tract contents and could not have contributed
significantly to the nutritional value of the sponges. The presence
of bacteria in prey sponges was not investigated.
Spongivory as a Feeding Niche
Sponges are eaten by other marine turtles, but apparently only to
a minor extent. Sponges represented an average of 2-9% of the dry
weight of fecal samples of 12 green turtles (Chelonia mydas) feeding
in an impounded tidal creek at Great Inagua, Bahamas (Bjorndal, 1979).
The sponge Haliclona rubens represented an average of 0.9% of the dry
weight of food samples from 243 green turtles captured off the eastern
coast of Nicaragua (Mortimer, 1981). Sponges were found in the
stomachs of three Pacific green turtles by Carr (1952). Two species
of sponges were identified as minor components in the diet of green
turtles at Oahu, Hawaii (Balazs, 1980).
Loggerheads (Caretta caretta) have also been reported to feed on
sponges (Carr, 1952; Layne, 1952; Brongersma, 1972; Mortimer, 1982).
Moodie (1979) found no evidence of sponges in fecal samples from 29
loggerheads captured in Australian waters. Although the feeding
habits of this species deserve further study, there is no evidence at
present that sponges are an important element of the diet.
Sponges have been reported in the diet of three freshwater
turtles. Specimens of Podocnemis expansa, a common river turtle in
South America, have been known to eat appreciable quantities of the
sponge Spongil la, but sponges apparently are not a significant part of
-102-
the overall diet (Ojasti, 1971). Freshwater sponges were a minor
component of stomach contents of a recently described Australian
chelid turtle, Rheody tes leukops (Legler and Cann, 1980). This
species is also a river-dweller. Sponges are relatively important in
the diet of the southern black-nobbed sawback turtle, Graptemys
nigrinoda delticola, which occurs in the Mobile and Tensaw rivers of
Alabama (Lahanas, 1982). The sponges Trochospongi 1 la leidyi and
Corrospongilla becki occurred in 46.7% of the males examined (N = 15)
and in 35.3% of the females (N = 17). The average percent volume
contribution of the sponges was 36.5% for males and 27.6% for females
(Lahanas, 1982). As far as I am aware, no other reptile eats sponges.
There are a number of generalizations about spongivory in the
literature that are worthy of mention in the light of data from the
present study. Spongivory is believed to be more common in tropical
than in arctic and temperate waters (Sara and Vacelet, 1973; Bakus,
1969). The hawksbill is, interestingly enough, the most confirmedly
tropical of the seven species of sea turtles. Toxicity of sponges, as
determined by bioassays with fish, shows a latitudinal gradient, with
the highest incidence being found in tropical waters (Bakus and Green,
1974; Green, 1977).
Bakus (1969, 1981) advanced the hypothesis that toxicity ±a
sponges, as well as in other sessile invertebrates, is negatively
correlated with crypticity. Regional differences apparently exist in
the abundance of cryptic vs. noncryptic sponges, the Caribbean being
noted for its diversity and abundance of exposed species (Randall and
Hartman, 1968; Bakus, 1964, 1969).
-103-
Another generalization that is made about spongivory is that in
temperate and arctic latitudes invertebrate spongivores predominate,
whereas in the tropics, both invertebrate and vertebrate spongivores
are present. Along with sponges, asteroid echinoderms and dorid
nudibranch mollusks dominate the epifaunal community at McMurdo Sound,
Antarctica (Dayton et al., 1974). There are apparently few, if any,
truly spongivorous fish in cold waters (Bakus, 1969). Spongivorous
angelfishes, filefishes, and the moorish idol are tropical in distri-
bution, as is the hawksbill.
Reiswig (1973) described the niche of spongivory as being made up
of a mosaic of species, with major predation within large geographic
regions being restricted to single taxa. He lists as examples the
asteroid echinoderms in benthic communities in Antarctica, echinoids
on coral reef sponges in Jamaica, and gastropods on temperate coastal
sponges. According to Reiswig (1973), no single taxon is responsible
for sponge predation throughout the world. If the hawksbill proves to
be spongivorous throughout its range, which seems likely, this concept
would need modification. The species occurs throughout the tropical
oceans of the world. As the largest known spongivore, the hawksbill
probably has had a significant evolutionary impact on sponge
populations and on the reef community.
There seem to be major differences in feeding strategies among
spongivores. The invertebrate spongivores, particularly the nudi-
branchs, tend to be highly specialized. Many live on the surface of
their prey and have highly coevolved relationships. The morphology of
the digestive tract is highly correlated with structural characteris-
tics of the prey sponge. Some species use spicules and chemicals
-104-
derived from their host for their own defense; others mimic the color
of their host by incorporating pigments.
