COMPARISONS OF SOCIAL-STRUCTURE OF COLUMBUS CRABS LIVING ON LOGGERHEAD SEA-TURTLES AND INANIMATE FLOTSAM

Columbus crabs, Planes minutus (Crustacea: Brachyura) live in the open ocean attached to floating substrata including Sargassum, flotsam, an d sea turtles. Eighty-two percent of 128 loggerhead turtles (Caretta c aretta) checked, harboured a total of 172 crabs. Crabs inhabiting flot sam occurred in mixed sex groups of adults and juveniles, with a mean group size of 8.2 +/- 11.8 (N = 17). In contrast, crabs living on sea turtles around Madeira occur in smaller groups 1.6 +/- 0.5 (N = 105) w hich are typically adult heterosexual pairs (61 out of 105). Crabs on turtles were bigger than on flotsam and a higher proportion of female crabs were brooding eggs. Columbus crabs exhibit different life histor y strategies depending on the substratum they inhabit.