Sally Lightfoot Crab

Grapsus grapsus is a typically shaped crab, with five pairs of legs, the front two bearing small, blocky, symmetrical chelae (claws). The other legs are broad and flat, with only the tips touching the substrate. The crab's round, flat carapace is slightly longer than 8 centimetres (3.1 in). Young G. grapsus are black or dark brown in color and are camouflaged well on the black lava coasts of volcanic islands. Adults are quite variable in color; some are muted brownish-red, some mottled or spotted brown, pink, or yellow.

Grapsus grapsus is found along the Pacific coast of Mexico, Central America, and South America (as far south as northern Peru), and on nearby islands, including the Galápagos Islands. It is also found along the Atlantic coast of South America, but is replaced in the eastern Atlantic Ocean (Ascension Island and West Africa) by its congener Grapsus adscensionis.

This crab lives among the rocks at the often turbulent, windy shore, just above the limit of the sea spray. It feeds on algae primarily, sometimes sampling other plant matter, sponges, mollusks (such as clams), crustaceans (including other crabs), fish, young sea turtles, bird eggs and droppings, bat guano and dead animals (mainly seals and birds). As larvae, they feed on phytoplankton. They have been known to resort to cannibalism when populations densities are high or food is scarce. It is an agile crab, capable of leaping, and consequently hard to catch. Not considered very edible by humans, it is used as bait by fishermen. It is preyed upon by the chain moray eel, Echidna catenata, as well as by octopuses.

G. grapsus has been observed in an apparent cleaning symbiosis taking ticks from marine iguanas on the Galápagos Islands.