As you stand at the edge of the ocean, letting your feet sink into the sand as the waves wash back and forth, your toes are not the only thing being buried and unburied by the constant motion. There are thousands of little critters that make their home just below the surface of the sand where the waves wash ashore.
Coquinas (Donax variabilis) are one of the most abundant macroinvertebrates in this zone. They live in dense colonies (up to 1,500 clams per square foot!) beneath the sand at the water’s edge. These tiny wedge-shaped clams come in almost any color and pattern imaginable, making them a favorite of beachcombers.
Coquinas, also known as butterfly clams, can make a rainbow out of the wrack line, but the truly fascinating thing about these bivalves is watching them in action. Dig your hand into the wet sand right after a wave recedes, and coquinas will seem to pop out of the sand as they pull themselves back down under the cover of the sand. During the summer and into early fall, you can find live coquinas by the handfuls on sandy beaches from Virginia all the way to Texas.
Coquinas have a muscular foot that they extend from their shell to maneuver through the sand and two straw-like siphons that are used to pull seawater through their body and expel it after filtering out food particles and oxygen. Watch the video below to see these clams doing their thing. If you look closely, you can see their foot and siphons as they scoot around in our bucket.
It seems logical that these small coquinas, need to bury themselves in the sand to avoid being haphazardly tossed around in the waves, but coquinas actually use the flow of the waves to their advantage. Colonies of coquinas make daily migrations shoreward and seaward as the tides move in and out each day. As the tide rises, when a coquina senses the right incoming wave, it will “jump” out of the sand and be pushed further up the beach where it buries in the sand again. As the tide falls, the coquinas will pop out of the sand again and ride the waves out towards sea. If you look closely as the waves wash up and down the beach, you might witness some of the coquinas taking a free ride.