Found from Southern California to Baja California on soft sandy bottoms, burrowing to depths of forty centimeters, but usually found near the surface. Found down to 195 feet depth. Up to ten inches long. Smooth slippery skin, with yellowish-brown mottling. Lacks tube feet. Ingests bottom material.
The pea crab Pinnixa barnharti, the large pea crab Pinnixa faba, and the mottled pea crab Opisthopus transversus live commensally in the cloaca of the sea cucumber C. arenicola, with the mottled pea crab also being found in the intestine. The cloacus is the terminal chamber of the sea cucumber intestine, located just inside its anus; the sea cucumber is not harmed by the crab's use of this safe house from predation. These crabs shelter in various species of animals, including tube worm tubes, and the size of the shelter may limit the size of the resident crab. 52 of 53 sea cucumbers C. arenicola collected in San Diego for one study had commensal crabs, with 13 being the mottled pea crab Opisthopus transversus.
The largest, most mature mottled pea crab Opisthopus transversus are the egg-bearing females, and they can be found in the intestine of the sea cucumber C. arenicola. The mottling of the mottled pea crab Opisthopus transversus can be dependent on the food and its pigment gathered by their host, since the crabs may feed on this gathered food. Sea cucumbers ingest mud rich in carotenoid pigments, and mottled pea crabs living within the sea cucumber C. arenicola maintain a reddish brown and white color pattern, while those living within other hosts may have a paler color with less discernible mottling.
-- Peter Brueggeman