What are those little gray bug-like things that dig into the sand where the waves break on shore?
Those are mole crabs (Emerita talpoida), often called sand fleas, sand fiddlers, sand bugs or sand hoppers. They are early spring arrivals on our beaches.
Pinkish-gray in color, these little crustaceans measure only 1 to 1-1/2 inches long and live in large groups. They’re harmless and are often sought after by fishermen, who use them as bait.
As you mentioned, mole crabs live in the pounding surf zone – a tough habitat. They move up and down the beach with the rise and fall of the tide, and their lives are fairly short, made up of a summer, a winter and a summer.
Looking more like insects than crabs, they begin life as tiny, free-swimming larvae, hatched from an orange-colored egg mass carried beneath the flap-like tail of the female. As larvae, they can be carried as far as 200 miles off shore.
After their first molt, they seek the bottom in the turbulent surf zone near shore. Toward summer’s end, they molt again, this time transforming into adults.
Mole crabs have an interesting way of feeding. Each time a wave approaches, they quickly burrow backward into the sand. Because their eyes are small and practically useless, they orient toward the sea so their antennae can filter water for plankton as the waves go out.
After the wave crashes on shore, the crab extends its antennae into the backwash to trap plankton, then rapidly draws the antennae through the appendages surrounding its mouth to pick off captured food.
Several times during the rising tide, entire beds of mole crabs shift position. The sand “bubbles” as a wave sweeps over them and carries them higher onto the beach. When the wave slackens, they quickly dig into the sand using a whirling motion of their tail.
Spring is mating season for the little crabs, and by July most or all of the males hatched the preceding summer have died. The females carry their egg masses for several months until the young hatch. Before winter, all the females will have died and only a single generation remains on the beach. Mole crabs are active until the sand becomes too cool. As winter approaches, they go out beyond the low tide zone to pass the cold months beneath a fathom or more of water.