Spongivorous fishes exhibit a very different, less specialized
pattern. Although they are all morphologically advanced teleosts, no
particular adaptations for sponge-feeding are known. Moreover, their
diet is not specialized. As previously mentioned, the diets of
angelfishes of the genera Pomacanthus and Holacanthus are remarkably
diverse.
Digestive tract samples from hawksbills suggest yet another
strategy. The breadth of the diet compares more closely with that of
angelfishes than nudibranchs, although it is decidedly less diverse.
It is, however, narrowly restricted to three orders of sponges.
Within these orders, a relatively large number of species are eaten.
No morphological adaptations for spongivory were noted in this study.
SUMMARY
1. Sponges were the predominant food item in digestive tract
contents of hawksbill turtles larger than 23 cm in carapace length.
The high percentage of sponges in the samples (x = 94.2% of dry
weight) and the high degree of homogeneity among samples from turtles
of different sexes, sizes (over 23 cm), and geographic origins provide
evidence that the species is a strict spongivore.
2. The presence of pelagic species of the alga Sargassum,
pelagic fish eggs, and other flotsam in digestive tract contents of
hawksbills smaller than 23 cm provides evidence that turtles of this
size class are associated with the Sargassum raft community.
3. Gravid hawksbills had little or no food in their digestive
tracts (x = 15.4 g dry weight vs. 616.8 g for nongravid adult females
and adult males), suggesting that they do not actively feed during the
reproductive period. Calcareous substrate material may be
purposefully ingested, possibly to replenish calcium reserves depleted
by egg shell production.
4. The sponge diet was found to be narrowly restricted to three
orders of tetractinomorph demosponges: Astrophor ida, Spirophor ida,
and Hadromerida. Representatives of these orders accounted for 98.8%
of the total dry weight of all identified sponges. Four major orders
of sponges with reef-dwelling representatives are poorly, if at all,
represented in the diet. Ten species accounted for 87.4% of the dry
weight of all identified sponges.
-106-
5. Siliceous spicules do not appear to serve as a feeding
deterrent to hawksbills. Digestive tract contents contain a high
percentage of ash, composed primarily of siliceous spicules. Scanning
electron micrographs show that spicules become embedded in the
intestinal epithelia. Prey sponges show a wide range of spicule
contents (0-51.6% of dry weight) and include species with stony
cortices. Astrophorid sponges are among the most highly silicified
demosponges.
6. Prey sponges lack spongin fibers, providing circumstantial
evidence of a deterrent function of spongin. The mechanism by which
spongin fibers could deter predation by hawksbills is not understood.
7. Prey sponges are characterized by a high content of collagen
fibrils. Carbohydrate-rich compounds associated with the fibrils
probably impart nutritional value.
8. Several prey sponges are toxic to fish and other laboratory
animals and contain compounds with antibiotic activity. Toxicity to
hawksbills is not known. Prey sponges do not belong to orders that
are notable producers of brorainated compounds and terpenoids,
metabolites which have been implicated as feeding deterrents. The
propensity of some classes of secondary compounds of sponges for being
transferred through the diet suggests a possible explanation of the
occasional toxicity that is exhibited by hawksbill flesh.
9. Most prey sponges are low in organic matter. Energy
contents of a few representative prey genera and species ranged from
7.64-15.66 kJg"-'- (dry weight basis). Nitrogen contents of representa-
tive prey genera and species ranged from 4.05-9.44% of dry weight.
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BIOGRAPHICAL SKETCH
Anne Barkau Meylan was born in St. Louis, Missouri, on February 9,
1952- In 1969, she graduated from Solebury School in New Hope,
Pennsylvania. She entered the University of Florida in 1970, and
received her Bachelor of Science degree in zoology in December, 1974.
In that same month she married Peter Andre Meylan. She completed the
requirements for the Master of Science degree in zoology at the
University of Florida in June, 1978.
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
f\\ (Z-tx.tlc^^ C^a^L^
Archie Carr, Chairman
Graduate Research Professor of Zoology
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
Jo
P
olm/CJ . Ewe
rofessor o
Ewel
f Botany
I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.
/:
^yt-^iy^ M
Ck^Mnt
QtRt
Carmine A. Lanciani
Professor of Zoology
This dissertation was submitted to the Graduate Faculty of the
Department of Zoology in the College of Liberal Arts and Sciences and to
the Graduate School and was accepted as partial fulfillment of the
requirements for the degree of Doctor of Philosophy.
December, 1984
Dean for Graduate Studies and Research
UNIVERSITY OF FLORIDA
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