Oedignathus inermis (Stimpson, 1860)

Common name(s):  Paxillose crab, Granular claw crab, Soft-bellied crab
Synonyms: Hapalogaster inermis, Hapalogaster brandti, Oedignathus gilli
Phylum Arthropoda
Subphylum Crustacea
Class Malacostraca
Subclass Eumalacostraca
Superorder Eucarida
Order Malacostraca
Suborder Pleocyemata
Oedignathus inermis from Sares Head.  Found intertidally in a sea cave
(Photo by: Dave Cowles, August 2007)
Description:   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 (photo).  The first (basal) segment of the abdomen has calcified plates, as do the two terminal ones. Oedignathus inermis has chelipeds very unequal in size (the right cheliped is largest) and the dactyl is shorter than the "palm" of the propodus.  The rostrum is sharp but has no spines (photo).  The legs are rounded (not heavily flattened).  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 (photo).  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 (the chelipeds). 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.
 

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.

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

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

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.


 
 
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References:

Dichotomous Keys:
  Coffin, 1952
  Hart, 1982
  Kozloff 1987, 1996
  Smith and Carlton, 1975
  Wicksten, 2009
 

General References:
  Harbo, 1999
  Jensen, 1995
  Johnson and Snook, 1955
  Kozloff, 1993
  Morris et al., 1980
  Niesen, 1997
  O'Clair and O'Clair, 1998
  Sept, 1999

Scientific Articles:

Web sites:

General Notes and Observations:  Locations, abundances, unusual behaviors:

Abdomen
The abdomen of this species is thick and soft.  The basal segment and the two distal segments have some calcified plates, which are not evident in this view.
Unequal claws
One claw is much larger than the other.  The "palm" of the chela on the large claw is longer than the dactyl.  The upper surface of the chelipeds is covered with prominent, granular tubercles but with no obvious spines.


Head
The rostrum is short.  The carapace has orange-red tubercles with a white spot in the middle.
 


Legs
The walking legs have setae and granules but no spines.


Authors and Editors of Page:
Dave Cowles (2007):  Created original page
CSS coding for page developed by Jonathan Cowles (2007)