Fish Identification Photos: Rough Swim Crab, Portunus asper. The Rough Swim Crab is characterized by having nine tooth-like extensions (or spikes) along the side of its shell that commence at the eyes and run to about half way back on the shell, or carapace.

The last of these “teeth” is very long without significant curvature. These crabs have unique and characteristic fifth legs with a spike on the posterior of the fourth segment, the mero, and claw shape.

The Rough Swim Crab reaches a maximum size of 4 inches and is normally found up to 300 feet deep in the water column. In Mexico, the Rough Swim Crab is found in the lower one-third of the Sea of Cortez, around the oceanic islands and along the coast of the mainland south to Guatemala; it is absent from the Pacific side of the Baja California peninsula and in the upper portions of the Sea of Cortez. The Rough Swim Crab can be easily confused with the Sand Swim Crab, Arenaeus mexicannus (longer curved ninth “tooth”).

The Rough Swim Crab is a member of the Portunidae Family of swimming crabs, which are known in Mexico as jaibas. The family has 13 individual genuses of which five genuses with 16 individual species are found in Mexican waters. They are decapods with ten limbs. Swim crabs are colorful and active crabs that are among the few crabs that are swift and agile swimmers.

They swim sideways utilizing their paddle-like fifth pair of (dactyl) legs that rotate like propeller blades when they swim. However, they are essentially bottom dwellers. They have a streamlined profile that allows for rapid swimming and long pincers armed with sharp spines to snag food. They consume clams, fish, snails, worms and other crustaceans. The commercially important Blue Crab, Callinectes sapidus, is a member of this family.

Rough Swim Crab, Portunus asper: Male (upper two photos), and femaie (lower two photos with orange egg cluster.) Shows up when one purchases a “scoop of bait.” Caught in the surf zone, La Playita, San Jose del Cabo, Baja California Sur, Mexico, during the month of November with water temperatures around 78 degrees, early morning, in 5 to 10 feet of water, utilizing a cast net by panguero bait salesmen. Size approximately 4 inches and they “bite like hell!” Not viewed by locals to be of great value. Does survive a panga “bait tank” at a high level. Worth sending “down deep” to see what materializes. Photos and description courtesy John Snow.