George Chernilevsky (Public domain)
Gorilla crabs / xanthid crabs
Family Xanthidae (hairy xanthid hitchhikers, several genera)
Opportunistic omnivores that grow bolder with size, picking at coral polyps, snails and shrimp, and capable of catching small sleeping fish.
Identification
A stout crab, carapace roughly 1-5 cm across in tanks, mottled brown, tan or dark, with conspicuously hairy or bristly legs and carapace edges and heavy claws tipped in black or dark brown. Strongly nocturnal: by day only eyes or claw tips show at a rock hole, and damage appears overnight. The keeper rule of thumb quoted across sources: hairy body plus dark claw tips means trouble.
Often confused with
Emerald crabs (Mithraculus sculptus), which have smoother legs and flattened, spoon-shaped claw tips built for grazing algae rather than black pointed tips. Also confused with beneficial Acropora crabs (Trapezia and Tetralia), which are small, smooth and pale with banded legs, live openly between Acropora branches and defend the coral; hairy crabs hiding in rock or down in a millepora colony are a different matter.
What it targets
Coral polyps (SPS polyps, zoanthids, soft corals), snails, hermits, shrimp, and small bottom-resting fish as the crab grows. Even when young and mostly scavenging, appetite shifts toward meat with size.
How it spreads
Classic live rock hitchhiker, also arriving in coral colony bases. A single hidden crab is the usual story; multiple crabs turn up when a whole batch of wild rock goes in uninspected.
Control (safest first)
Leaned-glass or jar trap at night: bait a smooth-sided glass with a piece of shrimp or prawn and lean it against the crab's rock; the crab slides in for the bait and cannot climb the glass. Check and rebait nightly, and remove the bait in the morning before it fouls the water. Simple, safe and the most recommended first method. Bottle trap with an inverted neck, baited with shrimp and sunk slightly into the sand near the hole: works on many crabs, although at least one keeper reported the bait vanishing nightly for a month while the crab stayed free, so persistence matters. Rock removal and eviction: lift the crab's home rock out, and flush the hole with RO water or prod the crab out with a skewer over a bucket. Invasive but decisive, and keepers often use the teardown to find other hitchhikers. Rehome to a sump or refugium, or return to a store, rather than keeping it in the display: a sump exile keeps the animal alive with no risk to livestock. Never release aquarium animals to the wild.
Natural predators
No natural predators are listed.
Field notes
A WetWebMedia correspondent battled a suspected gorilla crab for almost a month: the pill-bottle trap was emptied of mussel every morning with no crab inside, and the win finally came from removing the home rock, flushing the hole with RO water and poking the crab out from behind with a toothpick. A Reef2Reef keeper found a hairy, black-tipped crab deep inside a millepora colony; after removal another member reported the same crab type had started eating SPS polyps in their tank, and the first keeper exiled theirs to the refugium, noting the colony had looked irritated while the crab lived in it. An Ultimate Reef keeper reported a xanthid killing two cleaner shrimp within twelve hours, even dragging one into its rock hole, and advised setting a crab trap nightly for the first weeks with new rock — that keeper caught five xanthids in total.