Hemigrapsus oregonensis (Dana, 1851) Common name(s): Yellow shore cab, Mud-flat crab, Oregon shore crab, Hairy shore crab, Green shore crab
Synonyms:  Pseudograpsus oregonensis, Brachynotus oregonensis
Phylum Arthropoda  Subphylum Crustacea   Class Malacostraca    Subclass Eumalacostraca     Superorder Eucarida      Order Decapoda       Suborder Pleocyemata        Infraorder Brachyura (true crabs)         Family Grapsidae
	Hemigrapsus oregonensis from Sares Head.
	(Photo by: Dave Cowles July 2005)

How to Distinguish from Similar Species: Pachygrapsus crassipes, which lives mostly farther south, has transverse ridges on the dorsal surface of its carapace and 2 teeth on the anterolateral margin of the carapace.  Hemigrapsus nudus has few or no setae on the legs and purple spots on the chelipeds.

The first zoeal stage of H. oregonensis can be distinguished from zoeae of H. nudus because H. oregonensis has lateral projections on only abdominal segment 2 while H. nudus has lateral projections on abdominal segments 2 and 3 (Lee and Ko, 2008)

Geographical Range: Resurrection Bay, Alaska to Bahia de Todos Santos, Baja California.

Depth Range:  (mainly intertidal)

Habitat: Open mud flats, algal mats and eelgrass beds, in bays and estuaries and on open beaches where there is plenty of fine sediment.

Biology/Natural History: This species is a better osmoregulator than the other local shore crabs (H. nudus), which is probably associated with its being more common in estuaries.  It also often digs burrows and is capable of withstanding more hypoxic conditions than the other shore crabs are.  Feeds mainly at night, mostly on diatoms and green algae, but will eat meat if it has opportunity.  Predators include shorebirds.  May have the parasitic isopod Portunion conformis in the perivisceral cavity (not evident unless dissected).

References:

General References:
  Kozloff, 1993
  Morris et al., 1980
  Ricketts et al., 1985

Scientific Articles:
 Lee, Seok Hyun and Hyun Sook Ko, 2008.  First zoeal stages of six species of Hemigrapsus (Decapoda: Brachyura: Grapsidae) from the northern Pacific including an identification key.  Journal of Crustacean Biology 28:4 pp. 675-685

The chelae in this species does not have purple spots, as does H. nudus.

The legs of H. oregonensis have setae, as visible here.

View of the underside of a female (left) and a male (right).  Note the broader, more rounded abdomen on the female and the narrower, pointed abdomen on the male.
 

Below is another series of dorsal and ventral views of a male Hemigrapsus oregonensis. Notice how narrow the abdomen is in the ventral view, and that they have a very limited number of pleopods on the abdomen, used for transferring sperm duing fertilization.	Below is another series of dorsal and ventral views of a female Hemigrapsus oregonensis. Notice in the ventral view that their abdomen is much broader than the male's. They have a series of feathery pleopods on their abdomen, used for carrying the eggs while they are brooding them.

Closeup of the underside of the abdomen of a female.  Females in brachyuran crabs have pleopods on their abdomens which function to attach eggs to.

View of the underside of a male Hemigrapsus oregonensis.  Male brachyuran crabs have no pleopods on their abdomen except for two hemipenes used for transferring sperm.

The right chela of a male.  There is a tuft of setae on the palm of the chela on males.

This crab has many color variations.  A common form is a white or nearly white carapace, as seen in this individual.  Photo by Dave Cowles, July 2005

Light green is another common color.  Photo by Dave Cowles, July 2005

This individual from Kalaloch also shows the hairy legs (actually setae not hairs) common on this species, plus the three teeth on the anterolateral margin of the carapace which distinguish genus Hemigrapsus from Pachygrapsus crassipes (which has only two teeth).
Although abundant setae on the legs is characteristic of H. oregonensis and are evident in this individual, most individuals found near the Rosario Marine Labs have very few setae on the legs.
Photo by Dave Cowles, July 2009