The Purple Shore Crab
Size: up to 4 - 5.6cm when fully grown.
Range: Rocky and gravely beaches of western North America, from Alaska to Baja California.
Habitat: The Hemigrapsus nudus is most commonly found in the inter-tidal and sub-tidal areas of the beaches. Since these crabs main predator are birds, it usually stays under rocks and other forms of shelter. In Washington, the H. nudus is most commonly found on the shores of the Pacific coast and the Straight of Juan de Fuca.
Misidentification: The Hemigrapsus nudus can range in color from greenish yellows to deep purples. Also it shares the same habitat with a number of other small crabs like the H. oregonensis, and the Pachygrapsus crassipes. These two crabs can be misidentified with H. nudus, especially Pachygrapsus crassipes who has a similarly round shape carapace. Though the H. nudus does stand out because of its purple to reddish spot on each cheliped.
Life History: As the Hermigrapus nudus grows to around 5 cm, it spends the majority of its time feeding on algae and dead animals. The males, of this species and other shore crabs, are often seen with a sponge like tissue on the inside of each cheliped. It is unknown as to what this tissue is for. As any scavenger, these crabs are essential in maintaining a stable habitat.
Predators: most of the predators of the Hemigrapsus nudus are Gulls and other see birds.
Links and References:
Marine Discovery Center http://www.woodbridge.tased.edu.au/mdc/default.htm
Kozloff N. Eugene. Marine Invertebrates of the Pacific Northwest.
1987,1996 University of Washington Press. p. 416
Prepared by: Kwasi Addae