Purple Shore Crab

Hemigrapsus nudus

Marine Life - Brazil

Quick Facts

Nocturnal
  • Scientific Name: Hemigrapsus nudus
  • Spanish Name: Cangrejo de costa púrpura
  • Family: Varunidae
  • Known Nicknames: Naked shore crab
  • Average Length: 4.0–5.6 cm / 1.6–2.2 in (carapace width)
  • Average Weight: 15–30 g / 0.5–1.1 oz
  • Wingspan: N/A
  • Key Feature: Purple or reddish-purple carapace with characteristic dark purple spots on the chelipeds (claws)
  • Primary Diet: Omnivore (primarily Algae, Sea Lettuce, and small invertebrates)
  • Range: Pacific Coast of North America, from Alaska to Baja California
  • Habitat: Middle and lower intertidal zones, typically under rocks or in crevices
  • Social Structure: Solitary but gregarious (found in high-density groups)
  • Nesting/Breeding: Females carry eggs under the abdominal flap (pleopods)
  • Conservation Status: Least Concern (LC)
  • Population Trend: Stable

The Purple Shore Crab (*Hemigrapsus nudus*) is a vibrant and resilient inhabitant of the rocky intertidal zones along the Pacific coast of North America. Easily identified by its striking carapace, which ranges in color from deep purple to reddish-brown or olive green, this small crustacean is most notably distinguished by the prominent purple spots decorating its large, powerful claws. Unlike its close relative, the hairy shore crab, *Hemigrapsus nudus* features smooth, hairless legs—a characteristic reflected in its scientific name, as "nudus" translates to "naked." These agile scavengers are master survivalists, capable of staying out of water for extended periods as long as their gills remain moist, and they are frequently spotted by tide-poolers darting between crevices or hiding beneath seaweed-covered rocks.

Fun Facts

The Purple Shore Crab is easily identified by the bright purple spots on its claws, though its body color can surprisingly range from olive green to deep mahogany. Its scientific name, *Hemigrapsus nudus*, literally means "naked," referring to its smooth, hairless legs that distinguish it from its "hairy" relatives. These hardy crustaceans are remarkable intertidal survivors, capable of staying out of the ocean for up to 36 hours as long as they can keep their gills damp. Interestingly, they are also known for their "scrappy" personality; when threatened, they often choose to stand their ground and brandish their pincers rather than immediately retreating into a crevice.

Habitats & Distribution

This species is primarily distributed along the Pacific coast of North America, ranging from the Gulf of Alaska near Seward down to Turtle Bay in Baja California, Mexico. It is a common inhabitant of the temperate waters along the western seaboard, maintaining a consistent presence across diverse coastal latitudes. It predominantly inhabits the mid-to-low intertidal zones of rocky coastlines, where it seeks shelter beneath stones, within deep crevices, or among dense mats of seaweed to avoid desiccation and predators during low tide. While it favors sheltered bays and estuaries with mixed substrates of gravel and cobble, it is also frequently found on more exposed shores, demonstrating a high tolerance for varying levels of salinity and wave action.

Behaviours & Reproduction

These crustaceans are primarily nocturnal and maintain a solitary but competitive social structure, frequently congregating in high densities beneath rocks to avoid predators and desiccation. They exhibit territorial behavior, using their distinctively spotted claws for defensive displays and physical sparring against rivals. Despite their density in intertidal zones, interactions are largely centered on securing optimal shelter and food sources rather than complex social bonding. Reproduction typically occurs during the winter months. After mating, the female carries thousands of fertilized eggs attached to her pleopods beneath her abdomen, a state referred to as being "berried." These eggs are protected for several months until they hatch into planktonic zoea larvae. This reproductive strategy relies on high-volume offspring production and a mobile larval phase, allowing the young to disperse via ocean currents before eventually settling and metamorphosing into juvenile crabs in rocky coastal habitats.

Diet

The Purple Shore Crab is primarily an opportunistic omnivore with a diet that leans heavily toward herbivory, focusing on various types of green algae such as sea lettuce and diatoms. Using its sturdy claws, it scrapes film and plant matter from the surfaces of rocks in the intertidal zone, but it also functions as an effective scavenger and predator by consuming small invertebrates like snails, limpets, and worms. An interesting aspect of its feeding behavior is its dietary flexibility; while it prefers green algae when available, it can readily shift to eating carrion or animal tissue to meet its nutritional needs. These crabs are most active during high tide when they can forage submerged, though they are also known to feed in moist crevices during low tide to avoid drying out. This varied diet allows them to thrive in the competitive and fluctuating environment of the rocky Pacific coastline, where they play a crucial role in controlling algal growth and recycling nutrients.

Colors

The Purple Shore Crab features a smooth, square carapace typically colored deep reddish-purple, though it can also appear olive-green or brown. Its most defining marking is the presence of distinct purple or red spots on its pale, cream-colored claws. Lacking hair on its legs, the crab’s varied pigmentation provides effective camouflage against the dark rocks and seaweed of its intertidal habitat.