BULLETIN
OF MARINE SCIENCE. 32(2): 584-594, 1982
CORAL REEF PAPER
ECHINODERM SPINE STRUCTURE, FEEDING
AND HOST RELA TrONSHIPS OF FOUR
SPECIES OF DISSODACTYLUS
(BRACHYURA: PINNOTHERIDAE)
Malcolm Telford
ABSTRACT
Stomach contents of four species of Dissodactylus living on different host echinoids were
examined. Estimates were made of the relative degrees of host dependence of these crabs.
Dissodactylus primitivus, collected on the spatangoid urchins, Meoma ventricosa and
Plagiobrissus grandis, takes about 50 to 60% of its food from the hosts. Both D. crinitichelis
and D. mellitae, symbiotic with the c1ypeastroids Mellita sexiesperforata and M. quinquiesperforata respectively, obtain over 80% of their food from host tissues whilst D. calmani appears to feed exclusively on the tissues of its c1ypeasteroid host, Clypeaster rosaceus.
Differences in behavior and feeding habits can be attributed partly to the structure of host
spines, Allometric analysis and scanning electron microscopy indicate that the spines of C.
rosaceus are less porous than those of the other species examined. The spines of Mellita are
significantly more porous than others, and those of Plagiobrissus grandis are hollow. On
host species with porous spines, considerable areas are denuded by the feeding activity of
the crabs.
Morphometry of crab chelae is clearly related to feeding activity. Dissodactylus calmani,
with slender claws, has not been found with spines in the stomach whereas D. me/litae has
relatively small but very robust chelae and was always found to include spines in its diet.
Differences in feeding habits, morphometry and life cycles indicate that D. primitivus is
truly primitive, D. calmani the most specialized, and that D. crinitichelis and D. mellitae
occupy an intermediate position.
With few exceptions the known species of the genus Dissodactylus are commensal with or parasitic upon echinoids, particularly clypeastroids. The exact
relationship between these little crabs and their hosts has been the subject of
some speculation. Food preferences have been reported for only a single species.
In 1935 Glassell (cited by Hyman, 1955 and Dexter, 1977) observed that D. locking/oni eats the feces of its host and therefore appears to be a benign commensal.
On the other hand, Dexter (1977) reports that D. nitidus may cause extensive
damage to its clypeastroid host, Encope stokesi, by clipping away spines around
the lunules. Under laboratory conditions the clipped areas may become very large
and impair movement and feeding of the host. Presumably D. nitidus feeds, at
least in part, on the clipped spines, in which case it should be regarded as truly
parasitic. Although there are a dozen or more species of Dissodactylus reported
as symbionts on as many or more host echinoids (Schmitt et aI., 1973), except for
the above citations, no other mention has been made of damage to hosts nor offeeding activity. However, it has been generally accepted that the relationship presents
some advantages to the crabs.
In common with other pinnotherids, species of Dissodactylus are small and
have greatly reduced eyes. They show little morphological adaptatiQJ1 to the parasitic habit except for minor species differences in morphometry which adapt them
to individual host species. Many species, including those of Dissodactylus, are
host-specific or occur on only a small number of related host forms. This paper
presents data on the stomach contents of four Atlantic and Caribbean species
584
TELFORD:
HOST RELATIONSHIPS
OF D1SS0DACTYLUS
SPP.
585
which have different degrees of host dependence. Allometric analysis and SEM
study of host spines combined with measurements of crab chelae, provide some
explanation of feeding activities.
Note on Crab Species and Hosts
Dissodactylus primitivus Bouvier.-This
species is a common symbiont of Meoma ventricosa (Spatangoida) in Barbados (Telford, 1978) and in Jamaica, where
it is probably the unidentified species of Dissodactylus to which Chesher (1969)
referred. In the study reported here, material from both Barbados and Jamaica
was used. In the latter location D. primitivus was found also on Plagiobrissus
grandis (Spatangoida). This is a new host record for the species, which remains
the only one known to infest spatangoids. At Discovery Bay (Jamaica), the two
hosts occur in mixed flocks in very shallow water (1.5-5.0 m), and both harbored
abundant crabs. On Meoma the crabs generally produce no visible areas of damage but on Plagiobrissus they denude large, circular patches on the oral surfac(:.
The Jamaican specimens differed from those collected in Barbados by being
sparsely spotted and mottled with purple-brown.
