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. quin- quiesperforata respectively, obtain over 80% of their food from host tissues whilst D. cal- mani appears to feed exclusively on the tissues of its c1ypeasteroid host, Clypeaster rosa- ceus. 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 com- mensal 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. lock- ing/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 offeed- ing 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 para- sitic 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 Meo- ma 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 dam- age 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 Mel- lita (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 (Cly- peastroida), 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 col- lection 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. Subjec- tive 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 586 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 Scien- tific). 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.