Sancia E. T. van der Meij / B. T. Reijnen (CC BY 4.0)
Gall crabs (coral gall crabs)
Family Cryptochiridae (genera include Lithoscaptus and Hapalocarcinus)
Obligate coral dwellers that live in a pit or gall in the skeleton and feed on coral mucus and trapped particles; many hosts grow around them unharmed, while some corals show chronic irritation and decline.
Identification
A very small crab, females about 5-15 mm, living in a neat round hole, pit or tunnel in a stony coral, often with only claws and eyes showing at the opening. Colours vary with the species — white, pink, brown, or translucent claws with bright orange or pink markings have all been reported by keepers. The coral grows around the dwelling, so a perfectly round hole with a resident crab, in an LPS that inflates oddly or stays irritated in one spot, is the classic presentation.
Often confused with
Beneficial Acropora crabs (Trapezia, Tetralia), which live openly among branches and never occupy a sealed pit in the skeleton. Also confused with barnacles embedded in coral, which show a volcano-shaped shell and feathery feeding legs and have no claws or eyes at the opening.
What it targets
Stony corals: Trachyphyllia, Catalaphyllia (elegance coral), brain corals, Pocillopora and other LPS and SPS. The crab feeds on mucus and food the coral catches and may graze tissue at the pit edge; females are effectively trapped in the dwelling for life and carry eggs there.
How it spreads
Arrives inside its host coral from the wild — the crab and coral come as a package, and the first sign is usually the hole rather than the crab. Free-swimming larvae need a suitable host coral, so spread beyond the original colony in a tank is uncommon.
Control (safest first)
Observe first when the coral feeds and cycles normally: WetWebMedia guidance quoted for decades is that these crabs are rarely a problem and the coral usually grows around them; removal becomes the right call when that specific coral is declining. Catch it outside the burrow: some gall crabs wander at night, and one keeper simply waited for a pink gall crab to leave its Catalaphyllia burrow and grabbed it then — patience avoids surgery on the coral, although the crab hung on hard once caught. Tweezer extraction when the crab sits on the flesh or at the hole mouth: work gently, stop if the crab braces and tissue starts tearing, and flush the empty burrow clean with a turkey baster afterwards. One keeper removed a lima-bean-sized crab with eggs this way with the elegance coral unharmed and fully expanded afterwards. Last-resort probing of the burrow with a needle or rod is described in forum threads, but it kills the crab inside its hole and risks the coral; reserve it for a coral clearly failing from the irritation, and expect a round cylindrical scar hole to remain in the skeleton. No dip reliably evicts a gall crab from a deep burrow without stressing the host coral, so dips rank last despite being the first thing most keepers ask about.
Natural predators
No natural predators are listed.
Field notes
A Reef2Reef keeper spotted bright orange claws sticking out of an elegance coral, waited until the coral retracted slightly and the crab moved onto the flesh, and caught it before it could settle into a burrow; the elegance bounced back, although the same keeper reported an earlier removal from a brain coral ended badly for both animals. An Ultimate Reef keeper puzzled for months over why a green Trachyphyllia kept bloating while a blue one never did, then found a 5 mm crab with translucent, orange-striped claws in a hole; another member answered with a bright pink gall crab in a Catalaphyllia that seemed to do no real harm and was removed mainly to be safe. A WetWebMedia case from Lisbon involved a female gall crab carrying eggs, evicted with a needle from a distressed brain coral, leaving a clean round hole; the expert reply balanced the story by noting most gall crabs are harmless symbionts, Pocillopora being their most common host, and advising watchfulness rather than a tank-wide purge.