Walking Rock Crab, Aethra scutata

Walking Rock Crab, Aethra scutata. Crab provided by the commercial fishermen of the greater Los Cabos area of Baja California Sur, October 2010. Size: 9.3 cm (3.7 inches) x 5.8 cm (2.3 inches). Identification courtesy of Christine Ewers, Athens, Georgia.

General information: The Walking Rock Crab, Aethra scutata, is member of the Aethridae or Brachyuran Family of True Crabs. They are known in Mexico as cangrejo roca caminante. There are four known global members of the Aethra Genus. They are so named due to their appearance as rocks and are only noticed when they move.

Identification: The Walking Rock Crab has a broadly elliptical shell (carapace), with a toothed curved front, with short fat walking legs and large wide claws with the dactyl being much smaller than the fixed finger. The crab is a molted light brown with a few dark brown spots. The eyes are exceedingly small and hard to locate. The Walking Rock Crabs reach a maximum size of 12.5 cm (4.9 inches) being sexually dimorphic with the males being slightly larger than the females. I had a crab in my possession that measured 16.0 cm (6.3 inches) x 10.2 cm (4.0 inches) extending the maximum known size for this species.

Habitat and Range: The Walking Rock Crab are found in shallow coastal waters in an around rock rubble with sand, boulders, and ledges at depths up to 30 m (100 feet). They are found throughout the Sea of Cortez, around the tip of the Baja, and along the coast of the mainland south to Guatemala.

Diet: The Walking Rock Crab is an omnivorous predator and scavenger feeding on algae detritus and other small invertebrates including gastropods, barnacles, worms, and other small crustaceans.

Conservation Status:  From a conservation perspective the Armed Box Crab has not been formally evaluated. However, they are common and of limited interest to most and should be considered to be of Least Concern.