Purple shore crab

Geographical Origin and Habitat: Hemigrapsus nudus, the purple shore crab, is native to the Pacific coast of North America, ranging from southern Alaska to Baja California, Mexico. It is commonly found in the rocky intertidal zone, sheltering under stones, within seaweed beds, and in tide pools. It is highly adapted to survive fluctuating salinity levels and temporary exposure to air.

Taxonomy: Classified within the family Varunidae and the genus Hemigrapsus. This small crab features a sub-square carapace that measures approximately 4 to 5 centimeters (1.5 to 2 inches) in width. It has hairless, smooth legs (unlike the closely related hairy shore crab) and robust claws.

Social Behavior: It is a highly active scavenger, primarily nocturnal or crepuscular, that frequently climbs out of the water onto damp rock surfaces. While they tolerate high density in nature, in the home aquarium, they can be territorial and aggressive over hiding spots and food sources.

Coloration and Sexual Dimorphism: The carapace color ranges from dark purple to reddish-purple or reddish-brown. The claws (chelipeds) are lighter, typically white or yellow on the underside, and marked with distinctive purple or red spots. Males have a narrow abdomen and larger claws, while females have a broad, rounded abdomen used to carry eggs.

Tank Setup: A paludarium or aquaterrarium is required, providing a dry land area using rocks or driftwood that rise above the water line. The water can range from brackish to marine (specific gravity 1.015 to 1.024). A tight-fitting lid is absolutely critical, as they are exceptional escape artists.

Diet and Feeding: An opportunistic omnivore and scavenger. Feed a varied diet including sinking shrimp pellets, algae wafers, pieces of fish or clam, and blanched vegetables. They will also graze on naturally occurring green algae in the tank, helping with overall maintenance.

Water Quality: This species prefers cooler water temperatures, ideally between 12 and 18 °C (54 to 64 °F), and does not tolerate tropical heat (above 22 °C / 72 °F) well. Maintain pH between 7.8 and 8.2. Good filtration is necessary to keep waste levels low.

Compatibility and Tankmates: Not suitable for standard tropical reef tanks due to temperature requirements. Best kept in a species-specific setup or with quick, cold-water fish. They will prey on slow-moving fish, snails, and other small invertebrates if they can catch them.

Aquarium Breeding: During breeding season, females carry bright orange eggs under their abdomen. Rearing the larvae through their pelagic zoeal stages is extremely difficult in a home aquarium, requiring specialized planktonic setups and microscopic live foods.

Risks and Diseases: Molting failure is a common risk if calcium or iodine levels are depleted in the water. High temperatures can cause heat stress and mortality. During the molting period, the crab is highly vulnerable to predation by conspecifics, so multiple hides must be provided.