SELECTION BY 2 DECAPOD CRABS (PERCNON GIBBESI AND STENORHYNCHUS-SETICORNIS) ASSOCIATING WITH AN URCHIN (DIADEMA-ANTILLARUM) AT TOBAGO, WEST-INDIES

We studied the relationships among two species of decapod crabs, Percn on gibbesi and Stenorhynchus seticornis, associating with an urchin, D iadema antillarum, in shallow water along the coasts of Tobago, West I ndies. The crabs did not associate with other shorter-spined urchin sp ecies, suggesting that the crabs gained protection by associating with the relatively long-spined D. antillarum. Most D. antillarum (83.4%; n = 259) lacked an associating crab; P. gibbesi associated with D. ant illarum more often (11.6% of urchins) than did S. seticornis (3.5%) an d an unidentified crab (1.5%). We found no evidence that I? gibbesi an d S. seticornis differed in their selection of D. antillarum hosts. We also found no evidence that the crabs selected D. antillarum hosts pr oviding a potentially safer microhabitat by having longer spines, occu rring in denser populations, occurring nearer to each other, or occurr ing in deeper water (within the depth range of this study). These resu lts suggest that selection of D. antillarum hosts by P. gibbesi and S. seticornis is random, with minimal interspecific competition.