This species does not decorate itself as much as some other majid crabs do. It does have two rows of hooked setae just behind its rostrum, to which it sometimes attaches algae, etc. The items it attaches may be mainly food, which it detaches and eats later. This crab eats algae such as Fucus
, and red algae. Where algae is scarce they may eat barnacles, mussels, hydroids, and bryozoans. Predators include staghorn sculpins, gulls, cabezon, and sea otter. Velella velella
, the by-the-wind sailor, will readily capture and eat the pelagic larvae (zoeae). The species is sometimes parasitized by the rhizocephalan sacculinid barnacle Heterosaccus californicus
, which causes the crabs to be sluggish and to have a brownish mass (the reproductive parts of the barnacle) protruding from under the abdomen. The crab molts only once after being parasitized, and during that molt the barnacle's reproductive sac pushes out through the surface. The crab's gonads are damaged or destroyed and males exhibit some female-like characteristics such as a broad abdomen and small claws. He may even become a hermaphrodite and produce eggs as well as sperm. Females seem less affected other than speeding up the development of mature female characteristics. This species cannot osmoregulate so it cannot tolerate diluted seawater. The species has a terminal molt, after which the carapace may become partly overgrown with barnacles, etc. In the fall adults migrate to deeper water where they congregate, feed, and mate. Females may be carrying eggs during most seasons of the year. In southern Puget Sound females could not be found during May, September, and October. Clutch size ranges from 34,000 to 84,000 eggs. Freshly extruded eggs are bright orange, maturing to red, and to grayish-purple at hatching. Embryonic development may require nearly a year. Note: The long legs and claws of these crabs are dextrous and they can cling tenaciously and pinch hard, so be warned.