Dissodactylus crinitichelis Moreira.-Although
reported to occur on several
species of hosts (Schmitt et al., 1973), specimens were only available from Mellita (Leodia) sexiesperforata (Clypeastroida) collected in Barbados.
Dissodactylus mellitae (Rathbun).-Specimens
were taken from the five-Iunuled
sand dollar M. quinquiesperforata in the region of Beaufort, North Carolina. This
species has also been reported to occur on other hosts. Like D. crinitichelis, D.
mellitae has never been reported free-living.
Dissodactylus calmani Rathbun.-Although
previously regarded as free-living
(Rathbun, 1918; Voss and Voss, 1955) a common host, Clypeaster rosaceus (Clypeastroida), has now been found. The first specimens were obtained from a
solitary host collected at Mosquito Island (B.V.I.). Many additional specimens
were collected (by Gerhard Pohle) at Bahia Honda, Florida, and by the author
at Discovery Bay, Jamaica. Subsequently the writer has examined a small collection in the Smithsonian Institution, Washington (cat #91220), consisting of
nine males and four females found by F. M. Bayer in 1950 " ... clinging to the
bottom of Clypeaster rosaceus" at Indian Key, Florida. The crabs were always
found on the oral (ventral) surface, usually close to the mouth. In Jamiaca an
estimated 25-30% of the sea biscuit population harbored crabs, usually only one
per host but the maximum number observed on a single host was seven. Subjective estimates suggest that the Bahia Honda population was more heavily infested,
probably over 50%. Dissodactylus calmani differs from all other species in having
a flatter, more quadrangular carapace with dark brown pigmentation and white ..
banded legs. The Caribbean specimens are distinctly rugose, those from Florida
smoother. If this species spends time freeliving, as suggested by Voss and Voss
(1955), its dark coloration might provide a significant measure of protection from
predation. Further, since C. rosaceus is dark colored and does not burrow, th(:
crabs remain inconspicuous when in situ on the hosts.
MATERIALS
AND METHODS
Collection and Preservation.-Host
organisms were colected by diving and placed in individual plastic:
bags. The associated crabs were picked off, chilled in a freezer to prevent autotomy and fixed in 10%
neutral butTered formalin for 48 h before transfer to 80% ethanol for storage. Prior to fixation, the
posterior margin of the carapace was raised slightly to ensure rapid penetration. Specimens of the
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BULLETIN OF MARINE SCIENCE, VOL. 32. NO.2.
1982
various hosts were injected peristomially and immersed in the same fixative for 48 h, then stored in
80% ethanol.
Crab Stomach Contents.-Following
dissection the stomach contents were strewn on microscope
slides in 50% ethanediol (ethylene glycol) which retards evaporation. Some permanent mounts were
made by air drying and heat fixing slides which were then covered with "Permount" (Fisher Scientific). Stomach contents were identified as of host origin (spines, pedicellariae, podia, spicules etc);
or of other origin (diatoms, algal filaments, crustacean setae and fine granules of sand).
Host Materials.-Temporary
mounts of surface tissues, food groove and gut contents were prepared
as above, for comparison with the contents of crab stomachs. Cleaned preparations of spines and
ossic1es were made by dissolving the soft tissues in commercial bleach (sodium hypochlorite). The
residual calcite structures were carefully washed in distilled water and rinsed in ethanol to promote
rapid drying.
Spine Allometry.-Individual,
uniformly shaped spines of several sizes, from each host species, were
measured by eye-piece micrometer and weighed on a Mettler ME 22 microgram balance. For each
type of spine the volume was modelled geometrically according to its shape. The volume-weight
regression line was calculated on log-transformed data, assuming a power curve relationship
W
= aVh
(I)
or,
W = ala. Vh
(2)
where "a" in equation 1is a complex constant comprised of two parts: a" the shape coefficient, depends
on how closely the geometrical model of volume fits the spines and a., the density coefficient, depends
on porosity. Calcite has a density of2.71 gcm3• Provided that at and a2are truly constant, the slope of the
line, "b," should be 1.00.
Scanning Electron Microscopy.-Washed
spines were dried, broken to expose the cross section,
mounted on stubs and sputter coated by a SEMPREP 2 (Nannotech Thin Films Ltd., Cambridge,
England) before scanning with a Cambridge 180 SEM. The outlines and pore spaces of cross sections
were traced from micrographs on to squared paper and porosities were estimated by counting squares.
