Harbor Spider Crab, Amphithrax armatus

Harbor Spider Crab, Amphithrax armatus. Collected off the rocks at the beach at Cerro Colorado, Baja California Sur, May 2007. Size: 5.1 cm (2.0 inches) x 5.4 cm (2.1 inches). Note the moss growing on the body. Identification courtesy of Dr. Greg Jensen, Seattle, Washington.

General information: The Harbor Spider Crab, Amphithrax armatus, is a brachyuran crab and a member of the Majidae Family, which includes the spider crabs that are known in Mexico as cangrejos. It is known locally as Mexicano cangrejo araña or araña proteña. The majority of spider crabs live in deep water on the continental shelf. Spider crabs are poorly studied and very limited information about this species has been documented. All have long legs and shells that are longer than they are wide and have a point at the front. Most have bristles that they utilize to attach algae and other items for camouflage which gives rise to the common name Decorator Crabs. They are one of sixty different crabs that live in the Sea of Cortez.

Identification: The majority of Harbor Spider Crabs have shells (carapaces) that are less than 2.5 cm (1.0 inch) in width. They are not well known and rarely studied. All have long legs and shells that are longer than they are wide and have a point at the front. Most have bristles that they utilize to attach algae and other items for camouflage which gives rise to the common name Decorator Crabs. The Harbor Spider Crab is a dark red-brown decapod with short legs and a triangular body that is exceedingly well camouflaged to blend in with its surroundings. They have eyes on short stalks and a broad, flattened carapace with a small abdomen folded under the thorax and pincers. The claws are quite small and of equal size. The Harbor Spider Crab achieves a maximum length of 5.0 cm (2.0 inches), with the females being slightly larger than the males.

Habitat and Range: In Mexico, the Harbor Spider Crab are found in the lower two-thirds of the Sea of Cortez and along the coast of the mainland south to Guatemala. These small crabs are found in rocky, inter-tidal or shallow sub-tidal environments where they crawl among algae. They are also found in tidal pools. In contrast, the majority of spider crabs live in deep water on the continental shelf.

Diet:  The Harbor Spider Crab diet is not well known due to lack of research and related publications.

Conservation Status:  The Harbor Spider Crab has not been formally evaluated from a conservation status.  However, they are common and of limited interest to most and should be considered to be of Least Concern.

Synonyms:  Mithrax aramatus and Mithrax orcutti