Animals Clam Dancing

Monday, April 01, 1996
Although it is probably not possible to underrate the intelligence of clams, their athletic skills apparently deserve more respect. Olaf Ellers, a marine invertebrate biomechanic at the University of California at Davis, has found that some clams jump out of the sand and ride the waves, the better to stay near the water’s edge as the tide changes.

Most researchers who have studied clam locomotion (they are not a large group) hold that waves erode clams from the sand, carrying the mollusks along. And indeed, that is true for most clams. But according to Ellers, coquina clams in North Carolina are not so passive. They surf and seek out the largest waves.

Ellers took 234 dead clams and painted them pink. During an ebb tide, he planted each one in the sand in shallow water about an inch away from a live clam painted red. When the tide was out, he counted how many were left on the beach. The results: Fifty to 80 percent of the dead clams were still buried, compared with 5 percent of the live ones. The live clams had propelled themselves out of the sand with their muscular foot after a retreating wave had passed over them, and had let it carry them seaward. Their foot is like our tongue, Ellers says. It’s as if you had your head in the sand and stuck your tongue out and pushed against the sand with it.

During a rising tide, Ellers observed clams in the sand jump in front of a breaking wave and then ride the churning water up the beach. The clams chose only the waves that would carry them farthest--a strategy that no doubt saves them energy. But how do they pick the waves? In an aquarium in his lab, Ellers played them wave sounds and found that most jumps coincided with the pre-arrival rumble of the loudest waves. Ellers guesses the clams may sense the vibrations with small, hairlike cilia on their neck, which is really the siphon they breathe through.

Surfing keeps coquinas near their food--the breakers stir up plant and animal matter that the clams filter out of the water. But they may also be trying to keep out of the sun. Unlike clams in New England, which rest comfortably in cool, wet sand between tides, these coquinas live on hot, dry beaches. If you heat them up, they die, says Ellers.
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