Swash Riders of the Summer Surf

Natural History, July, 2000 by Peter J. Marchand

Tiny but sturdy, bean clams live on the edge.

With breakers surging past and the sand washing out from under my feet, I stared intently into the water. Through the choppy reflections of a golden Pacific sunset, I soon spotted a small wedge clam. And it was not alone: it had a hydroid hitchhiking on its back.

The wave-washed edge of a sandy shore, devoid of permanent physical structure and subject to constant change, is a difficult habitat for any living thing. Yet this so-called swash zone--defined by the run-up and backwash of the frothy surf--is home to a number of organisms. One requirement for residency is the ability to adjust to daily tides as well as to lunar and seasonal cycles that continually shift sand and water levels. And if the potential swash-zone resident is a filter-feeding organism that needs to stay anchored on the sandy bottom, it must have the agility to quickly regain a favorable position if dislodged by wave action or temporarily buried by sand.

The list of common swash-zone animals includes a number of diminutive burrowing crustaceans (shrimplike organisms such as ostracods) and annelids (worms) that live below the sand surface. But one of the most familiar inhabitants that stays up near the action is a small clam known on Atlantic and Gulf of Mexico beaches as the coquina and on the Pacific coast as the bean, or wedge, clam. (It is also called the sea butterfly for the resemblance of its empty shell, when agape, to butterfly wings.) Though these clams (all in the genus Donax) are subject to population swings at specific locations, one or more species can be found almost anyplace that has warm water and a sandy beach, from the Americas to Africa, Asia, and Australia.

The success of this cosmopolitan mollusk has much to do with its prowess as a swash rider. Using its foot and siphons much like a sail, a Donax clam surfs landward and seaward with flowing and ebbing tides, then digs in quickly between waves to maintain itself in the wet sand just above the tide line. It has been demonstrated in the laboratory that Donax respond actively to loud wave sounds and that this sensitivity is cued to an internal biological clock. This may be how the clam knows when it is time to push out of the sand and ride the surf up and down the beach, remaining within the swash zone regardless of the tide cycle. For animals that spend most of their time sitting in the sand feeding, wedge clams can move surprisingly quickly. Some investigators have reported seeing Donax jump out of the sand with a single thrust of the foot, and I have seen many small individuals bury themselves from a standing start in less than four seconds.

Despite such impressive abilities, the clam I was currently watching had been upstaged by an even more unlikely actor--the small hydroid on its back. Organisms that look more like plants than animals, hydroids are tubelike filter feeders classified in the same phylum as jellyfish and sea anemones. They are free swimming only in their brief larval stage. Early in their adult lives, hydroids anchor themselves in one place and grow thereafter by clonal duplication, forming branching chains of polyps that soon resemble small bushes. Remarkably, a few species of hydroids have found their way into the rich, turbulent waters of the swash zone by attaching themselves to the posteriors of swash-riding clams.

I mused over this adaptation while my clam, with hydroid attached, dug into the sand until only the hydroid remained visible. Though I could no longer see the clam, I knew it had pushed its siphon to the surface for feeding, and it occurred to me that the hydroid, by creating an eddy in the swash, might actually help the clam obtain food. But the advantage seemed mostly the hydroid's. As I watched a well-scoured cobble and pieces of larger shell tumbling in the constant wave action, I could see that a hydroid attached to either of these inanimate objects would soon be ground into organic matter for other filter feeders. I knew, too, that sometime in the night, long before this spot was left high and dry by the outgoing tide, the hydroid's "anchor" would have pulled itself up and moved, with its hitchhiker, back down the beach to continue the incessant task of filtering seawater. Getting hooked up with an animal like Donax seemed a very clever trick of hydrozoan evolution.

The next day I returned to the same beach at low tide and walked out to the edge of the swash zone. The sand was firm and glistening, almost metallic. Spent waves lapped at my feet. The beach appeared devoid of life, as if everything had returned to the sea during the night. Then a large breaker shattered the illusion. As the roiling backwash streamed past, it scoured around me, and when the froth cleared, there beneath my feet were multitudes of clams, all small and without hydroids. I dropped to my knees and scooped frantically before the next wave rushed in. Two hundred fifty-three clams in a single square foot of sand--and six feet farther up the beach from the tide line, not one.