Observations of Feeding.-Specimens
of D. primitivus were held individually without access to hosts
for 48 h, after which they were placed on living fragments of tests with the spines still moving.
Observations on whole urchins were very difficult because the crabs are photophobic and seek shelter
beneath the urchins, which are themselves often entirely buried. Crabs were observed for 1 hand
either sacrificed immediately or at intervals during the following 5 h.
RESULTS
Dissodactylus primitivus from M. ventricosa, Barbados .-Stomach and intestine
contents of 23 individuals were examined. Nineteen of them contained fragments
of spines from the host (Fig. 1), many with tissue still attached; two also had
recognizable fragments of pedicellariae. All of the crabs had ingested diatoms,
most often naviculoids but sigmoidal and discoidal forms were not uncommon.
A few had conspicuous plumose setae, probably from their own exuvia. Ingested
fragments of echinoderm spines ranged from 70 to 120 JLm in diameter and were
mostly short, in the range of 100-200 JLm, with the ends crushed, not cleanly cut.
Only one whole spine was found, 540 JLm in length. Stomach and gut contents
always formed a reddish-brown paste.
Dissodactylus primitivus from M. ventricosa, Jamaica.-Contents
of the digestive tracts of seven Jamaican crabs were examined. In all there were fewer host
spines, more diatoms, forams etc., and the contents formed a light gray paste.
Despite this it was apparent that they had all been feeding on their hosts at some
time.
Dissodactylus primitivusfrom P. grandis, Jamaica.-Gut
contents were available
for only four crabs from this host urchin. The stomachs were filled with a whitishgray paste which included numerous diatoms, forams, much granular debris pos-
TELFORD:
HOST RELATIONSHIPS
OF D/SSODACTYLUS
SPP.
587
Figure I. (Top left). SEM of stomach contents of D. primitivus (scale divisions = 0.10 mm). Material
includes granules of sand, numerous fine fragments and two large fragments of host (M. ventricosa)
spines. (Top right). SEM of transverse break of spine from C. rosaceus, (scale divisions = 0.10 mm).
Thick ribs and branches of stereom leave less tissue space (lower porosity) than in other spines
examined. (Bottom left). SEM of spine from M. ventricosa (scale divisions = 0.10 mm). Although
ribs appear closer than in previous example, the total pore space is greater. (Bottom right). SEM of
locomotory (type I) spine of P. grandis (scale divisions = 0.10 mm). Because it is entirely hollow
this type of spine is the least dense or most porous one examined.
sibly from finely pulverised spines but only two specimens included clearly recognizable pieces of echinoderm skeletal structures.
Observations on Feeding.-Of
six starved crabs, which could be seen externally
to have empty stomachs, (brownish stomach contents are usually visible through
the translucent carapace), only one specimen appeared to feed directly on a spine
held in front of the mouth by scraping it with the maxillipeds. Two collected
masses of mucus and debris on the chelae and third maxillipeds. Frequent movements of the maxillipeds and chelae moved the debris to the mouth where it was
held in place and slowly swallowed. This material included trapped diatoms, very
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BULLETIN OF MARINE SCIENCE, VOL. 32, NO.2,
1982
Table I. Echinoderm spine weight-volume relationships and porosities determined a1lometrically
and estimated from SEMs (*ranges of porosities estimated from trial and error simulations, see text)
Porosity
Species
Weight-Volume
C. rosaceus
M. ventricosa
P. grandis I
II
M. sexiesperforata
W
W
W
W
W
=
=
=
=
=
1.52Yl.O'
1.31Yl.03
1.05Yl.O'
0.82yo.90
0.nYO.77
e"
Allometric
SEM
0.985
0.974
0.963
0.856
0.871
42%
52%
61%
51-58%*
50--60%*
36%
44%
57%
small granules of sand and soft debris, possibly from epidermal cells. In color it
was indistinguishable from crab stomach contents and scrapings of Meoma tissues. The remaining three crabs were not seen to feed at all but when sacrificed
later they had their stomachs packed with fresh spines and tissue.
