Trapezia cymodoce
Small guardian crab hiding deep in Acropora branches, most reliably spotted in Sabah, Malaysia and the Visayas.
A small, flattened crab that wedges itself into the branching structure of Acropora coral, usually visible only as a pair of eyes and claw tips poking out from between the branches. Body colour ranges through mottled reds, browns and purples that roughly match the host coral, with a broad, squarish carapace and legs adapted for gripping branches rather than walking on open reef. Look for the movement of a claw waving out from inside a coral head rather than a crab out in the open.
Spends nearly its entire life tucked inside the branches of a single Acropora coral colony, rarely if ever venturing onto open reef where a diver would easily spot it. When threatened, it retreats deeper into the coral rather than swimming off, so slow, careful looking into branching coral heads is the way to find one. Most sightings happen during a considered scan of coral structure rather than a passing glance, which is likely why it reads as uncommon rather than common at most sites.
This crab lives in a real two-way relationship with its host coral: it defends the colony against crown-of-thorns starfish and other predators by pinching and harassing them, and in return gets shelter and a share of the coral's mucus as food.
Comfortable across a wide 23 to 30C range, so it's a year-round find on the tropical reefs where it lives rather than a seasonal target. There isn't enough data to point to a specific best month, so any healthy Acropora coral is worth a look whenever you're diving.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most commonly logged on reefs in Sabah, Malaysia, with strong showings in the Visayas in the Philippines and across Sulawesi in Indonesia. Divers also find it reliably in the Central Atolls of the Maldives, plus scattered records in Queensland, Bali and Nusa Tenggara.
Mapping where Coral Crab is logged…
It turns up most reliably on reefs in Sabah, Malaysia, and is also a solid find in the Visayas in the Philippines, Sulawesi in Indonesia, and the Central Atolls of the Maldives.
There's no strong season in the data. It tolerates a wide temperature range, so it's present on tropical reefs year-round wherever its host coral is healthy.
Look inside branching Acropora coral heads rather than on open reef. You'll usually see just eyes or a claw tip poking out, with a body colour that blends into the coral's reds and browns.
It spends its life wedged deep inside a single coral colony and retreats further in when disturbed, so it rarely appears in the open. Divers usually need to slow down and scan coral branches closely to find one.
No. It's a small, shy crab that hides inside coral and poses no threat to divers.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
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