Sand dollars are unique marine creatures often discovered as bleached, disc-shaped remnants on beaches. These intriguing animals are a type of flattened sea urchin, belonging to echinoderms like sea stars and sea cucumbers. While the familiar white objects found on sandy shores are the skeletons of deceased individuals, live sand dollars have a distinct appearance and inhabit sandy ocean floors.
The Unique Features of a Sand Dollar’s Body
A live sand dollar’s body is disc-like and flattened, appearing circular or slightly pentagonal. Most species measure approximately 2 to 4 inches (5 to 10 cm) in diameter, resembling a large coin. This flat form allows them to burrow into the sand for survival.
The natural coloration of a living sand dollar ranges from purplish or brownish to reddish-brown or gray. This darker hue comes from thousands of tiny, dense spines covering their body. These spines give the live animal a velvety or fuzzy texture, different from a dead specimen’s smooth, hard surface.
The upper surface, known as the aboral side, features a distinctive “petaloid” pattern. This design resembles a five-petal flower, formed by specialized pores through which tube feet extend. These tube feet facilitate gas exchange, allowing the sand dollar to respire in its sandy habitat.
On the underside, or oral surface, a central star-shaped groove leads to the mouth. This mouth contains five teeth-like structures, known as Aristotle’s lantern, used for grinding microscopic food particles. The sand dollar uses its tiny spines and hair-like cilia on its underside to move food particles towards the mouth.
Some sand dollar species also have oval or elongated holes, called lunules, that pass entirely through their body. These perforations serve multiple functions, including acting as pressure drainage channels to help the animal remain buried in strong currents. Lunules can also aid in food gathering by assisting the passage of food particles from the upper to the lower surface.
Telling the Difference: Live vs. Dead Sand Dollars
Distinguishing between a live sand dollar and a dead one is straightforward. A live sand dollar displays a natural, darker color, such as purplish, brown, or gray, unlike the white or bleached appearance of a dead test. This color comes from its living tissue and covering spines.
The texture provides another clear indicator. Live sand dollars are covered in tiny, fine, movable spines, giving them a soft, velvety feel. Dead sand dollars are smooth and hard, as their spines have fallen off and their skeletal “test” is exposed and dried. Observing these small spines for movement is a reliable way to check for life.
If gently placed in shallow water, a live sand dollar may exhibit subtle movements of its spines or tube feet, indicating its active state. Dead specimens remain rigid and inert. A live sand dollar, or one that has recently died, might have a faint, distinct marine odor, while a fully bleached test will be odorless.
Some living sand dollars can also secrete a harmless substance called echinochrome, which may leave a temporary yellowish stain on human skin upon contact. Live sand dollars are more robust and less prone to breakage than the dried, fragile skeletons found on beaches. If you encounter a live sand dollar, return it gently to the ocean, as they cannot survive long out of water and are often protected by local regulations.