Tetralia nigrolineata
A pinhead-sized crab living inside Acropora coral, most reliably found across the Central Atolls of the Maldives on warm reef flats.
A tiny, flattened crab that wedges itself between the branches of Acropora coral, usually pale cream to white with fine dark lines or mottling across the carapace that break up its outline against the coral skeleton. Its body is broad and shield-shaped, built to slot into the narrow gaps between coral branches rather than swim or walk in the open. You will not spot it by scanning the reef at large; you spot it by looking closely at a single healthy Acropora colony and waiting for the small pale shape to shift.
Look for it tucked among the branches of live Acropora coral, where it spends its time grazing on the coral's mucus and fending off small threats to its host colony. It tends to hold still or retreat deeper into the branches when a torch or hand comes close, rather than bolt across open reef, so patience and a slow approach with a torch or macro lens pays off. Divers most often find it during a deliberate close look at a single coral head rather than while cruising past.
It lives its whole adult life on a single Acropora coral colony, cleaning the coral's surface and chasing off small predators like coral-eating snails, effectively acting as a tiny bodyguard for its host.
Sightings show no clear seasonal pattern, and with water temperatures in its range running from a comfortable 25.7 to 31.7 C, it is effectively a year-round find on healthy Acropora coral wherever that coral is present.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
This crab turns up on Acropora-rich reefs, and the Maldives is by far the strongest place to find it, with sightings concentrated in the Central Atolls. It has also been logged in the Visayas region of the Philippines, so it is worth checking staghorn and table coral heads there too, though the Maldives is the more consistent bet.
Mapping where Acropora Crab is logged…
The most reliable place is the Central Atolls of the Maldives, where it has been logged on Acropora coral at numerous sites. It has also turned up in the Visayas region of the Philippines, though sightings there are fewer.
There is no strong seasonal pattern in the sightings, and it tolerates a wide warm-water range of roughly 25.7 to 31.7 C, so it is present on healthy Acropora reefs throughout the year.
Look closely at a single Acropora coral colony rather than scanning the wider reef. It is a small, flattened, pale crab with fine dark markings, wedged between the coral branches, and it tends to hold still rather than dart away when approached.
It is a small crab, sized to fit into the narrow gaps between the branches of Acropora coral, so a slow, close look at the coral head is needed to spot one at all.
It spends its entire adult life on one coral colony, feeding on the coral's mucus and actively defending it against small predators, which makes it a real symbiotic partner of the coral rather than just a passer-by.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
2,422 sites
