During a recent dive in Monterey Harbor, Xantus’ swimming crab was observed in 15 feet of water.
Crustacean expert Dr. Greg Jensen identified the species as Portunus xantusii, a portunid crab that prior to these observations was seen only as far north as Morro Bay. More typical of southern California and Mexico, this crab is strikingly different from native crabs, and lives primarily in sandy or muddy habitats, such as eelgrass beds and mudflats. As has been the case with several other species of fish and invertebrates, the last two years of abnormally high temperatures due to the warm-water “Blob” and the 2015-16 ENSO event facilitated the recruitment of species more commonly found in warm-temperate and subtropical waters.
The last pair of walking legs are flattened and facilitate rapid swimming, which the crab uses to avoid predators, or by burrowing quickly into the sand–backwards! These paddle-like appendages are one key feature that separates this species from many of the other crabs found in Monterey Bay. The carapace has a very sharp, elongate spine at the widest point, and the carpus of the claw has a prominent spine, which differentiates it from Callinectes crabs that also have flattened, rear appendages. (The carpus connects the cheliped [claw] to the merus [arm, so to speak])
It is not known when the crab recruited to Monterey Harbor (possibly 2014 and/or 2015), and it was most likely larvae that came northward with warm currents in the last 2-3 years. These larvae settled and developed into the fast-moving adults that were observed by divers.