Columbus crabs, Planes minutus live in the open ocean attached to floating substrata including Sargassum, flotsam, and sea turtles (Caretta Caretta). Crabs inhabiting flotsam occurred in mixed sex groups of adults and juveniles. In contrast, crabs living on sea turtles around Madeira occur in smaller groups which are typically adult heterosexual pairs. 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.