Overview
The Purple Shore Crab (Hemigrapsus nudus) is one of the most familiar, abundant, and charismatic invertebrates inhabiting the rocky intertidal zones of the Pacific Northwest. Instantly recognized by its glossy reddish-purple square carapace, completely hairless legs, and distinctive purple-spotted white claws, it is a beloved encounter for tidepoolers and marine naturalists from Alaska to California.
Biology and Physical Characteristics
Hemigrapsus nudus features a smooth, nearly square carapace ranging in color from deep purple and reddish-violet to dark mahogany or olive. Its species name ‘nudus’ refers to its completely naked, hairless walking legs, which distinguishes it from the related hairy-legged Oregon shore crab. Its large, white chelae (claws) are generously speckled with vivid purple or reddish-brown polka dots.
Behavior and Ecology
Remarkably hardy and adaptable, the Purple Shore Crab can tolerate broad fluctuations in water temperature, salinity, and aerial exposure. During low tide, dozens of crabs congregate beneath wet, seaweed-covered boulders to maintain gill moisture. It feeds primarily on green algae and sea lettuce, using its delicate claw tips to scrape films of nutritious diatoms off rocky surfaces.
Conservation and Human Impact
Unassessed by the IUCN Red List due to its immense intertidal abundance, the Purple Shore Crab is completely secure across its North American range. It plays a foundational role in coastal food webs, serving as a critical staple food source for sea otters, gulls, octopuses, and coastal fishes.
Interesting Factoid
The Purple Shore Crab’s scientific name ‘nudus’ literally means ‘naked’ because unlike its shaggy crab cousins, its walking legs are completely bald without a single hair.