There’s a lot more to the western sand dollar (Dendraster excentricus) than meets the eye. Most people who spend any time at the beach are familiar with the sand dollar’s skeleton, or test—the rigid, white flattened disk that commonly washes up on local beaches after the animal has died.
But a living sand dollar leads a much more interesting life than one would expect. In life, western sand dollars are covered with thousands of very short purple spines that move in coordinated waves like fields of grain. The surface of the sand dollar also contains an orderly arrangement of five paired rows of tube feet and an army of pedicellariae, small pincher-like organs with moveable jaws. With so many moving parts, the surface of a western sand dollar can be quite a hub of activity.
Found only along the northeastern shores of the Pacific Ocean, the aptly named Dendraster excentricus has evolved some eccentric traits and behaviors that are unique among living sand dollars.
However, the Latin name is not based on the creature’s strangeness, but rather on another meaning of “eccentric”: off-center. This sand dollar’s mouth and its ambulacral radii—special tube feet that extend from the familiar flower-shaped pattern on the top surface of a sand dollar test—are shifted off-center, toward the dollar’s back end.
That shift is integral to this sand dollar’s peculiar feeding habits. All other species of sand dollar make their living as deposit feeders, lying flat in the sand, mouth side down, using tube feet and mucous filled channels to deliver food to their oral cavity where nutrients are collected from digested sand. Dendraster, which can feed in this way, has evolved its off-center body plan to take advantage of a completely different feeding strategy as well, suspension feeding. While standing up on its edge, the animal plucks small organisms and other organic matter out of the passing current instead of extracting nutrients from digested sand. Scientists aren’t exactly sure what specific environmental factors caused the evolution of this unusual feeding behavior, but they do know that a good part of the Dendraster body plan has adapted to facilitate feeding in an upright position.
Dendraster uses its spines to push up onto its front edge, anchoring itself by burying its front end in the sand, a process that takes about five minutes. It orients itself parallel with the flow of the current, exposing both sides of its test to the passing water. In this position, tube feet and pedicellariae on both surfaces of the animal can reach out into the passing current to grab tiny bits of food. Because the creature’s “business end” is offset, all of its critical parts remain freely exposed to the current and above the sand line even as the animal anchors itself in the sand.
Eccentric positioning of body parts isn’t the only adaptation that sets Dendraster apart from other sand dollar species. It also uses a unique conveyor belt-like system of spines to pass food from the top surface of its test right over the edge to its mouth on its underside, where mucous-filled grooves carry the food to the oral cavity. Because of this adaptation, Dendraster can eat food caught on either its top or bottom surface.
So, the next time you come across a bleached-out sand dollar on a beach, remember that eating while standing up isn’t necessarily bad manners … if you’re a Dendraster.
Links to information on sand dollars and related species:
SFSU page on the western sand dollar
Echinoderm Directory, Natural History Museum-London
Thanks to Rich Mooi, curator in the department of invertebrate zoology and geology at the California Academy of Sciences, for his help with this article.