Polybius henslowii
A small paddle-legged crab logged along Portugal's Lisbon Coast and Costa Verde, usually turning up as an uncommon find in cool Atlantic water.
A small crab, only around 3 cm across the shell, with a smooth, slightly fan-shaped carapace in shades of reddish-brown to olive. The last pair of legs are flattened into paddles, which it uses to swim just off the bottom rather than only walking. Look for it hovering or scuttling low over sand and mixed seabed rather than tucked into a crevice like many crabs.
Divers usually find it moving over open sand or mixed bottom rather than hiding under rock, using those paddle-shaped rear legs to swim in short bursts just above the seabed. It tends not to react much to a diver's presence, carrying on foraging or shuffling along the bottom. Most logs describe it as an uncommon, low-key encounter rather than something divers actively seek out.
The flattened swimming legs set it apart from most shore crabs, letting it move through open water in short swimming bursts instead of relying only on walking across the bottom.
Sightings don't show a clear seasonal pattern, so treat it as a year-round possibility wherever it's found. It sits in a cool-water niche of roughly 13.6 to 19.6 C, which matches Atlantic conditions off Portugal and Galicia for much of the year.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged along Portugal's Lisbon Coast and Costa Verde, with a smaller but consistent presence in Galicia, Spain, just across the border. These cool Atlantic coastlines are the core of its known diver-observed range.
Mapping where Henslow's Swimming Crab is logged…
It's most commonly logged along Portugal's Lisbon Coast and Costa Verde, with additional sightings in Galicia, Spain. These cool Atlantic coastal sites make up the core of its known diver-reported range.
There's no strong seasonal pattern in the sightings, so it can turn up year-round. It favours cooler water between roughly 13.6 and 19.6 C, typical of the Atlantic coasts where it's found.
Look for a small reddish-brown to olive crab, about 3 cm across, with paddle-shaped rear legs. It's often seen swimming or hovering just above sand rather than hiding under a rock like most crabs.
They're small, typically reaching around 3 cm across the carapace, making them one of the more modest-sized crabs a diver will spot on Atlantic dives.
No, they pose no danger to divers. They're small, tend to keep moving over the seabed, and show no defensive behaviour toward divers.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
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