Purple shore crab

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.[1] 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.[8] The purple shore crab is the prey of seagulls, seabirds, white-winged scoters, anthopleura anemones, larger crabs, staghorn sculpins, and tidepool sculpins.[1][4][9] Nucella lamellosa has not been observed predating H. nudus but is attracted to its scent. Portunion conformis will parasitize the perivisceral cavity of the purple shore crab.[1] Like other species of the Hemigrapsus and Pachygrapsus genuses, H. nudus' eggs are vulnerable to parasitization by Carcinonemertes epialti.[1][3]

Front facing image of H. nudus

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.[1][4][10] 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.[4][10] 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.[3] 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 insteads prefers natural shelters. The crabs have been observed sheltering on acrylic surfaces, however.[11] Unlike other members of the Hemigrapsus genus, the purple shore crab does not live in burrows.[1] H. nudus can tolerate temperatures up to 33.6 °C (92.5 °F) but prefers colder environments.[3] Generally, H. nudus avoids temperatures above 26.9 °C (80.4 °F) and strongly prefers water temperatures of 17 °C (63 °F).[12]

Hemigrapsus nudus is commonly found together with Hemigrapsus oregonensis.[9] Unlike other shore crab species, H. nudus is slow moving and will often play dead. H. nudus is nocturnal. Male and female crabs often display different defense behaviors; males will often fight when disturbed while females will autotomize and flee.[3] Different sexes will also display different autonomy patterns, with both males and females generally missing at least one limb on average.[13] H. nudus engages in behavioral thermoregulation by scuttling in and out of the water.[12]

The purple shore crab is an osmoregulator, thus it can tolerate hyper-osmotic and hypo-osmotic environments.[1] H. nudus is more tolerant to salinity changes relative to other Hemigrapsus species and is resistant to desiccation. This allows H. nudus to habitate in seawater, brackish water, and hyper-saline estuarine marshes, with their tolerance to low salinities highly dependent on water temperature.[3] This is achieved by the heavy regulation of Na K-ATPase in the gills. The posterior gills in H. nudus engage in the majority of osmoregulation and will increase their filtration activity as salinity increases.[14]

Carapace removed, showing the internal gills of H. nudus

Hemigrapsus nudus is amphibious and capable of surviving in both aquatic and out-of-water environments.[12] H. nudus is capable of staying out of water for up to 8 hours and will engage in hyperventilation prior to exiting the water in an effort to increase hemolymph O₂ concentrations.[15]

When hypoxic, H. nudus will undergo rapid metabolic alkalosis to compensate, unlike most crustaceans. H. nudus is able to tolerate hemolymph pH levels of 8.19, indicating this compensation is an evolved response. Even in water oxygen concentrations as low as 10 mmHg PO₂, H. nudus will increase cardiac output but will not hyperventilate or increase heart rate. This is achieved by increasing haemocyanin O₂ affinity and urate concentration, with internal alkalosis prompting these changes. Under normal oxygen conditions, H. nudus relies predominately on dissolved arterial O₂ for adequate profusion but will transition to haemocyanin reliance in low oxygen environments. Furthermore, H. nudus has shown the ability to limit its use of urate oxidase in a temperature dependent manner, thus limiting its overall oxygen use. These adaptations indicate a strong evolutionary drive to survive in hypoxic waters and have been used to contest the theory that crustations evolved air breathing as a means to avoid hypoxia.[16]

Hemigrapsus nudus can digest glucose, galactose, fucose. maltose, maltotriose, and maltotetraose, making its metabolism similar to other crabs.[17] H. nudus is unable to digest trahalose and blood analysis indicates a preference towards acid-soluble glycoproteins. Maltose derivative metabolites serve as one of the primary energy sources for H. nudus. While H. nudus is generally able to metabolize maltose oligosaccharites, maltose levels in the blood can vary depending on the individual's diet and the time of year. H. nudus uses glucose-6-phosphate, a maltose derivative, as an intermediate in maltose oligosaccharide formation which, in turn, is used as an intermediate in glycogen synthesis. Glucose levels are largely independent of these factors since it can be further synthesized from circulating maltose or during glycogen breakdown. This glucose is used by the hepatopancreas as a major component in pre-molt chitin synthesis.[18]