NC Aquariums – Ask the Aquarium – mole crabs

PINE KNOLL SHORES, NC August 18, 2014:

Q. My kids like catching those little gray, bug-like things in the sand where the waves break on the beach. Sometimes we find them and sometimes we don’t. We’ve heard them called sand fleas. Can you tell us what they are?

A. Those are mole crabs (Emerita talpoida), often called sand fleas, sand fiddlers, sand bugs or sand hoppers. Looking more like insects than crabs, these busy little crustaceans show up in early spring in the pounding surf zone – a tough place to eke out a living.

As you’ve seen, mole crabs are small, measuring only an inch to an inch and a half long. They are harmless and live in large groups. Spotting them is tricky in the wet surf line because they burrow quickly and move up and down the beach with the rise and fall of the tide. Their lives are fairly short, made up of a summer, a winter and a summer.

Spring is mating season for the little crabs. The young hatch from an orange-colored egg mass carried beneath the flap-like tail of the female. Newborns begin life as tiny, free-swimming larvae. Waves and currents can carry them as far as 200 miles offshore. After their first molt, they seek the sandy bottom in the turbulent surf zone near shore. Toward summer’s end, they molt again and transform into their adult stage.

Mole crabs have an interesting way of feeding: Each time a wave approaches, they quickly burrow tail-first into the sand. When the wave crashes on shore, they extend their plumed antennae into the backwash to trap plankton, then draw the antennae through appendages surrounding their mouth to pick off the captured food.

Entire beds of mole crabs shift position several times during the rising tide. The sand “bubbles,” as waves sweep over and carry them higher onto the beach. As each wave recedes, they use a whirling motion of their tails to quickly burrow back into the sand.

Mature crabs are active until the sand becomes too cool. As winter approaches, adults go beyond the low tide zone to pass the cold months beneath a fathom or more of water.

You can spot mole crabs in the beach surf zone by looking for patches of “bubbling” sand and a small “V” made by their protruding antennae as the waves go out.

You can spot mole crabs in the beach surf zone by looking for patches of “bubbling” sand and a small “V” made by their protruding antennae as the waves go out.


Information provided by the North Carolina Aquarium at Pine Knoll Shores. The state operates three public aquariums; one in Pine Knoll Shores, another at Fort Fisher and a third on Roanoke Island, as well as Jennette’s Pier in Nags Head.The facilities are administered by the North Carolina Department of Environment and Natural Resources and are designed to inspire appreciation and conservation of North Carolina’s aquatic environments. For more information, log onto, or call 800-832-FISH.