Cymo quadrilobatus
A small reef-dwelling crab logged in Fiji's Lomaiviti and Queensland, hiding among coral in warm 26-32C water year-round.
A small, knobbly crab that hides deep in the branches of its host coral, usually spotted by torchlight or a close look at a coral head rather than swimming in open water. Its carapace (shell) is broad and lumpy with four distinct lobes across the front edge, giving the species its name, and colouring tends to blend with the coral it lives on, ranging from pale cream to mottled brown. Legs are short and thick, built for wedging into coral gaps rather than crawling across open reef.
Spends most of its time wedged among coral branches, rarely venturing onto open reef where divers would spot it easily. Divers typically find it by slowing down and scanning coral heads closely rather than by seeing it move, and it tends to stay still or retreat further into the coral when disturbed. Most sightings come from divers deliberately searching branching coral rather than spotting it in passing.
Living permanently within a single coral colony, this crab likely helps its host by picking off small parasites or competing organisms, a common pattern among small xanthid crabs that shelter in branching coral.
No clear seasonal pattern shows up in the sighting data, and with a comfortable range of 25.7 to 31.7C it stays active on warm tropical reefs all year round. Plan around good visibility and calm conditions for close coral inspection rather than a particular month.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Logged as an uncommon find across scattered Pacific sites, including Lomaiviti in Fiji, Queensland in Australia, Saipan in the Northern Mariana Islands, and Kwajalein Atoll in the Marshall Islands. Each of these regions offers a real but low-probability sighting, so this is a species you find while working a coral head carefully rather than one that turns up on every dive.
Mapping where Coral Four-lobed Crab is logged…
Sightings come from scattered Pacific locations including Lomaiviti in Fiji, Queensland in Australia, Saipan in the Northern Mariana Islands, and Kwajalein Atoll in the Marshall Islands. It is an uncommon find at each, so treat any sighting as a bonus rather than a guarantee.
There is no detected seasonal pattern, and its comfortable temperature range of 25.7 to 31.7C means it can be found on tropical reefs throughout the year.
Look for a small, lumpy-shelled crab wedged deep inside branching coral, with a carapace showing four distinct lobes along its front edge. Colouring is usually pale or mottled to match its coral host, and it rarely moves into open view.
Exact size data is not available, but like most xanthid crabs living inside coral branches, it stays small, likely only a few centimetres across, well suited to squeezing between coral gaps.
It spends its life embedded in a single coral colony, likely helping keep the coral clear of small parasites or competing organisms in exchange for permanent shelter.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
2,422 sites
