The crab Grapsus grapsus (known variously as "red rock crab", "abuete negro", and, together with other crabs such as Percnon gibbesi, as "Sally Lightfoot") is one of the most common crabs along the western coast of South America. It can also be seen along the entire coast of Central America and Mexico, and nearby islands. It is one of the many charismatic species that inhabits the Galápagos Islands, and is often seen in photos of the archipelago, sometimes sharing the seaside rocks with the marine iguanas (Amblyrhynchus cristatus).

G. grapsus is a typically-shaped crab, with five pairs of legs, the front two bearing small, blocky, symmetrical chelae. The other legs are broad and flat, with only the tips touching the substrate. The crab's round, flat carapace is just over 8 cm (3 inches) in length. Young G. grapsus are black or dark brown in color and camouflage 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. The ones seen on photographs of tropical island fauna are often bright orange or red with stripes or spots dorsally, blue and green ventrally, and sporting red claws and pink or blue eyes.

This crab lives amongst the rocks at the often turbulent, windy shore, just above the limit of the seaspray. It feeds on algae primarily, sometimes sampling plant matter and dead animals. It is a quick-moving and agile crab, and hard to catch, but not considered very edible by humans. It is used as bait by fishermen.

The species Grapsus grapsus and G. adscensionis were not separated until 1990. The latter is found in the eastern Atlantic, while the former is not .

They were sighted by Charles Darwin during his voyages on HMS Beagle, and also by the first comprehensive study of the fauna of the Gulf of California, carried out by Ed Ricketts, together with John Steinbeck and others. Steinbeck records :

Many people have spoken at length of the Sally Lightfoots. In fact, everyone who has seen them has been delighted with them. The very name they are called by reflects the delight of the name. These little crabs, with brilliant cloisonné carapaces, walk on their tiptoes, They have remarkable eyes and an extremely fast reaction time. In spite of the fact that they swarm on the rocks at the Cape [San Lucas], and to a less degree inside the Gulf [of California], they are exceedingly hard to catch. They seem to be able to run in any of four directions; but more than this, perhaps because of their rapid reaction time, they appear to read the mind of their hunter. They escape the long-handled net, anticipating from what direction it is coming. If you walk slowly, they move slowly ahead of you in droves. If you hurry, they hurry. When you plunge at them, they seem to disappear in a puff of blue smoke—at any rate, they disappear. It is impossible to creep up on them. They are very beautiful, with clear brilliant colors, red and blues and warm browns.

Man reacts peculiarly but consistently in his relationship with Sally Lightfoot. His tendency eventually is to scream curses, to hurl himself at them, and to come up foaming with rage and bruised all over his chest. Thus, Tiny, leaping forward, slipped and fell and hurt his arm. He never forgot nor forgave his enemy. From then on he attacked Lightfoots by every foul means he could contrive and a training in Monterey street fighting has equipped him well for this kind of battle). He hurled rocks at them; he smashed at them with boards; and he even considered poisoning them. Eventually we did catch a few Sallys, but we think they were the halt and the blind, the simpletons of their species. With reasonably well-balanced and non-neurotic Lightfoots we stood no chance.