Calcinus seurati
Spot Seurat's hermit crab tucked in shallow reef rubble across Hawaii and Taiwan's Orchid Island, in warm water year-round.
A small hermit crab that shelters in a compact, rounded snail shell rather than showing much of its own body in the open. What you actually see underwater is usually just a pair of dark eyestalks and banded legs poking out of the shell mouth, with the legs often showing a mottled or ringed pattern against a pale to reddish base. Size in the water is modest, closer to a fingertip than a fist, so most divers spot it while scanning shallow rubble and rock crevices rather than swimming past it in open water.
Look for it wedged into rubble, small rock crevices and shallow reef nooks in the top few metres of water, where it moves slowly over the substrate scavenging for scraps. It tends to be uncommon rather than abundant at any given site, so divers usually find one or two individuals per dive rather than groups. It withdraws into its shell readily if disturbed, so a slow, patient approach with a torch gives the best look at the legs and eyestalks before it retreats.
Like other hermit crabs it does not grow its own shell, so as it matures it has to find and move into a series of increasingly roomy abandoned snail shells, which is why size and shell choice can vary a lot between individuals at the same site.
No clear seasonal pattern shows up in the sightings, and its comfort range of roughly 22.6 to 28.6C matches Hawaii's reef water for most of the year, so it is a fair year-round find rather than a seasonal target.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged in Hawaii, where it turns up across a large number of shallow sites, with a second, smaller population documented off Orchid Island in Taiwan. Outside these two areas it has not been logged with any regularity, so Hawaii is the strongest bet for a dedicated look.
Mapping where Seurat's Hermit Crab is logged…
Hawaii is by far the strongest area for sightings, logged across a wide spread of shallow sites there. Orchid Island in Taiwan is the other documented location, though in smaller numbers.
There is no strong seasonal pattern in the sighting data, and its warm-water niche of about 22.6 to 28.6C is present on Hawaiian reefs year-round, so any month is reasonable.
Look in shallow rubble and crevices for a small hermit crab peeking out of a rounded snail shell, with banded or mottled legs and dark eyestalks visible at the shell opening. It rarely leaves its shell fully exposed, so the legs and eyes are the main clue.
No, it poses no risk to divers. It is a small, retiring scavenger that withdraws into its shell rather than confronting anything nearby.
Exact size figures are not well documented for this species, but divers describe it as small, roughly finger-sized including the shell it occupies rather than a large crab.
It has a soft, unprotected abdomen and relies entirely on a scavenged snail shell for protection, upgrading to a bigger shell as it grows rather than building its own.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
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