Hemigrapsus nudus

Distinguished from the similar Hemigrapsus oregonensis (yellow shore crab) by its purple coloration and smoother carapace; H. oregonensis typically shows yellow or greenish tones with more granular texture. Separated from Pachygrapsus crassipes (striped shore crab) by lack of transverse stripes on walking legs and more rounded carapace margin. Carcinus maenas (European green crab), where ranges overlap, has five prominent spines on each side of the carapace and green coloration.

Small crab with a square to slightly rounded carapace, typically reaching 3-4 cm in carapace width. Coloration ranges from deep purple to olive-brown, often with lighter mottling. The walking legs are relatively slender and may show banding. The carapace surface is smooth or finely granular, lacking the pronounced anterolateral teeth found in many related species. Chelipeds (claws) are roughly equal in size and moderately robust.

Rocky intertidal zones, particularly in the mid to high intertidal region. Shelters under rocks, in crevices, and within algal holdfasts during low tide. Tolerates moderate wave exposure but more common in protected to semi-protected shores. Substrate typically rocky with cobble or boulder fields.

East Pacific: Alaska (Aleutian Islands) south to Baja California, Mexico. Continuous distribution along the Pacific coast of North America.

Active year-round in temperate portions of range; may reduce activity during winter in northern populations. Breeding season extends from spring through fall, with peak activity varying by latitude.

Primarily herbivorous, feeding on sea lettuce (Ulva spp.) and other green algae. Occasionally scavenges dead animal material.

Females carry fertilized eggs on the abdomen until hatching. Larvae pass through zoeal and megalopal stages in the plankton before settling to the benthos as juvenile crabs. Multiple broods per year possible in warmer portions of range.

Seeks shelter under rocks during low tide to avoid desiccation and predation. Forages on exposed rock surfaces and in tide pools during high tide. Generally slow-moving and cryptic; relies on shelter rather than flight for predator avoidance.

Herbivore and detritivore in rocky intertidal food webs. Serves as prey for shorebirds, fish, and larger crabs. Contributes to algal community structure through grazing pressure on green algae.

No direct economic importance. Occasionally collected for educational purposes or as bait. Not targeted for human consumption due to small size. May be incidentally affected by shoreline development and pollution.

• Hemigrapsus oregonensisSimilar size and habitat; distinguished by yellow-green coloration, granular carapace texture, and presence of small teeth between the eyes
• Pachygrapsus crassipesOverlapping range and habitat; distinguished by striped walking legs, more rectangular carapace with distinct anterolateral teeth, and generally larger size
• Carcinus maenasWhere introduced ranges overlap; distinguished by five prominent lateral spines on each side of carapace, green coloration, and more aggressive behavior

• intertidal
• rocky shore
• herbivore
• Pacific coast
• crab

• BugGuide
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• Bug Eric: Wasp Wednesday: Wasp Workshop
• Bug of the Week