Sand dollars are a common sight for beachgoers, often found as smooth, white discs washed ashore. These intriguing objects lead many to wonder if they were once living organisms. The answer is yes; sand dollars are indeed living marine animals, though the familiar white form represents their skeletal remains.
The Living Sand Dollar
A living sand dollar is a type of flattened sea urchin, belonging to the phylum Echinodermata, which also includes starfish and sea cucumbers. When alive, its appearance differs significantly from the bleached disc typically found on beaches. Live sand dollars are covered in a dense layer of tiny, velvety spines and cilia. These spines give the animal a color ranging from purplish, brown, or greenish, rather than white.
The spines and cilia are not merely for coloration; they are essential for the sand dollar’s movement and feeding. Coordinated movements of these structures allow the sand dollar to burrow into the sand or creep along the seafloor. On its underside, a central mouth is present, to which food particles are transported by the spines and cilia. Sand dollars feed on microscopic algae, detritus, and small organic matter.
The Shell You Find
The familiar white, disc-shaped object discovered on beaches is not a shell in the traditional sense, but rather the “test” or endoskeleton of a dead sand dollar. This test is composed of calcium carbonate plates fused together, providing a rigid yet lightweight structure. After the animal dies, currents and sunlight clean and bleach the test, removing the spines and revealing its distinctive features.
The surface of the test displays a petal-like pattern, often resembling a five-pointed star. This pattern, called petaloid ambulacra, contains tiny pores where tube feet were once located in the living animal. Some species of sand dollars also feature small holes, known as lunules, which help the animal remain anchored in sandy environments by allowing water to pass through.
Their Ocean Home
Sand dollars inhabit shallow, sandy, or muddy seafloors in temperate to tropical coastal waters around the world. They prefer areas free of dense vegetation, allowing them to burrow effectively. These animals live in large groups or “beds,” sometimes with hundreds of individuals per square yard, which can aid in reproduction and food acquisition.
Sand dollars exhibit burrowing behavior, partially or fully burying themselves in the substrate to hide from predators and strong currents. Their flat bodies and specialized spines enable them to dig into the sand efficiently. While feeding on microscopic particles, their presence and feeding habits contribute to the marine ecosystem by filtering organic matter from the seafloor. They also serve as a food source for certain fish, such as cod and flounder, and other marine organisms.