Patterns of resource use within a population of xanthid crabs occupying bryozoan colonies

Heads, i.e. colonies, of the bryozoan Schizoporella pungens are discrete resource units which protect the xanthid crab Pilumnus sayi from predation. The heads were spatially clumped, temporally stable, and differed both qualitatively and quantitatively. The number of adult P. sayi was equal to the number of heads with preferred characteristics, yet a relative shortage of this resource is suggested with concomitant intraspecific competition. Adult crabs occupied heads individually except for infrequent cases of double occupancy by a male and female. Further spatial pattern among crabs was consistent with a hypothesis that males defend clumps of heads occupied by females.