DissodactyLus criniticheLis.-Material
from the stomachs and intestines of 17 in-
dividuals was inspected, all of which included at least one fragment of a sand
dollar spine. In five the stomachs were fully distended with curved spines ranging
from 200 to 400 /Lm in length and about 60 /Lm in diameter. In addition to spines,
most individuals had a few diatoms in the stomach and in two cases the stomachs
were about half filled with diatoms and sand grains. The food grooves of living
M. sexiesperforata transport mucus-bound strands of fine sand grains, forams,
algal cells and general debris, which are moved towards the mouth by the sweeping action of podia. Located on either side of the four tracts of locomotory spines,
and flanking the anal lunule, the grooves are partially covered by curved spines.
The curved spines, averaging 420 /Lm in length, are broad at the tips (90 /Lm) and
narrow at the base (45 /Lm). It is these spines that occur most commonly in the
stomachs of the crabs. Shallow pressure drainage channels (Telford, 1981) lead
into the lunules and are equipped with two sizes of slightly sinusoidal spines
300-500 /Lm long: one type is 70 /Lm in diameter, the other only 30 /Lm. Fragments
of these are also common in the crab stomach contents.
Dissodactylus mellitae.-Only
two of five individuals of this species had fresh,
recognizable stomach contents. They were densely packed with Mellita spines,
curved and sinusoidal types, with an average length of 280 /Lm and diameter of
50 /Lm. One immature female (carapace length 2.5 mm) had ingested 22 spines.
Intestinal contents of all five individuals included small pieces of spines, diatoms
and unidentifiable granular debris.
Dissodactylus calmani.-Among
27 specimens dissected, only 11 were found to
have material in the stomach or intestine. The most abundant item in the digestive
systems was a fenestrated disc from the tips of the podia of C. rosaceus; some
individuals contained more than fifty of them. An oval shaped ossicle, notched
at one end, with or without a perforated plate was occasionally found. Neither
its role nor location in the host is known. In no cases were spines from the host
found. No differences in gut contents were seen between the Caribbean and
Florida specimens; in every case the material was a reddish-brown paste, exactly
the color of C. rosaceus tissues.
Allometry of Spines
Clypeaster rosaceus (Fig. 1).-Ambulatory spines are elongated cones with hard,
flattened tips and basal collars. Fifteen spines of this type ranging from 55 to 364
TELFORD: HOST RELATIONSHIPS OF DISSODACTYLUS
SPP.
589
Figure 2. (Left). SEM of non-locomotory (type II) spine of P. grandis, (scale divisions = 0.03 mm).
This type of spine is not hollow but its porosity increases with size. (Right). SEM of spine from M.
sexiesperforala (scale divisions = 0.03 mm). Spines of this sand dollar and of E. micropora are almost
identical. As in previous example, these spines appear to become more porous as their size increases.
were measured. In determining the volumes they were treated simply as cones,
ignoring the basal collars. The volume-weight relationship is shown in Table I,
with two estimates of porosity. The allometric determination was calculated assuming that the conical model was perfect, which probably yields an over estimate
(see below). Tracing SEMs, on the other hand, tends to give an under estimation
of porosity because of the difficulty in deciding precisely which parts of the calcite
structure are exactly in the plane of focus in the micrographs.
f.Lg
Meoma ventricosa (Fig. I).-Fifteen
small spines, from spaces between ambu-·
latory spines, ranging from 24 to 169 f.Lg were selected for uniformity of shape.
They were modelled as slightly tapered cylinders with bluntly rounded tips and
the volume was calculated using a mean diameter between that of the base and
the tip; estimated porosity is shown in Table 1.
Plagiobrissus grandis (Figs. 1, 2).-This species presented some problems be··
cause of major differences between spine types. Locomotory and similarly shaped
spines are elliptical in cross section, only slightly tapered and they are hollow.
Volumes were calculated as for ellipsoids, using major and minor diameters and
lengths (type I spines in Table 1): data are given for ten spines, 47 to 342 f.Lg.
Non-ambulatory (type II) spines are not hollow and are best treated as simple
cylinders. Both types of spines are curved: length was measured as the cord of
a circle and the arc length was calculated later. Data for 15 spines are shown in
Table 1. The slope of the regression line departs significantly from 1.00, and this
appears to be due to decreasing density as the spines become larger. By trial and
error a similar line was generated (W = 0.840 VO.907),assuming that the geometrical model was 90% correct and that, over the size range in question, porosity
increases uniformly from 51% to 58%. A one-sided t-test indicated that there was
no significant difference between this hypothetical line and the observed regression (P > 0.999).
