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. (But read the discussion below that the Sares Head population may be an unusual one)
(Photo by: Dave Cowles July 2005)
Description:  This grapsid crab (rectangular carapace, wide-set eyes and no teeth on the carapace between, no rostrum) is a common intertidal crab.  The merus of legs 2-5 is not flattened.  The dorsal surface of the carapace does not have transverse ridges or lines, and is variously colored, often with light greenish spots on a dark reddish-brown background, but may be a pale green (photo), yellow-green, gray-green, or even nearly white (photo).  There are 3 teeth on the anterolateral margin of the carapace (photo).  The legs have abundant setae (photo)(photo),  and the chelipeds have no purple spots (photo), but have yellow or white on the tips.  Carapace width to 34.7 mm in males and 29.1 mm in females.

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 carapaceHemigrapsus 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).

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Dichotomous Keys:
  Coffin, 1952
  Flora and Fairbanks, 1966
  Hart, 1982
  Kozloff 1987, 1996
  Smith and Carlton, 1975
  Wicksten, 2009

General References:
  Hart, 1982
  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

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

The chelae in this species do 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.

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 fine 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
Exploring a Hemigrapsus nudus-Hemigrapsus oregonensis mystery (2017):  One of the key features of H. oregonensis is the fact that, unlike H. nudus, the species has prominent setae on its walking legs. However, the putative H. oregonensis that I find on Sares Head next to Rosario Beach Marine Laboratory has few if any setae on the legs, even though their coloration pattern matches that of H. oregonensis. This seems to be the case specifically for this restricted area-even the H. oregonensis a few hundred meters away across Rosario Bay at Urchin Rocks seem to have plenty of setae on their legs. Hart (1982) lists a variety of features that might be helpful for distinguishing H. oregonensis from H. nudus. The photos below explore four of those features comparing an individual from Sares Head with one from Urchin Rocks: Overall coloration, setae on legs, spots on chelae, and shape of anterior carapace border. My tentative conclusion is that the putative H. oregonensis from Sares Head may either be an odd subpopulation of H. nudus without chelae spots or hairy legs, or it may be a hybrid between the two species. I wonder if odd populations like this exist elsewhere in these species' ranges?
SaresHeadLegs UrchinRocksLegs
The legs of individuals from Sares Head have few if any setae. Note the three distinct teeth at the anterolateral corner of the carapace, which is characteristic of Hemigrapsus. Notice the H-shaped depression in the middle of the dorsal carapace (partly obscured by my finger) that is characteristic of both species. The legs of individuals from Urchin Rocks have many setae. Note that these individuals also have the three anterolateral  carapace teeth, and that this individual is a male, as shown by the tuft of fine setae on the palm of its chela. Here the H-shaped depression in the middle of the dorsal carapace is clearly visible.
SaresHead_Chelae UrchinRocks_Chelae
The chelae of individuals from Sares Head have no trace of purple spots, indicating it is H. oregonensis. The same is true of individuals from Urchin rocks.
SaresHead_AnteriorCarapace UrchinRocks_AnteriorCarapace
The anterior margin of the carapace of individuals from Sares Head is nearly straight but slightly convex anteriorly. This matches the description and illustration given in Hart (1982) for H. nudus, NOT for H. oregonensis. In contrast, the anterior margin of the carapace from individuals from Urchin Rocks is clearly lobed, which corresponds to the description and illustration given in Hart (1982) for H. oregonensis.
My conclusion, based on these features, is that the green Hemigrapsus population on Sares Head, while keying clearly to H. oregonensis, may either be an aberrant population of H. nudus that has lost its leg setae and acquired a strong green color, or it is a hybrid between H. nudus and H. oregonensis.

Authors and Editors of Page:
Dave Cowles (2005):  Created original page