Echinoderm Spine Structure, Feeding and Host Relationships of Four Species of Dissodactylus (Brachyura: Pinnotheridae)

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 clypeastroids 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 clypeasteroid 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. mellitae 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.