Mellita sexiesperforata (Fig. 2).-Data from 18 spines, 8 to 66 f.Lg, provided a
result similar to type II spines of P. grandis (Table 1) and were amenable to the
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BULLETIN OF MARINE SCIENCE, VOL. 32, NO.2,
b
1982
c
a
d
Figure 3. Chelipeds of a, D. primitivus; b, D. crinitichelis; c, D. mellitae; and d, D. calmani. Despite
the general similarity of appearance, there are appreciable differences in proportions: c is much more
robustly built than the others whilst d is more slender. See Table 2 for details.
same trial and error simulation, indicating a porosity of 50-60% and 90% agreement with a cylindrical model. Estimates from the SEMs are in accordance with
this figure and comparison of SEMs for Encope micropora (not figured) show the
two species of sand dollars to be indistinguishable in this regard.
Following the analysis of those spines with varying densities, both of which
indicated that the chosen geometrical model was only 90% correct, porosities
were recalculated for C. rosaceus (38%), M. ventricosa (47%) and P. grandis
(55%), assuming that volumes had likewise been overestimated by the geometrical
models chosen. These values for porosity agree reasonably well with estimates
Table 2. Carapace measurements (mm) and relative size of chelipeds for four species of Dissodactylus (* relative size = propodus length/carapace length; t robustness = circumference/propodus
length)
Carapace
Species
D. calmani
D. primitivus
D. crinitichelis
D. mellitae
Sex
Length x Width
F
M
4.35
3.95
X
F
M
6.95
6.50
X
F
M
3.65
3.65
X
F
M
3.65
3.70
X
X
X
X
X
Relative Size-
Robustnesst
4.64
4.35
0.46
0.47
1.18
1.17
8.90
8.35
0.47
0.51
1.35
1.53
3.94
4.06
0.51
0.54
1.50
1.52
3.82
3.96
0.35
0.30
1.69
1.98
TELFORD:
HOST RELATIONSHIPS
OF DISSODACTYLUS
SPP.
591
Figure 4. SEM of aboral surface of juvenile M. sexiesperforata with second crab instar of D.
crinitichelis posed beside devastated area where crab had been feeding (scale divisions = 0.30 mm).
from SEMs. The differences between species in spine porosity were shown to be
highly significant by t-test: comparing M. ventricosa with P. grandis (P = 0.01)
and with C. rosaceus (P < 0.001).
Cheliped Morpholgy
The chelae of all four species are essentially alike in form (Fig. 3). The palms
and fingers curve inwards towards the midline; the finger tips are hooked and
overlap with the movable finger inside. Dissodactylus primitivus has two or three
small teeth near the base of the movable finger and three or four more extending
to the mid-point or beyond on the immovable finger. The fingers and teeth gape
when the claw is fully closed. There is only a single tooth at the base of the
movable finger of D. crinitichelis, with three or four on the immovable one. The
rest of the finger length is sharp edged and the fingers meet along their lengths
when the claw is closed. Neither D. mellitae nor D. calmani have distinct teeth:
in both, the fingers are sharp edged, slightly irregular and overlap along most of
their length when the claw is closed.
Despite their similarity in form, however, the chelipeds differ in shape and
proportions when compared to the rest of the body. Table 2 shows carapace
measurements for both sexes of each species. The cheliped measurements for
these individuals are expressed in terms to show the relative size of the claw
(propodus length/carapace length) and the robustness of the claw (circumference!
propodus length). Circumference was used here instead of width because the
propodi are elliptical in cross section and both axes are variable.
DISCUSSION
It is evident that these four species of Dissodactylus feed upon the tissues of
their various hosts and are, therefore, to be regarded as parasites. The nature and
extent of their depredations on the hosts varies. On Meoma, D. primitivus ap-
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BULLETIN OF MARINE SCIENCE, VOL. 32, NO.2,
1982
pears to act as both parasite and scavenger; some of its food is obtained by
clipping healthy spines and some consists of material foreign to the host. Fewer
specimens were available for examination from Plagiobrissus and their stomach
contents suggested that they might perform more scavenging here. However, on
this host the crabs cause extensive damage by clipping spines. It is interesting to
note that D. primitivus has been reared to maturity in the total absence of a host
(Pohle and Telford, In Press). When laboratory reared individuals were presented
to a host Meoma, they immediately proceeded to feed on the spines and cleared
away considerable areas. These individuals, in fact, caused more host damage than
has been observed in the field. A conservative estimate suggests that normally, under
field conditions, D. primitivus probably takes 50-60% of its diet from host tissues.
