Hemigrapsus nudus (Dana, 1851) Common name(s): Purple shore crab, Naked shore crab
Synonyms:  Pseudograpsus nudus  Heterograpsus nudus  Brachynotus nudus
Phylum Arthropoda  Subphylum Crustacea    Class Malacostraca     Subclass Eumalacostraca      Superorder Eucarida       Order Decapoda         Suborder Pleocyemata          Infraorder Brachyura (true crabs)           Section Brachyrhyncha    Family
	Hemigrapsus nudus, San Simeon, CA.  About 4 cm carapace width.
	(Photo by: Dave Cowles, May 1995)

How to Distinguish from Similar Species: Hemigrapsus oregonensis also has 3 anterolateral teeth but no purple spots on the chelipeds and the legs have abundant setae.  Pachygrapsus crassipes (Oregon and south) has transverse lines and 2 anterolateral teeth on the carapace.

Larvae in the first zoeal stage can be distinguished from zoea of H. oregonensis 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: Yakobi Island, Alaska to Bahia de Tortuga, Mexico.  Uncommon below central CA.

Depth Range: Mostly intertidal

Habitat: Under rocks and in cracks.  Also high in some estuaries.

Biology/Natural History: Does not live in burrows, as Hemigrapsus oregonensis often does.  The chela of males, as of H. oregonensis and P. crassipes, have a prominent tuft of hairlike setae on the palm.  This species is an osmoregulator and can tolerate both hypo- and hyperosmotic conditions.  In Puget Sound feeds on diatoms, desmids, and small Ulva and Enteromorpha green algae scraped from rocks with the tips of the chelae.  May also feed on a few animal products, such as amphipods and the eggs of Nucella emarginata and other whelks.  In Puget Sound, females carrying eggs are found from January to mid-July; especially in April.  Female may carry from 400 to 36,000 eggs.  This species sometimes has the pasasitic isopod Portunion conformis in the perivisceral cavity, and the eggs may be attacked by the tiny Nemertean worm Carcinonemertes epialti.  Predators include gulls white-winged scoters, Anthopleura anemones, and staghorn and tidepool sculpins.  Nucella lamellosa seems to be attracted to the scent of this crab but is not known to be a predator.

References:

General References:
Jensen, 1995
Kozloff, 1993
Morris et al., 1980
Niesen, 1994
O'Clair and O'Clair, 1998

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
 
 

This species seems to be less tolerant of hypoxia than is is H. oregonensis.  In places where their range overlap it is usually found higher in the intertidal and on more sandy/less muddy substrate.

A related species, H. sanguineus (Asian shore crab) on the New England coast was shown to prefer animal prey such as small mussels and barnacles, even though it could also feed on algae.  When starved or in crowded conditions it ate algae, but if given a free choice it chose invertebrates.  The authors speculated that the species may have an important effect on competition and succession among intertidal attached species.  Source:  Brousseau, Diane J. and Jenna A. Baglivo, 2005.  Laboratory investigations of food selection by the asian shore crab, Hemigrapsus sanguineus: algal versus animal preference.  J. Crust. Biol. 25(1): 130-134 (abstract)

This individual is from Sares Head.  Photo by Dave Cowles, Sept 2005

Notice that the purple spots on the chelae are less distinct on this individual than on some.  Photo by Dave Cowles, Sept 2005

Many crabs "slobber" bubbles when out of water because they are pumping air and water in their gill chambers, whose outlets are near the mouth.  Photo by Dave Cowles, Sept 2005
 
 
This individual is a female, as can be seen from the wide, round abdomen (left picture) and the pleopods she uses for carrying her eggs (right picture)  Photos by Dave Cowles, Sept. 2005

Males of this species have a tuft of fine setae on the inner palm of their chelae, as seen here.  Note also the much narrower abdomen than is seen in the female.
Photo by Dave Cowles, July 2006

This male also shows the narrow abdomen and the tufts of setae on the chelae.
Photo by Dave Cowles, July 2009
 

In an August 2007 trip to Shi Shi beach we found that Hemigrapsus nudus range over flat sandy beach areas, leaving a long set of crab tracks behind.  Several seemed to have gotten stranded by the retreating tide and buried themselves at the end of their tracks far above the water line at low tide, like this individual.  Many gulls were present on the beach so burying doubtless contributes to the crabs' survival.