Hemigrapsus nudus (Dana, 1851)
Common name(s): Purple shore crab, Naked shore crab
Infraorder Brachyura (true crabs)
|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.
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
and can tolerate both hypo- and hyperosmotic conditions. In
Sound feeds on diatoms, desmids, and small Ulva and
green algae scraped from rocks with the tips of the chelae.
feed on a few animal products, such as amphipods and the eggs of Nucella
and other whelks. In Puget Sound, females carrying eggs are
from January to mid-July; especially in April. Female may
400 to 36,000 eggs. This species sometimes has the pasasitic
conformis in the perivisceral cavity, and the eggs may be
by the tiny Nemertean worm Carcinonemertes epialti.
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.
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Morris et al., 1980
O'Clair and O'Clair, 1998
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
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.
|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?|
|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.|
|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.|
|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.|