Portunus sayi
Uncommon find in Florida's drifting sargassum mats, 23-29C water, this paddle-legged crab rewards divers who check the weed lines.
Small, mottled brown to olive carapace that blends almost perfectly with floating sargassum weed, usually only a few centimetres across. Like other swimming crabs, its last pair of legs are flattened into paddles for swimming rather than walking. Look for it clinging upside down or sideways within loose weed rafts rather than out on open reef or sand.
Divers find it tucked into loose sargassum rafts drifting near the surface or caught against a reef, gripping the weed with its front legs while its paddle-shaped rear legs keep it steady in the current. It tends to sit still and rely on camouflage rather than bolt when a diver approaches, though it will scoot sideways into denser weed if disturbed. Most logs describe it as an uncommon to rare find, so spotting one usually means someone took the time to search a weed line closely.
Its entire life is tied to floating sargassum rather than the seafloor, so it lives, feeds and hides within drifting weed mats that can carry it far from any reef.
Sightings run year-round with no clear seasonal pattern in the data, which fits its comfort range of roughly 23 to 29C found on warm subtropical and tropical dives. Your best odds come down to finding sargassum weed in the water that day rather than picking a particular month.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most consistently logged off Florida in the United States, with additional sightings around San Juan in Puerto Rico, the Yucatán Peninsula in Mexico, and Grand Cayman. It turns up wherever drifting sargassum mats collect near dive sites rather than at any single fixed reef feature.
Mapping where Sargassum Swimming Crab is logged…
Florida in the United States is the most reliable region, with further sightings around San Juan in Puerto Rico, Mexico's Yucatán Peninsula, and Grand Cayman in the Cayman Islands.
There is no strong seasonal pattern in sightings, so it is treated as a year-round find on warm reefs between roughly 23 and 29C. Look for drifting sargassum weed rather than waiting for a particular month.
It is a small, mottled brown crab with paddle-shaped rear swimming legs, usually spotted clinging inside a raft of floating sargassum weed rather than out on open reef.
No, they pose no danger to divers. They are small, shy within their weed cover, and have no interest in confrontation.
Unlike most crabs tied to the seabed, it spends its whole life drifting with floating sargassum mats, using them for food, shelter and transport across open water.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sally Lightfoot Crab
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