Selection by two 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, Percnon gibbesi and Stenorhynchus seticornis, associating with an urchin, Diadema antillarum, in shallow water along the coasts of Tobago, West Indies. The crabs did not associate with other shorter-spined urchin species, 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. antillarum more often (11.6% of urchins) than did S. seticornis (3.5%) and an unidentified crab (1.5%). We found no evidence that P. gibbesi and S. seticornis differed in their selection of D. antillarum hosts. We also found no evidence that the crabs selected D. antillarum hosts providing a potentially safer microhabitat by having longer spines, occurring in denser populations, occurring nearer to each other, or occurring in deeper water (within the depth range of this study). These results suggest that selection of D. antillarum hosts by P. gibbesi and S. seticornis is random, with minimal interspecific competition.