Trapezia digitalis
A rare find on Hawaii reefs: this small coral-guarding crab hides in branching coral heads in warm 25.5 to 31.5 C water.
A small, compact crab, only a few centimetres across, with a mottled brown body that blends into the branching coral it lives on. Look for it tucked deep between coral branches rather than out on open reef, with squarish flattened claws held close to the body. It rarely ventures far from its coral head, so spotting one usually means peering into the coral rather than scanning open water.
Spends its time wedged inside the branches of its host coral, rarely straying onto open reef where divers would spot it easily. Divers who find one usually do so by deliberately shining a light into coral heads rather than by chance during a normal swim-by. It reacts to disturbance by retreating deeper into the coral rather than swimming off.
Guard crabs like this one live in a tight partnership with their host coral, defending it from predatory snails and other threats in exchange for shelter, a relationship that makes them one of the more overlooked reef bodyguards.
No clear seasonal pattern shows up in the sightings, and its comfort range of 25.5 to 31.5 C matches Hawaii's reef water year-round, so any month is fair game.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Logged diving in the United States, specifically Hawaii, where it turns up tucked inside branching coral heads. Data on this species is thin outside Hawaii, so treat it as a Hawaii-focused find for now rather than a widespread reef species.
Mapping where Brown Guard Crab is logged…
The only confirmed diver sightings come from Hawaii, United States, where it hides inside branching coral heads. It is logged as rare even there, so finding one takes deliberate searching rather than a lucky glance.
There is no detected season for this species, and its warm-water niche of 25.5 to 31.5 C is met by Hawaii's reefs all year, so there is no specific month to target.
Look for a small mottled brown crab wedged inside the branches of a coral head rather than out on open reef. Its compact body and squarish claws help it stay hidden, so a dive light aimed into coral branches is often needed to spot one.
Exact size data is not confirmed for this species, but like other guard crabs in the Trapeziidae family it stays small, typically only a few centimetres across, which suits its life wedged between coral branches.
It lives permanently inside a single coral head and defends it from predators such as crown-of-thorns snails, earning it the guard crab name. In return, the coral gives it constant shelter from open-reef predators.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
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