Resource quality, dispersion and mating prospects for crabs occupying bryozoan colonies

Abstract

Responses by the xanthid crab Pilumnus sayi Rathbun to varied resource conditions were tested in a series of laboratory and field experiments and from those results aspects of mating strategies were inferred. Crabs discerned differences in both quality and dispersion of their critical resource, the bryozoan Schizoporella pungens Canu et Bassler. Crabs preferred bryozoan heads > 17-ml volume displacement, heads attached to the substratum and heads with cavities. Crabs moved quickly to heads along straight paths and occupied heads alone. These results, combined with previously published data, implicated refuge from predation as the primary resource value of heads. A preference for clumped rather than dispersed heads was sex-dependent and males moved between heads more frequently than did females. Movement by males was inhibited by the presence of a larger male, with or without females present. When resource quality was controlled in the field and only head dispersion varied, clumped heads favored a spatial pattern among crabs consistent with large males amidst several females. Overdispersed heads resulted in that pattern plus individual females amidst several males. Smaller males on clumped heads also had apparently greater access to prospective mates than did their counterparts on overdispersed heads. A working model of flexible mating strategies for P. sayi is proposed.