H. nudus was first described in 1847 by Adam White, and in 1851, James Dwight Dana formally classified the species.

H. nudus is a small, amphibious crab that is similar physically and behaviorally to Pachygrapsus crassipes and Hemigrapsus oregonensis.

H. nudus prefers inter-tidal and sub-tidal zones, and it can oftentimes be found sheltering under rocks or other debris.

H. nudus demonstrates complex compensatory mechanisms to counteract fluctuating salinity and water oxygen concentrations, permitting it to live in a variety of different environments.

The first documentation of H. nudus occurred in 1847 when zoologist Adam White described the species as Grapsus marmoratus, a name that was not accepted taxonomically and declared a nomen nudum.

[3] Its carapace is generally a dark purple in color, although it may be olive green or red, with white or cream markings.

Furthermore, the purple shore crab is generally less aggressive and slower moving compared to the genus Pachygrapsus.

[4] While uncommon, fully white and yellow coloration has also been noted in some specimens, which has been observed throughout the Hemigrapsus genus.

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.

Juvenile H. nudus presents with a shallow depression along the frontal carapace and the spines are generally rounded.

Post-molt crabs have a soft shell that will gradually harden and will also involve the regeneration of previously amputated limbs.

[3] 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.

[1] Littorina scutulata sensu lato are also predated by H. nudus and will employ chemical cues to avoid the crabs.

[1] Like other species of the genera Hemigrapsus and Pachygrapsus, H. nudus' eggs are vulnerable to parasitization by Carcinonemertes epialti.

[1][3] 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.

[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.

This is achieved by increasing haemocyanin O₂ affinity and urate concentration, with internal alkalosis prompting these changes.

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.

[17] H. nudus is unable to digest trahalose and blood analysis indicates a preference towards acid-soluble glycoproteins.

Glucose levels are largely independent of these factors since it can be further synthesized from circulating maltose or during glycogen breakdown.