Beach Walks with Dr. Joe Life in the Swash Zone By Dr. Joe Richardson Believe it or not, there is a section of the beach known as the “swash zone.” It is right there where very shallow water washes in, due to the waves, but then immediately washes back out. I still remember one of my first experiences at the ocean was standing in the swash zone and feeling that weird sensation as the sand washed from around and under my bare feet. I still like to do that! With the constantly moving sand and water, the swash zone would seem to be a rather unstable habitat for any animal to live in. And it is. Also, consider 26 TYBEE BEACHCOMBER | SEPT 2017 this: as the tide comes in, the swash zone moves up higher on the beach; and as the tide goes out, the swash zone moves out with it. So this is a really dynamic, ever-changing habitat. Why would anything choose to live here? I’ll give you my guess later. As unstable as it is, there is a very small clam that actually prefers the swash zone. It is called a Coquina (Donax variabilis). The “variabilis” refers to the fact that their shells come in a variety of colors. On Tybee you can find them with white, yellow, orange, blue, lavender and purple shells. They are small, less than an inch, and somewhat triangle shaped. To find Coquinas in the swash zone, just stir the sand with your hands. They will be just under the surface, probably less than an inch. This summer, I’ve seen more Coquinas than I ever remember seeing on Tybee. When you find one, lay it back down on the wet sand and watch. Often it will dig itself right back down into the sand. It won’t go very deep, and once it is back in place, the Coquina will extend two tiny tubes out the top. Through one of these tubes it pulls water in, for oxygen and plankton (food). After passing through the clam, the water goes back out the other tube. So the Coquina needs to be underwater, but because it is so small with short tubes, it can’t be very far below the sand surface. And I think this is part of the answer to why it prefers the swash zone. If it lived out deeper where it is always under water, it would be easy prey for bottom feeding fish like Whiting and Pompano and for shallow crabs like Speckled Crabs. But in the swash zone, it has a refuge from these predators. So even though the Coquinas have to re-position themselves as the tides rise and fall and do a bit of digging every few minutes, they may have found a safer place to live. So the next time you are standing in the swash zone, stir up a few Coquinas and watch them dig back in. I think it is pretty impressive for a tiny little clam. Dr. Joe Richardson is a retired marine science professor with 35+ years of research and teaching experience along GA and the southeastern coast and Bahamas. Besides research, he conducts Tybee Beach Ecology Trips year round (www.TybeeBeachEcology.com) and frequently posts pictures of what they are finding on his Tybee Beach Ecology Trips Facebook page.
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