Sally Lightfoot Crab

Sally Lightfoot is the name of a type of crab that lives on the Atlantic and Pacific coastlines of the Americas, from Florida down to Brazil and Mexico down to Chile,(1,2,3,4) as well as on nearby islands such as the Galápagos (1,2,3,4,5) and along the Atlantic coast of Africa.(2,6) Their backs can measures about 5-8 centimeters across, and their body is bright red with various patterns (though their bodies are darker-colored when they are young), and have ten legs.(2,4) Sally Lightfoot crabs, also known as red rock crabs(1) (although the adults can really be many colors from red to orange to brown(4)), spend most of their time hiding away in rock crevices.(4,7) But when they come out to feed—mainly during low tide (4,5,7) and less sunny parts of the day (3)—they move with the remarkable agility and speed that give them their name, “Sally Lightfoot.”(2,4) While they eat, powerful waves often crash over them, but they survive this by flattening themselves against rocks and holding on tightly.(2,4) These crabs mainly eat red and green algae,(2,4,5) but will gobble down practically anything they can get,(4) including mussels, barnacles, other crabs, young sea turtles, dead fish, and the young of seabirds such as boobies.(3,4) Their big appetite means that they also help clean the beach of material such as broken eggs and dried bird and bat droppings.(3,4) Through their role as predators, grazers, and cleaners, and also through their role as prey for many animals including large birds, octopuses, Kemp’s Ridley sea turtles, and chain moray eels,(4) Sally Lightfoot crabs can be an important part of their environment.(3,5)

The Sally Lightfoot crab, sometimes called the red rock crab,(1) is a common sight on rocky beaches on the Atlantic and Pacific coastlines of the Americas, from Florida down to Brazil and Mexico down to Chile,(1,2,3,4) as well as on nearby islands such as the Galápagos (1,2,3,4,5) and along the Atlantic coast of Africa.(2,6) Adults, which have carapace widths of around 5-8 centimeters, are generally bright red, brown, or orange with various patterns, while young Sally Lightfoot crabs are darker-colored.(2,4) Sally Lightfoot crabs spend most of their time hiding away in rock crevices,(4,7) but when they come out to feed—primarily during low tide(4,5,7) and during less sunny parts of the day (3)—they move with the remarkable agility and speed that give them their common name.(2,4) While they feed, powerful waves often crash over them, but they are able to withstand these by flattening themselves against rocks and holding on tightly.(2,4) Although these crabs mainly eat red and green algae,(2,4,5) they will eat practically anything they can get,(4) including mussels, barnacles, other crabs, young sea turtles, dead fish, and the young of seabirds such as boobies.(3,4) They also clean the beach of other material such as broken eggs and bird and bat droppings.(3,4) Through their role as predators, grazers, and cleaners, and also through their role as prey for many animals including large birds, octopuses, Kemp’s Ridley sea turtles, and chain moray eels,(4) Sally Lightfoot crabs can play an important part in coastal ecosystems.(3,5)

Among other ecoregions, this taxon occurs in St. Peter and St. PaulRocks, a set of small rocky islands far out in the Atlantic Ocean between Brazil and the coast of West Africa. In particular, there are five islets, five very large rocks and a series of smaller skerries that comprise this formation.The rock type of this formation is ultramafic and not volcanic.

This is one of the few places on Earth where an underwater oceanic ridge breaks through the surface of the sea. This formation, also known as St. Peter and St. Paul Archipelago, is the second tallest deep sea mountain or megamullion. These isolated rocks function as a biogeographic oasis in the deep ocean, providing a prime niche for marine life to prosper nearer the ocean’s surface. While the islands are virtually devoid of terrestrial vegetation, the rich marine flora and fauna provides a food source to seabirds that reside and breed here.

Isolation from the mainland provides a habitat of significant ecological and biogeographical interest. More than 800 kilometers from South America, St. Peter and St. PaulRocks (0°56’N, 29°21’W) land area is estimated to be only 15,000 square metres. Composed of mylonitic peridotite, the submarine mountain of which these rocks are the pinnacles extends 4000 metres into the ocean depths. St. Peter and St. PaulRocks were visited by Charles Darwin in 1832 on the first Beagle expedition.

The structure of the islets is a rugged serpentine surface, consisting of numerous fissures, pinnacles and ridgelines.The meager terrestrial vegetation that does occur includes simplistic marine grasses, mosses, fungus and algae. A filamentous blue-green algae, Lyngbya spp, and a minute green algae as Stichococcus bacillaris are example flora.The ocean floor structure in the vicinity of the formation is very irregular as well as steep and rocky; silt bottoms do not occur to the north and south until attaining a depth of approximately three km.

While terrestrial flora is scarce, the isolated islands provide habitat for a rich benthic and littoral marine biota. This food source supports many seabirds, which are the only vertebrate wildlife found on the islands. Breeding seabirds found on the Rocks during the 1971 survey included Brown booby (Sula leucogaster), Brown noddy (Anous stolidus), and Black noddy (Anous minutus). All life-cycle stages of the booby were found during this survey, suggesting that their breeding was aseasonal. The bird eggs often fall prey to Sally Lightfoot Crabs (Grapsus grapsus), a marine invertebrate that is present in large numbers on the islets. The invertebrate element of the Rocks’ food chain primarily consists of microbial feeders. These include protozoa, nematodes (Acrobeloides, Diploscapter and Panagrolaimus genera), bdellodes rotifers, and certain mites (Scheloribates spp.).