Secrets of the Sand Dollar

Winter brings some of the year's best beachcombing to the Jersey Shore, and rougher seas kick up treasures onto the beach. One highly sought-after "shell," the sand dollar is actually the skeleton of a fascinating bottom-dwelling animal related to the sea urchin.

What it is: Sand dollars are tidal and ocean-dwelling invertebrates in the same relatively primitive animal phylum, Echinodermata, as starfish and sea urchins. Different species are found in seas all over the world, but individuals in the family of creatures share a number of characteristics.

Like a lot of other echinoderms, sand dollars exhibit radial symmetry – that is, their bodies look the same on the left and right, and their differences are top-and-bottom only. The flat-bodied creatures spend their time on sandy seafloors near the coast.

Most of us picture sand dollars as the white, rigid shells we find washed up on the beach on occasion. Alive, however, they look quite different. A living sand dollar is covered with a colorful, fuzzy-looking coating of spines that help it creep through sand and sediment and move food particles – algae and the larva of crustaceans and mollusks – to its mouth in the center of its underside. This time-lapse video shows a west-coast sand dollar moving through wet sand on its small, spiny feet.

The flowerlike petal shapes on the sand dollar’s upper side are actually pores that house specially adapted arms for breathing.

As miniscule larva themselves, sand dollars are easy fish food (though the journal Science explained a recent study a few years back that showed the tiny jelly-like larva can clone themselves when fish are nearby to increase their numbers and the chance of their genetic survival). Adult sand dollars have few, if any, natural predators.

Where to find it: The common sand dollar tolerates the cooler waters of the Jersey Shore, but it’s rare to see living ones. Still, beachcombers hunting along relatively calm beaches may come across the bleached white skeletons of the animals.

The sand dollar pictured here in the author's hand was collected on the beach in Asbury Park at low tide. 

Why bother: Besides being lovely beach gleanings, sand dollars are a reminder that the ocean holds a lot of strange and remarkable creatures. We may think of them more as décor than as living things, but sand dollars are complex, ancient animals that have remained largely unchanged over the last 50 million years or so.