No bigger than a thumb, a sand crab spends most of its time buried in shifting sand. Well camouflaged by its gray shell, a sand crab keeps its balance in the ever-moving sand with the help of a heavily armored, curved body and pointy legs. To stay put in the sand, a crab burrows quickly and often. While most crabs move in any direction—forward, backward and sideways—a sand crab moves only backward. And a sand crab has no claws on its first pair of legs—another unusual feature for a crab.
Sand crabs feed in the swash zone—an area of breaking waves. As the swash zone moves up and down the beach with the tide, so do sand crabs. To feed, the crabs burrow backward into the sand and face seaward, with only their eyes and first antennae showing. As a receding wave flows over them, the sand crabs uncoil a second pair of featherlike antennae and sweep them through the water to filter out tiny plankton. This movement happens very quickly, allowing the crabs to gather food several times in one receding wave.
Fishes, seabirds and shore birds are the main predators of sand crabs. Because a barred surfperch's diet is 90% sand crabs, surf fishermen use sand crabs as bait. Commercial bait fisheries keep sand crabs that are in the soft-shelled stage, which occurs after a crab molts its old shell and before its new shell hardens. Because the fisheries throw back hard-shelled crabs, sand crab populations haven't been affected by bait fishing.
Since sand crabs live in sand—the area of the ocean most often contaminated by toxins—they play an important role in the beach ecosystem. Domoic acid—a naturally occurring toxin produced by microscopic algae—causes serious amnesic poisoning in higher animals, including humans. Filter feeders, like sand crabs, ingest the toxin, and it progresses up the food chain. The amount of domoic acid in the crabs' flesh can indicate the amount of toxin in the water.
If dislodged while feeding, sand crabs—in a manner most unusual for crustaceans—can swim or tread water by beating their back legs.
Laboratories use sand crabs in neurological studies because the crabs' tails have the largest sensory neurons found in any animal.
Mating occurs mostly in spring and summer. A female may produce as many as 45,000 eggs. She carries them on her abdomen until the eggs hatch—about 30 days later. For two to four months, the larvae drift as plankton, and currents may carry them long distances.
Sand crabs can reproduce during their first year of life, depending on the water temperature, and may not live more than two to three years.