Description:
Hemigrapsus nudus is a small shore crab (Phylum Arthropoda, Order Decapoda). Like all crustaceans their entire body is covered by an exoskeleton that provides protection and support. Their square body is dorsoventrally compressed or flattened from top to bottom (<56 mm in width). You can distinguish males from females because they have a narrow abdomen ("light-house" like structure) on their ventral surface, while the females have a wider "bee-hive" shaped abdomen.
H. nudus have five pairs of jointed legs (hence a decapod deca = 10, poda = feet) that extend out from the main body. The first pair of legs is modified into large claws, or chelipeds, which the crabs use for defense and for handling food. Red-purple spots are usually present on the claws. The legs of H. nudus lack hairs (hence the name nudus). The last four pairs of legs are used for walking (never swimming). Crabs can slowly walk forward, but typically move sideways.
Sensory organs include a pair of compound eyes on the end of movable stalks, which can detect light intensity and may be able to discriminate form and colour. They also have two pairs of antennae, which is a distinguishing feature of crustaceans. These antennae can detect odours in the environment.
H. nudus are typically dark red or purple-brown, but yellow and whitish individuals have been reported. Their colouration allows them to blend in with the environment.
The green shore crab (Hemigrapsus oregonensis) is a similar species, but lacks the red-purple spots on the claws, and has fine hairs on its legs.
H. nudus have five pairs of jointed legs (hence a decapod deca = 10, poda = feet) that extend out from the main body. The first pair of legs is modified into large claws, or chelipeds, which the crabs use for defense and for handling food. Red-purple spots are usually present on the claws. The legs of H. nudus lack hairs (hence the name nudus). The last four pairs of legs are used for walking (never swimming). Crabs can slowly walk forward, but typically move sideways.
Sensory organs include a pair of compound eyes on the end of movable stalks, which can detect light intensity and may be able to discriminate form and colour. They also have two pairs of antennae, which is a distinguishing feature of crustaceans. These antennae can detect odours in the environment.
H. nudus are typically dark red or purple-brown, but yellow and whitish individuals have been reported. Their colouration allows them to blend in with the environment.
The green shore crab (Hemigrapsus oregonensis) is a similar species, but lacks the red-purple spots on the claws, and has fine hairs on its legs.
Ecology:
Hemigrapsus nudus inhabits rocky intertidal areas ranging from Alaska to California. They prefer protected or semi-protected shorelines with large rocks or stones overlying coarse sand or gravel. They are found from the high to the low intertidal zone.
As adults, H. nudus primarily forage at night primarily consuming diatoms and green macroalgae (e.g. Ulva sp.); but they will sometimes eat live prey (e.g. littorine snails, amphipods, small bivalves) and scavenge decaying algae and animals. They avoid predators such as shorebirds, larger crabs, fishes, and mammals by hiding under rocks or amongst seaweed (especially Fucus). If threatened, they can feign death. If attacked, males are more likely to fight; whereas, females are more likely to drop a leg and escape.
As adults, H. nudus primarily forage at night primarily consuming diatoms and green macroalgae (e.g. Ulva sp.); but they will sometimes eat live prey (e.g. littorine snails, amphipods, small bivalves) and scavenge decaying algae and animals. They avoid predators such as shorebirds, larger crabs, fishes, and mammals by hiding under rocks or amongst seaweed (especially Fucus). If threatened, they can feign death. If attacked, males are more likely to fight; whereas, females are more likely to drop a leg and escape.
Life History:
In the Pacific Northwest, H. nudus mate in mid-winter. The female carries 400-36,000 fertilized eggs until they hatch in May and June. The hatched free-swimming planktonic larvae stay in the water column for about five weeks before settling to the substratum and developing into juvenile crabs.
To learn more:
Low, C.J. 1970. Factors affecting the distribution and abundance of two species of beach crab, Hemigrapsus oregonensis and Hemigrapsus nudus. University of British Columbia. M.Sc. thesis.
Todd, M.E. and Dehnel, P.A. 1960. Effect of temperature and salinity on heat tolerance in two grapsoid crabs, Hemigrapsus nudus and Hemigrapsus oregonensis. Biological Bulletin. 118: 150-72.