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.
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