Sand dollar

Natural History

The familiar exoskeleton of a sand dollar—often found cast up on a beach—is white, with an obvious five-pointed shape on the back. But a live sand dollar has a different look. Densely packed, tiny, dark purple spines cover live sand dollars and hide the star design.

In their sandy seafloor habitat, sand dollars use their fuzzy spines, aided by tiny hairs (cilia), to ferry food particles along their bodies to a central mouth on their bottom side. They capture plankton with spines and pincers (pedicellariae) on their body surfaces. A tiny teepee-shaped cone of spines bunched up on a sand dollar's body marks a spot where captive amphipods or crab larvae are being held for transport to the mouth. Unlike sea stars that use tube feet for locomotion, sand dollars use their spines to move along the sand, or to drive edgewise into the sand. On the upper half of the sand dollar's body, spines also serve as gills.

In quiet waters, these flattened animals stand on end, partially buried in the sand. When waters are rough, sand dollars hold their ground by lying flat—or burrowing under. In fast-moving waters, adults also fight the currents by growing heavier skeletons. Young sand dollars swallow heavy sand grains to weigh themselves down.

Conservation

The sandy seafloor seems to be barren—until you look closer. Diversity is low, but species concentration is high. Sand dollars are usually crowded together over an area—as many as 625 sand dollars can live in one square yard (.85 sq m).

Detritus and microscopic organisms settled on the sand provide food for scavengers and filter feeders—like burrowing anemones. Above the sand, crabs scurry for food. Flatfishes, skates and some sharks hide in the sand.

The sandy seafloor is a valuable resource and needs protection. Bottom trawling causes damage to seafloor habitats and accidentally catches and kills tons of marine life every year. The good news is that some states have enacted laws regulating trawling. Visit the Seafood Watch section of our web site to learn more about trawling and choosing seafood wisely.

Cool Facts

The sand dollar's mouth has a jaw with five teethlike sections to grind up tiny plants and animals. Sometimes a sand dollar "chews" its food for fifteen minutes before swallowing. It can take two days for the food to digest. 

Scientists can age a sand dollar by counting the growth rings on the plates of the exoskeleton. Sand dollars usually live six to 10 years. 

California sheepheads, starry flounders and large pink sea stars prey on sand dollars. When threatened by pink sea stars, sand dollars bury themselves under the sand. Observers have seen a pink sea star leave a wide path of buried sand dollars as it moves across a sand dollar bed.