Percnon gibbesi
Spot flat crabs year-round in the Canary Islands, Costa Blanca and Malta. Learn the key ID features that set this fast-moving shore crab apart.
Flat crabs live up to their name: the carapace (shell) is strikingly flattened and wide relative to its depth, allowing the crab to squeeze tight against rock faces. Colouration is typically olive to dark brown with reddish or orange-tipped legs that often spread in a star pattern, making the animal look larger than the body alone suggests. Look for the habit of pressing completely flush to the substrate and scuttling sideways at speed when disturbed, that combination of flat profile and rapid movement is the quickest give-away.
You will almost always find flat crabs pressed tightly against exposed rocky or rubble surfaces, rarely venturing far from a crack or overhang they can dart into. When a diver approaches, they respond quickly: a burst of sideways movement and then stillness, relying on their low profile to disappear against the background. Dawn and dusk tend to bring more activity, but they can be spotted at any time of day on shaded rock faces.
Percnon gibbesi is an invasive species in the Mediterranean, native to the eastern Atlantic and Caribbean, it first appeared in the Med in the 1990s and has since spread rapidly, outcompeting local herbivorous crabs on rocky reefs.
In the Northern Hemisphere it is present year-round with no concentrated season, reflecting its tolerance of a broad temperature range from roughly 16 to 29 °C. If you are planning a trip specifically around this species, any month works in the Mediterranean and Canary Islands.
Most commonly logged across the Mediterranean and the Atlantic island groups, Spain leads the count, with strong sightings concentrated in Costa Blanca, the Balearic Islands and Tenerife. The Canary Islands as a whole, plus Madeira, Malta (particularly Gozo) and Italy (Sicily, Lampedusa and Sardinia) are all reliable areas. Outside Europe, Bonaire in the Caribbean has a meaningful presence too.
Mapping where Flat Crab is logged…
Flat crabs are most reliably found in the Mediterranean and Atlantic island groups. Costa Blanca and the Balearic Islands in Spain, Tenerife and Gran Canaria in the Canary Islands, Gozo in Malta, and Sicily and Sardinia in Italy all have strong sighting records. Bonaire in the Caribbean is also a good bet.
In the Northern Hemisphere, flat crabs are present throughout the year with no obvious peak season. Their wide temperature tolerance, observed from about 16 to 29 °C, means they cope with both warm summer and cooler winter conditions across Mediterranean and Atlantic sites.
The flattened, wide carapace is the key feature, it lets the crab lie completely flush with rock. Combined with long, orange-tipped legs spread wide and a tendency to sprint sideways when disturbed, the flat crab is fairly distinctive once you know what to look for. Olive to dark-brown body colouration helps it blend into rocky surfaces.
Flat crabs are small rock-dwelling crabs. The carapace typically measures a few centimetres across, though the widely spread legs can make the animal appear considerably larger. They are not easily spotted until you are looking at the reef surface carefully.
Yes. Although native to the Atlantic and Caribbean, flat crabs colonised the Mediterranean in the late 1990s and have spread rapidly since. They are now found at rocky reef sites throughout the sea, and researchers consider them one of the more successful marine invertebrate invaders in the region.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
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