Identification Photos: Harbour Spider Crab, Mithrax armatus: The Harbour Spider Crab is a dark red-brown decapod with short legs and a triangular body that is exceedingly well camouflaged to blend with its surroundings. It has eyes on short stalks and a broad, flattened carapace with a small abdomen folded under the thorax and pincers.

The claws of the Harbour Spider Crab are quite small and of equal size. The Harbour Spider Crab achieves a maximum length of just over two inches, with the females slightly larger than the males. It is normally found in rocky, intertidal environments.

In Mexico, the Harbour Spider Crab is found in the lower two-thirds of the Sea of Cortez and along the coast of the Mexican mainland south to Guatemala. The Harbour Spider Crab is not an easy species to identify.

The Harbour Spider Crab is a brachyuran crab and a member of the Majidae Family, which includes the spider crabs, known in Mexico as cangrejos. There are 60 different crabs that live in the Sea of Cortez, the majority with shells (carapaces) of less than one inch. The small crabs are found in rocky, intertidal or shallow subtidal environments, or tidal pools where they crawl among algae.

Harbour Spider Crab, Mithrax armatus: Collected off the rocks in May, 2007, at the beach at Cerro Colorado, Baja California Sur, Mexico. Size approximately 2 inches. Note the moss growing on the body. A tentative identification courtesy of Dr. Greg Jensen, University of Washington, was confirmed by Dr. Michel E. Hendrickx, Unidad Academica Mazatlan. Description and photos courtesy of John Snow.