Distribution and abundance of Dissodactylus mellitae Rathbun (Pinnotheridae) on Mellita quinquiesperforata (Leske) (Echinodermata)

Abstract

The distribution and abundance of a symbiotic crab, Dissodactylus mellitae Rathbun, were determined by monthly collections of its sand dollar host Mellita quinquiesperforata (Leske). Infestation of sand dollars varied from 36 to 96% and mean burden of crabs per sand dollar ranged from 0.4 to 2.7 (burden ranged from zero to eight). Both percent infestation and mean burden increased in the summer during juvenile recruitment, decreased in early fall, and remained steady throughout the winter. Three size classes of crabs were tested for randomness of their dispersion on sand dollars. Although analysis of all size classes together suggested that the choice of a host by D. mellitae is independent of the number of crabs already present, analysis of separate size classes revealed that D. mellitae is dispersed differently at different sizes. While juveniles showed gregarious settlement and young adults tended toward a solitary existence, mature adults exhibited random dispersion. Although the dispersion of the number of mature crabs was random, the distribution by sex was not. Of those adults living in groups of two, significantly more lived with a member of the opposite sex than would be expected in a random distribution. The possibility exists that the overall analysis of distribution may mask distributions which change with age. It is hypothesized that for D. mellitae changes in dispersion from uniform young adults to a nonrandom distribution of male-female pairs can best be explained by frequent movements between hosts.