The Pacific sand crab is a small crab growing up to 35 millimetres (1.4 in) long and 25 mm (1.0 in) wide. The female is nearly twice as large as the male and may be identified by the orange egg mass often carried under the telson. The adult is sand-coloured and well camouflaged and has no claws or spines. There are five pairs of legs and three pairs of pleopods. The crab moults periodically so its exoskeleton may be found washed up on the beach.
The sand crab is well adapted to life in the sand, which presents an unstable substrate, and its shape is an elongated dome-shape designed for fast burrowing. The eyes are on long stalks and the antennules are also elongated so as to project above the surface of the sand. These form a tube which channels water downwards through the gills. The much longer antennae are retractable. When there is water overhead, they also project above the sand surface in order to collect food particles. The legs and uropods have hairy margins to assist in digging and for use in collecting food and transferring it to the mouth.
Biology
The sand crab always moves backwards when burrowing or crawling. It can also swim (backwards) and tread water using its back legs. It is a suspension feeder. It burrows backwards into the sand and faces the sea. As each wave retreats, it extends its antennae and catches floating organisms. It then retracts the antennae and scrapes the particles into its mouth. It can do this several times per wave. When it becomes uncovered by water, it coils its antennae and burrows backwards deeper into the sand. The diet is plankton, mostly consisting of dinoflagellates.
Ecology
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 has adopted the strategy of retreating under the sand surface as each wave goes out. By this means it may be out of reach of the bird's beak. In order to maximise the bird's chance of feeding on sand crabs, it needs to scurry around in the edge of the surf.
