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, and sea turtles. Eighty-two percent of 128 loggerhead turtles (Caretta caretta) checked, harboured a total of 172 crabs. Crabs inhabiting flotsam 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) which 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 history strategies depending on the substratum they inhabit.