Sally Lightfoot Crab

The Sally lightfoot crab has a remarkably thin, flat body that lets it squeeze into narrow rock crevices. Its carapace is olive green to brown with a thin gold line along the edge; its long, flattened legs have yellow-orange rings. The carapace reaches at most 3–4 cm across, but with its legs spread it covers a much larger area. In the hobby it is sold as the "Sally lightfoot"; because this name is actually also used for the much larger Grapsus grapsus, which lives semi-terrestrially on shoreline rocks, check the scientific name when buying.

In the aquarium it is known for grazing hair algae and other filamentous algae. When young it is largely herbivorous; toward adulthood its claws grow stronger and it turns to meaty foods.

Warning: this species is an invasive alien species that became established in the Mediterranean in 1999 and has now spread from Spain to the coasts of Türkiye. Under no circumstances should aquarium individuals or their young be released into the sea.

Its natural range is the Western Atlantic (Florida to Brazil), the Eastern Atlantic islands and the Eastern Pacific coasts from California to Chile. It hides on shallow, wave-swept rocky shores, mostly among the spines of sea urchins or under stones. When threatened it escapes sideways among the rocks with astonishing speed.

• Volume: at least 100 liters; a solidly built live rock layout with plenty of crevices.
• Water: salinity 1.023–1.026 specific gravity, pH 8.1–8.4, temperature 22–27°C.
• Reef compatibility: Reef-safe with caution. Most individuals leave corals alone, but hungry large adults may pick at polyps such as zoanthids and at food given to corals.
• Catching: because it escapes deep into the rocks, it is very hard to remove from the aquarium if it causes problems; the rockwork should be built with this in mind.
• Medication: copper-based medications are lethal; acclimate by the drip method for at least 1 hour.

Omnivorous. Hair algae and microalgae on the rocks are its staple food. When algae run short, give dried seaweed (nori), algae-based granules and, a few times a week, mysis or chopped seafood. A regularly fed individual does much less harm to other animals.

Active during the day as well, but skittish; it darts among the rocks when it sees an approaching shadow. Adults may catch very small snails that fit in their mouths, tiny hermit crabs, feather duster worms and small sleeping fish; this behavior is seen in hungry large individuals.

Good companions: clownfish, tangs, damselfish, medium-sized gobies and blennies; large snails.

Avoid: triggerfish, pufferfish and large wrasses, hawkfish; very small fish, dwarf shrimp, feather duster worms and other large crabs.

Kept singly. Two Sally lightfoot crabs in the same aquarium fight over territory and food; a molting individual may be killed at that time.

The female carries her eggs under her abdomen; the larvae live in the plankton for a long time. It is not bred in home aquariums. Its rapid spread in the Mediterranean is the result of this long larval stage.

• Molting: it sheds its shell as it grows; the discarded shell should not be mistaken for a dead crab.
• Lost legs: it may drop legs under stress or in fights; they are regenerated over the following molts.
• Sudden deaths: copper and rapid salinity changes.

Look at the folded tail section under the body (abdomen): it is wide and round in females, narrow and roughly triangular in males. Males' claws are usually slightly larger.