Purple Shore Crab

The body of the purple shore crab is divided into two major components, the cephalothorax and the abdomen. A small crab, H. nudus reaches sizes of approximately 4.0–5.6 centimetres (1.6–2.2 in) in width and approximately 4.8 centimetres (1.9 in) in length. Beneath the thorax, there are five pairs of thoracic appendages and three pairs of maxillipeds that fold ventrally. The eyestalks are angled outwardly. The mouth is compositely made of six pairs of appendages, which include one pair of mandibles located on either side of the mouth. Additionally, it is made up of two pairs of maxillae and three pairs of maxillipeds that attach posteriorly to the mouth and cover the mandibles.

The purple shore crab is most commonly found sheltering under rocks and in seaweed within the inter-tidal and sub-tidal regions along the western coast of North America. The purple shore crab's distribution ranges from Alaska to Baja California in Mexico but it is uncommon to find the purple shore crab south of Morro Bay, in central California. H. nudus strongly prefers semi-protected and protected rocky coasts and bays, and can commonly be found in tide pools, swift water, and under large boulders. Less commonly, H. nudus can be found under driftwood or in salt marshes. Despite the increase in anthropogenic plastic waste pollution in inter-tidal zones, H. nudus has been shown to generally avoid plastic, glass, and styrofoam shelters when able and instead prefers natural shelters. The crabs have been observed sheltering on acrylic surfaces, however.

The purple shore crab primarily feeds on sea lettuce and other green algae, and occasionally scavenges dead animals. Specifically, the purple shore crab feeds on diatoms, desmids, small Ulva, and Enteromorpha algae species. H. nudus most commonly obtains green algae from rocks by scraping it off with their chelae. When scavenging, H. nudus most often eats amphipods, whelks, and the eggs of Nucella emarginata. Littorina scutulata sensu lato are also predated by H. nudus and will employ chemical cues to avoid the crabs. L scutulata is capable of detecting exudate from both the purple shore crab and its prey, warning it that the purple shore crab is actively feeding in the area. The purple shore crab is the prey of seagulls, seabirds, white-winged scoters, anthopleura anemones, larger crabs, staghorn sculpins, and tidepool sculpins. Nucella lamellosa has not been observed predating H.

Hemigrapsus nudus begins mating between December and January. The mating process is considered very similar to pachygrapsus species, save for that pachygrapsus breed in the summer and not the winter. Mating occurs when a male holds the female by her chelipeds, and guides her via his walking legs. The male will use his swimming legs (first pleopods) to move his sperm to the female. Females become gravid (egg bearing) during January to mid-July, most commonly in April. Samples taken from Puget Sound, Washington have shown that roughly 70% of females are carrying fertilized eggs by late January and 99% are gravid by April. Females lay between 400 and 36,000 eggs annually, and second broods are rarely observed. Embyo crabs begin at a size of 380 μm and grow to 450 μm prior to hatching. Hatching typically occurs between May and July, but is highly dependent on water temperature.