Acanthonyx lunulatus
Small, algae-covered crab clinging to shallow reef in Valencia and Tuscany, comfortable in 16-22C water year-round.
A small, spider-like crab with a triangular, algae-textured shell that makes it almost disappear against weed-covered rock. Legs are thin and spindly compared to the body, and colour runs from dull green to brownish-red depending on the algae it is hiding among. Look for it sitting motionless in shallow rocky patches rather than swimming or scuttling across open sand.
Found sitting still on algae-covered rock or weed in shallow, current-sheltered spots, relying on its camouflage rather than fleeing when divers approach. It's the kind of animal you spot by slowing down and scanning weedy rock faces closely rather than swimming past looking for movement. Most logs come from uncommon, not-easy-to-spot encounters rather than crabs sitting out in the open.
Its shell texture and colouring closely match the algae it lives on, giving it one of the better camouflage tricks among small Mediterranean crabs, so most divers who log it do so by accident while looking at something else on the rock.
Sightings show no clear seasonal pattern, and the observed temperature range of 15.8 to 21.8C spans much of the Mediterranean diving calendar, so it's a reasonable year-round find on rocky reef dives.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged along Spain's Valencia coast, which accounts for the bulk of sightings, with further records from Catalonia and Andalusia. Italy's Tuscany region is the other strong spot, so divers working Mediterranean rocky reefs in either country have a fair shot at finding one.
Mapping where Surf Crab is logged…
Valencia in Spain is the strongest area for sightings, with further records from Catalonia and Andalusia. In Italy, Tuscany is the other notable region for finding this crab on rocky reef dives.
There's no strong seasonal pattern in the sightings, and it tolerates a wide temperature range from about 16 to 22C, so it can turn up on Mediterranean rocky reef dives at any time of year.
Look for a small, triangular-bodied crab with thin legs and an algae-textured shell that blends into weedy rock. It sits still rather than scurrying, so slow, close scanning of rocky, algae-covered surfaces is the best way to find one.
No, this is a small, harmless crab that poses no risk to divers. It won't react defensively and is easy to observe without disturbing it.
Exact size data isn't available, but like other members of its family it's a small crab, easily overlooked against the algae and rock it lives on.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
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