Both D. crinitiche/is and D. mellitae living on various species of sand dollars,
derive about 80% of their stomach contents directly from the hosts. The remainder, plant cells and sand grains, most likely comes from the food stream of the
sand dollars. Before metamorphosis the megalopa of D. crinitiche/is feeds on
host tissues and despite its small size, can cause appreciable damage, as shown
by the SEM picture of a juvenile M. sexiesperforata (Fig. 4). Finally, D. calmani,
at least when it occurs on Clypeaster, appears to be wholly dependent upon its
host and feeds exclusively on soft tissues. This latter species seems to be the
most highly adapted to life as a parasite, insofar as it specializes in taking the
richest food supply. Moss and Lawrence (1972) noted that the spines and epidermis of M. quinquiesperforata consisted of about 12% combined lipid, protein
and carbohydrate by dry weight and that this was typical of other echinoderms
investigated. It is, presumably, this nutrient material, distributed throughout the
spine stereom, which these species of Dissodactylus digest.
Differences in spine structure of the echinoids exploited by the crabs provides
one explanation of the differences in their feeding activities: spine depredation
is proportional to spine porosity. Echinoderm skeletal components have an average porosity of about 50% (Currey, in Wainwright et aI., 1976), which agrees
well with the values obtained here. Rough calculations from the data of Moss and
Lawrence (1972) for sand dollars also yield similar figures. Spines of those hosts
on which the crabs feed most abundantly have greatest porosity and, in the case
of sand dollars, are also very small. Selection of highly porous spines in feeding
specialization is probably based on two factors: nutrient content and the ability
of the crabs to clip them. Small spines of Mellita and Plagiobrissus become more
porous as they increase in size, which might be expected because the calcite
elements of the stereom appear to be approximately constant in size within any
one species. This increase in pore space implies a corresponding increase in
nutrient value. However, at the same time the spines will become less available
for clipping by the claws because of their increasing dimensions. There should
be an optimal size of claw for each species of host infested. Dexter (1977) has
reported that D. nitidus clears away spines from around the lunules of Encope
stokesi. This species was not available in this study but the similar E. micropora
was examined by SEM and shown to have very porous spines. It appears most
likely that all species of Dissodactylus parasitize their hosts and it is improbable
that D. lockingtoni (Hyman, 1955) derives any extensive nutrition from the feces
of its sand dollar host.
Associated with these differences in feeding habits, spine-clipping species have
more robust claws, reaching an extreme in D. mellitae which must also be one
of the smallest brachyuran crabs known. Dissodactylus calmani has slender,
sharp-edged claws, specialized for precisely excising the soft podia of its host.
It is perhaps significant that the host of this species, which has specialized more
TELFORD: HOST RELATIONSHIPS OF DISSODACTYLUS
SPP.
593
than any other in taking soft tissues, is the echinoid with the least porous and
most difficult spines to clip. The flattened chelae of D. calmani are the least
typical of the entire genus; all other species examined appear to have claws
adapted for cutting or crushing spines. The general proportions and tooth structure of the claws of D. primitivus, in keeping with its more generalized feeding
habits, are the least specialized chelae represented in this series and the species
is to be regarded as truly primitive.
In common with other species of pinnotherids, species of Dissodactylus are
small as adults. There is a rough correlation of adult size with size of host species.
Thus, D. primitivus, the largest species in this study and perhaps in the genus,
occurs on large spatangoid urchins, whilst D. mellitae and D. criniticheLis, the
smallest species in the genus, occur on relatively small species of sand dollars.
Individually, however, the correlation is much less satisfactory; small crabs may
often be taken on large hosts and vice versa. Further, it should be noted that D.
calmani is also very small, rarely, if ever, exceeding 5 mm in width, yet it occurs
on a large and robust host. All species have reduced eyes with very small visual
areas, especially D. calmani. Reduction of the eyes would certainly be expected
in internal parasites but only D. primitivus has been reported to enter the host
urchins via the mouth (Telford, ]978). Living on echinoids which burrow could
also lead to reduction in the eyes and this is the habit of most of the known hosts.
