Pacific Sand Crab

The Pacific sand crab is a small crustacean growing up to 35 mm (1.4 in) long and 25 mm (1.0 in) wide. The female is nearly twice as large as the male and can often be identified by the orange egg mass carried under the telson. The adult is sand-coloured and well camouflaged, and has no claws or spines. It has five pairs of legs and three pairs of pleopods. Sand crabs moult periodically, so their exoskeletons may be found washed up on the beach.

The sand crab occurs in North America from Alaska to Baja California and in South America, where it is found from Salaverry, Peru, southwards to around Cape Horn and into southern Argentina. The species is common on the beaches of California, but greater changes in population levels occur further north, probably as a result of variations in the coastal currents which in some years passively disperse the planktonic larval stages northwards. In Oregon, the populations seem not to be self-sustaining and recruitment is largely from larvae originating in California. There is an established population of sand crabs on the Twin Harbors Peninsula of Washington state.

The sand crab lives under the surface of the sand, moving up and down the beach according to the state of the tide. As each wave advances and retreats, the crab comes to the surface and extends its antennae to feed. This makes it vulnerable to predatory birds such as the sanderling. These birds actively patrol the part of the beach washed by incoming waves, probing the softened sand with their bills. The sand crab retreats under the sand surface as each wave goes out, maximizing its chance of being out of reach of the bird's beak. The bird maximizes its chance of feeding on sand crabs by scurrying at the edge of the surf.