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June 2011 cover image Those Remarkable Reds

Wild Thing: Surfing Clams

Intuitive coquinas are experts at riding the waves.

By Sheryl Smith-Rodgers

As a child on the beach, I’d spend hours crouched down, watching small wedge-shaped clams emerge from the surf. Their pearly, pastel hues mesmerized me. Then — wow! — they’d flip over and burrow back down into the wet sand.

These many years later, I’m even more impressed to learn that coquinas — classified as a bivalve mollusk with two shell halves — have some smarts.

Donax variabilis — sometimes called a butterfly shell or bean clam — live in dense colonies in the sand where waves ebb and flow.

To feed, a coquina extends two short siphons, which filter microscopic algae and debris from the water. For quick getaways, it digs its muscular foot into the sand. However, some coquinas aren’t fast enough to escape hungry shorebirds, crabs, drum and other fish.

It’s believed that coquinas spawn in the spring and summer. Females release eggs; males send out sperm. Fertilization happens in the water, and the larvae that hatch out are so tiny that they can be measured only in micrometers (a unit that equals one millionth of a meter).

As for smarts, waves don’t randomly sweep coquinas around on a beach. They decide where they want to go!

“Clams actively jump out of the sand and ride specific waves,” wrote researcher Olaf Ellers in the Biological Bulletin. If they didn’t, coquinas could be either stranded on dry ground or swept into the ocean.

How do they do that? As tides rise, coquinas — triggered by the sound of larger waves — emerge in front of a wave so they’ll be carried higher up the beach. As tides fall, the clams wait and jump from the sand as a wave recedes. Now that’s smart!

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