Abbreviation of larval life histories is probably another adaptation to parasitic
existence (Goodbody, ]960). For D. crinitichelis and D. primitivus larval life
spans are only about 8 days (Pohle and Telford, ]98]; In Press). Dissodactylus
primitivus can be reared in the absence of a host and is thus a facultative parasite
but D. crinitichelis is an obligate parasite, requiring a host in order to complete its
development. Host specificity might also be an indicator of the degree of parasitic
adaptation. Of the four species described here, only D. calmani has been reported
to have a single host but this might well be due to lack of published field observations.
In summary, several features suggest that these four species of crabs form a
progressive series of parasitic adaptation. Dissodactylus primitivus is a facultative
parasite, eking out an existence as both scavenger and parasite. The name given
the species by Bouvier (19]7) is well merited. Dissodactylus crinitichelis is an
obligate parasite, which is most likely the case for the very similar D. meLlitae.
The species which shows the greatest morphological adaptation to this life style,
D. calmani, is the only one which has been reported free-living. Further information on its larval life history and behavior away from the host would be especially interesting.
ACKNOWLEDGMENTS
This work has been supported by the Natural Sciences and Engineering Research Council of Canada
through Operating Grant # A4696. Thanks are due to the directors of the Bellairs Research Institute
of McGill University in Barbados and the Discovery Bay Laboratory of the University of the West
Indies, Jamaica for use of their facilities; to Mr. E. Lin, Department of Zoology, University of Toronto
for technical assistance in SEM; to Dr. F. A. Chace, Jr., for making available specimens in the
collection of the Smithsonian Institution, Washington, D.C., and to Mr. R. Mooi for the drawings of
chelipeds.
LITERATURE
CITED
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americaines du "Hassler" et du "Blake." Bull. Mus. Nat. Hist. Paris, 23: 391-398.
Chesher, R. H. 1969. Contribution to the biology of Meoma ventricosa (Echinoidea: Spatangoida).
Bull. Mar. Sci. 19: 72-110.
594
BULLETINOF MARINESCIENCE,VOL.32,NO.2, 1982
Dexter, D. M. 1977. A natural history of the sand dollar Encope stokesi L. Agassiz in Panama. Bull.
Mar. Sci. 27: 544-551.
Goodbody, I. 1960. Abbreviated development in a pinnotherid crab. Nature, Lond. 185: 704-705.
Hyman, H. L. 1955. The Invertebrates, vol. IV. Echinodermata. McGraw-Hill, New York, 763 pp.
Moss, J. E., and J. M. Lawrence. 1972. Changes in carbohydrate, lipid, and protein levels with age
and season in the sand dollar Mellita quinquiesperforata (Leske). J. Exp. Mar. BioI. Ecol. 8:
225-239.
Pohle, G. and M. Telford. 1981. The larval development of Dissodactylus crinitichelis Moreira,
1"901(Brachyura: Pinnotheridae) in laboratory culture. Bull. Mar. Sci. 31: 753-773.
--and ---.
In Press. The larval development of Dissodactylus primitivus Bouvier, 1917
(Brachyura: Pinnotheridae) reared in the laboratory. Bull. Mar. Sci.
Rathbun, M. J. 1918. The grapsoid crabs of America. Bull. U.S. Nat. Mus. 97: 1-461.
Schmitt, W. L., J. C. McCain, and E. S. Davidson. 1973. Pages 16-21 in H. E. Gruner and L. B.
Holthius, eds. Crustaceorum catalogus III. W. Junk, Den Haag.
Telford, M. 1978. Distribuution of two species of Dissodactylus (Brachyura: Pinnotheridae) among
their echinoid host populations in Barbados. Bull. Mar. Sci. 28: 651-658.
Voss, G. L., and N. A. Voss. 1955. An ecological survey of Soldier Key, Biscayne Bay, Florida.
Bull. Mar. Sci. 5: 203-229.
Wainwright, S. A., W. D. Biggs, J. D. Currey, and J. M. Gosline. 1976. Mechanical design in
Organisms. Edward Arnold Ltd., London. 423 pp.
DATE ACCEPTED: December 8. 1980.
ADDRESS:Department of Zoology, University of Toronto, Toronto M5S lAl, Canada.