It is found in the western Atlantic Ocean and the Caribbean Sea where it makes its home among floating mats of Sargassum seaweed.

[4] This crab, like other members of its family, is specially adapted for swimming rather than walking as its fourth pair of legs are modified with flattened, paddle-like surfaces.

[1][3] Portunus sayi is a swimming crab that lives among the tangled masses of Sargassum, a type of brown seaweed that floats in the Sargasso Sea.

[6] It is part of a community inhabiting these floating masses which includes fish, nudibranchs, crustaceans, hydroids, bryozoans and polychaete worms.

This is not surprising as young loggerhead turtles live among the floating mats of Sargassum algae feeding on the fauna found there.