Sharing the surname of a famous Canadian folk singer, of whom I’ve been an ardent fan for many, many years (once again confirming that my profound nerdiness knows no bounds), the Sally Lightfoot crab, a.k.a. the nimble spray crab, of the Tropical West Atlantic, Caribbean, and Indo-Pacific, is a common offering at local fish stores specializing in marine livestock. This crab can make for an interesting aquarium resident, provided its true nature is understood and tankmates are chosen with care.
The Sally Lightfoot crab (Percnon gibbesi)—not to be confused with Grapsus grapsus, a common semi-terrestrial crab species found along the Pacific coast of the Americas and on the Galapagos Islands that shares this same common name—is a small (reaching approximately 4 inches in diameter, from leg tip to leg tip), vertically flattened crab with brown to olive-brown overall coloration and tan to yellow or orange bands on its legs. Among the Sally Lightfoot’s more endearing traits are the tiny antennae located above its eyes, which continually flick up and down to the amusement of onlookers.
P. gibbesi is a startlingly fast-moving crab that scurries about the rockwork and substrate, stopping to pluck anything edible with its front claws, which seem to be in perpetual motion. One could argue that its body shape and movements might be a bit too reminiscent of a fast-moving spider for the comfort of arachnophobic aquarium hobbyists.
This species is commonly sold as a harmless herbivore/scavenger. When specimens are young, that’s generally true enough. Smaller specimens will content themselves with plucking algae and uneaten food from on and between rocks (a job made easy by the crab’s flattened body), largely ignoring any tankmates that pose no threat to them.
However, before bringing that cute Sally Lightfoot home and adding it to your system, it’s important to be aware that this harmless, “herbivorous” crustacean isn’t always so well behaved in aquariums once it matures. At that point, this species tends to become more aggressive and predatory. It will even grab small fishes and other invertebrates if it’s able.
Feeding in captivity
In addition to food items they graze or scavenge/capture for themselves, Sally’s will accept just about any standard fare you can offer to fish, so feeding them in aquaria is pretty much no challenge whatsoever. Offer a variety of meaty and algae-based foods. Keeping them intentionally well fed may help to reduce the predatory tendencies of mature specimens as well—but there are no guarantees.
Because P. gibbesi can’t be trusted in the long term around small fishes, crustaceans (including others of its own species), and other invertebrates, tankmates must be chosen carefully. Any fish sharing the tank must be large enough to avoid getting grabbed by the Sally and must not include crustaceans on their own natural menu. Mature Sally Lightfoots aren’t reliably trustworthy around sessile invertebrates either, so I would discourage including one in a reef system. Besides, with this crab’s flattened body and lightning quickness, it could prove to be pretty tough to capture and remove a troublemaking specimen from an established reef system.