Hemigrapsus sexdentatus
A rare but rewarding find in North Island, New Zealand, this cold-water crab turns up in rock pools and shallow reef year-round.
A stocky, square-shelled crab with a mottled purple to reddish-brown carapace and pale-tipped claws, usually spotted wedged into crevices between rocks rather than out in the open. The shell edge has small tooth-like notches along the front margin, a useful confirming feature if you get close enough to look. Size in the water reads as small to modest, comfortably fitting in a cupped hand.
Divers usually find this crab tucked under rock ledges or jammed into narrow crevices in shallow, rocky terrain rather than crawling across open sand. It tends to hold still or retreat further into its crevice when a diver's shadow passes over, rather than bolting or raising claws in defence. Most logs describe it as an incidental find while scanning rock faces at shallow depth, not a target sighting.
Its low sighting frequency at most sites, rare at the majority and only uncommon at a handful, suggests it spends most of its time wedged deep in rock crevices out of open view, so patient, close-in searching along rock faces pays off more than mid-water scanning.
Sightings show no clear seasonal pattern, and the crab's comfort range of roughly 10 to 16C spans much of New Zealand's coastal water temperature through the year, so it is a reasonable find on any dive to the right rocky habitat.
This one is a North Island, New Zealand speciality, most frequently logged there, with a smaller but real presence in South Island, New Zealand too. Look in shallow, rocky sites rather than deep reef, since every logged sighting so far comes from New Zealand's cooler coastal waters.
Mapping where Purple Rock Crab is logged…
It is most reliably reported in North Island, New Zealand, with a smaller number of sightings from South Island, New Zealand. All confirmed sightings so far are from New Zealand's coastal rocky habitats.
There is no detected seasonal pattern, so it is considered a year-round find. Its comfort range of roughly 10 to 16C covers much of New Zealand's coastal water temperature across the seasons.
Look for a stocky crab with a mottled purple-brown shell wedged into rock crevices in shallow water. The front edge of the shell has small tooth-like notches, and the claws often show pale tips.
No, this species is not considered dangerous to divers. It tends to retreat into crevices rather than react defensively when approached.
It is logged as rare at most sites where it has been recorded, with only a few sites reporting it as uncommon. It is more of an incidental find than a reliable target sighting.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
2,422 sites
