If you have walked a beach along the California coast, you’ve probably seen sand dollars washed up. You may have even picked one up and wondered what it was. A shell? Something that never was alive? If you happened to find an intact sand dollar—most are broken when they end up on the beach—you might conclude that it was something formerly alive. And you’d be correct! But what kind of organism was it?
Sand dollars are animals, a type of flattened sea urchin and more distantly related to sea stars and sea cucumbers in the phylum Echinodermata. Unlike their spiky sea urchin kin, which live on rocky shores at both intertidal and subtidal depths, sand dollars live primarily in shallow sandy areas past the low tide line, where they avoid most of the force of crashing waves.
The only sand dollar species in Northern California is Dendraster excentricus, the eccentric sand dollar. Alive, Dendraster is a grayish purple, covered with thousands of short spines that give its surface a fuzzy texture. Tube feet—the hollow appendages used for breathing, moving and feeding that are one of the defining characteristics of an echinoderm—protrude around the spines like transparent filaments, difficult to see with the naked eye.
In still water, Dendraster crawls on the surface of the sand, shoving sand grains aside with its spines as it moves forward. In currents, the sand dollar anchors part of its body in the sand and stands up on one edge. When the water gets too rough, the sand dollar lies flat or shelters beneath the surface of the sand. Though it digs in, it can’t burrow deeply enough to escape predators, including birds, fish, and short-spined sea stars.
The purple and red sea urchins of our coast prey on kelp and other algae. Sand dollars, on the other hand, eat tiny food. While they’re standing up in the sand their tube feet capture small bits of detritus, which the feet combine with mucus to form a sticky thread that they pass, bucket- brigade style, to a mouth on the underside of the body. Feeding this way and living where they do, sand dollars also ingest sand grains, which act as ballast to keep them from being carried away by currents.
Living just below the surface of the sand, Dendraster is protected from ordinary swells and waves. But winter storms churn the sand and wash the hapless sand dollars onto the beach, where they strand and will suffocate unless they can return to safety under water. The spines fade quickly in the sun, leaving behind the familiar white endoskeleton, or test. It is during winter that you will find the most Dendraster tests.
At first appearance, sand dollars may not seem to have much in common with sea urchins. Why, then, do biologists consider sand dollars to be sea urchins? Conceptually, if you take a globular sea urchin and flatten it for a life in the sand, you would end up with a sand dollar. To see the similarity, compare the naked tests of these animals.
Each of these tests shows a pentaradial, or five-way, symmetry. If you look carefully at an urchin’s test, what at first seems to be a mishmash of bumps and tiny holes resolves into an exquisite pattern. Spines grew out of the bumps, and tube feet emerged from the holes. On any urchin test you can recognize five sections, each consisting of two rows of former tube feet and spines, alternating with five sections indicating only spines. The sand dollar too has spines and tube feet, and the familiar five-petaled flower pattern on the test is a mark of its own pentaradial origins. This pattern, called a petaloid, is where thin, flat tube feet that were used for breathing protruded. Each “petal” corresponds to one of the five tube foot regions of the urchin.
Next time you find a sand dollar test on the beach, take a moment to study it carefully. Can you imagine how it came to live buried in the sand, so different from its rock-loving sea urchin brethren?
