Sally Lightfoot Crab

Fascinating facts about the Sally Lightfoot Crab

• Grapsus grapsus has a round carapace slightly longer than 8 cm (3.1 in) and legs whose tips touch the substrate. It lives among rocks just above the sea-spray limit and is agile, capable of leaping, making it hard to catch on turbulent shores.
• First catalogued by Carl Linnaeus in the 1758 10th edition of *Systema Naturae*, the crab was later collected by Charles Darwin aboard the HMS Beagle.
• When the Pacific coast of Mexico, Central America, and South America teems with these crabs, they can turn a barren rock face into a bustling market, munching primarily on algae but also sampling sponges, clams, other crabs, fish, and even bird eggs. In crowded or starving conditions they even turn on each other, a rare glimpse of cannibalism among coastal invertebrates.
• Scientists only split this crab from its Atlantic cousin, Grapsus adscensionis, in 1990 after spotting consistent color differences in their walking legs and distinct shapes of the first zoea larva. No genetic link has been found, confirming that two look-alike species live on opposite sides of the world.
• Observed on the Galápagos, these crabs sometimes hop onto marine iguanas to pick off ticks, acting as a tiny cleaning crew. Their ability to leap and sprint in any of four directions makes them almost impossible to catch, even for predators like chain moray eels and octopuses.

At a glance

About the Sally Lightfoot Crab

The Sally Lightfoot crab, Grapsus grapsus, is a strikingly agile inhabitant of the rocky intertidal zones along the Pacific coast from Mexico to northern Peru, including the Galápagos, and also the Atlantic coast of South America. Its round, flat carapace measures just over 8 cm (3.1 in) and shifts from the dark, camouflaged hues of juveniles to a dazzling palette of brown-red, pink, yellow or mottled patterns in adults. Broad, flat legs end in tips that barely touch the substrate, allowing the crab to sprint across slick stones and even leap, a feat that makes it notoriously difficult to catch. Its eyes swivel with remarkable speed, and its reaction time is so acute that early observers noted it seemed to anticipate a hunter’s move. A voracious omnivore, G. grapsus grazes primarily on algae but will sample sponges, mollusks, other crustaceans, fish, young sea turtles, bird eggs, droppings, bat guano and carrion, while its larvae feed on phytoplankton. In crowded or food-scarce conditions it may turn cannibal, and it even engages in a cleaning symbiosis, removing ticks from marine iguanas in the Galápagos. Though not prized as food, it serves as bait for fishermen and falls prey to chain moray eels and octopuses, completing its role in a dynamic coastal food web, a reminder of the delicate balance that defines these wind-blown shores

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