Oedignathus inermis

How to Distinguish from Similar Species: Of the Lithodid crabs with soft abdomens, Placetron wosnessenskii has a much thinner abdomen and has scales on the carapace and legs. Acantholithodes hispidus has large spines on the rostrum. Hapalogaster mertensii and H. grebnitzkii have strongly flattened cephalothorax and legs and have spines on the upper surface of their chelipeds.

Biology/Natural History: These crabs are often found in pairs, and may be in such a tightly secluded space that they appear to be trapped. They feed by straining plankton from the water with their third maxillipeds. Captive individuals also catch worms and small crustaceans with their small claw and crush mussels with the large claw. Predators include black oystercatchers.

Under encrusting coralline algae, under Mytilus californianus mussels or Anthopleura xanthogrammica anemones, in crevices, and in other protected areas

Geographical Range: Amchitka Island, AK to Monterey, CA; eastern Russia, Japan, Korea. Mostly on the open coast. Rarely seen in the San Juan islands and is said to not to occur in Puget Sound; rare in California.

Depth Range: Middle intertidal to 15 m

As a Lithodid crab, this species has no uropods and the abdomen is folded against the underside of the thorax. Its 5th walking legs are reduced. Oedignathus inermis has a thick, soft abdomen which is not tightly held under the thorax. The first segment of the abdomen has calcified plates, as do the two terminal ones. Oedignathus inermis has chelipeds very unequal in size and the dactyl is shorter than the "palm" of the propodus. The rostrum is sharp but has no spines. The legs are rounded. The chelipeds have large granular tubercles but no spines on the upper surface. The large cheliped has violet to gray or blue tubercles and a smooth tan to white tip to the claw with a spoonlike hollow where the propodus and dactyl contact one another. The shall cheliped has small, sharp granules and setae. The walking legs have sharp granules, setae, and dark brown and white spots. The anterior margins of legs 2-4 have some setae, tubercles, and small spines but no large spines. Note, however, that there are large spines on the anterior margins of leg 1. Carapace to 3 cm long and 2.5 cm wide in males, 2 cm wide in females; wider posteriorly than anteriorly, brown with scales on the dorsal surface. It has white-centered orange granules and dark brown spots, but these colors may be masked by mud. May have white on the sternum. There are few if any setae on the body.

Oedignathus inermis is a species of king crab found off the Pacific coasts of the United States and Canada, from California to Alaska, and disjunctly around the coasts of Japan. It is the only species in the genus Oedignathus, and is sometimes called the granular claw crab, paxillose crab or tuberculate nestling lithode crab.