For a chance to admire great resilience, go wading on a (safe) sandy ocean beach. Where shallow waves slosh onshore, you may find the Pacific mole crab, aka sand crab, Emerita analoga—a creature at home in impermanence. Found in patches on Pacific beaches from Alaska to Baja California and Ecuador to Argentina, this small crustacean is often super-abundant from spring through fall. At area beaches, like Drakes in Marin, a scoop of wet sand may wriggle and twitch in your hands.
Mole crabs live in perpetual motion. Unlike a barnacle, say, that clings to solid rock, a mole crab moves—fast—to stay within the ribbon of beach washed by waves. This mobile part of a sandy shore has a wonderful name: the swash zone! Not only can storms and currents relocate masses of sand; the swash itself perpetually shifts on the beach, with the tides. Mole crabs travel along with their dynamic habitat.
They’re ultra-efficient at this—always locomoting backward, whether paddling to body-surf in flowing water or digging to burrow into soaked sand. Emerita has a smooth carapace that’s egg-shaped for low resistance; five pairs of legs for swimming and steering; and a tail-like flap, the telson, for rapid shoveling. Place an individual you’ve examined on wet sand and watch it vanish in seconds, leaving only a dimple.
To feed on the ocean’s abundant plankton, a mole crab positions itself just below the sand’s surface, facing the surf. It extends its eyestalks and breathing antennae and unfurls a pair of longer, feathery antennae. With these it quickly combs the receding water, then draws the antennae through its mouthparts to glean food. It can do this more than once on a single wave.
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Mole crabs reproduce from spring to early autumn. Gravid females (up to 1.6 inches long, twice the size of males) secure broods of bright-orange eggs to their abdomens with specialized thread-like legs and their tucked-under telson. Several times each year a female Emerita may release dozens to thousands of minuscule larvae into the ocean, where they drift, feed, grow, and change through eight to 11 larval molts. After about four and a half months at sea, those that survive the journey try to land on beaches as masses of tiny adults.
Huge populations and a wide distribution make this little decapod important as food for nearshore fishes and migratory shorebirds. Though Emerita seeks a zone in the swash that’s too deep for birds and too shallow for fish, its own feeding activity can reveal its location. Troops of sanderlings arrive in the sheen and motor back and forth, rapidly probing for crab treats. Shorebirds as large as the long-billed curlew also stalk mole crabs. Surf scoters may dredge the shallows for Emerita, while nearshore fishes (mole crabs’ main predators) hunt beneath the breakers or even ride them into the swash.
Yet mole crabs persist. Even when winter surf gouges sand from a beach, mole crabs can go along for the ride and survive the winter in offshore sandbars. In spring the hardy adults move, with the sand, back onshore. A mole crab can keep up this rugged lifestyle for two to three years.
Though its numbers fluctuate, Emerita’s regular appearances on California’s coast make it a useful indicator of beach and nearshore conditions. Since 2002, students in the Greater Farallones Association’s LiMPETS program have gathered mole crab data to build a baseline for evaluating shoreline phenomena like beach restoration or ocean toxins. You can survey crabs for yourself at the beach, at least unofficially. A handful-sample of wet sand may hold female crabs clutching tangerine-colored eggs; a carpet of oddly rumpled sand may house a throng of tiny recruits, newly landed to begin life in the swash.
