Sand dollars, with their distinct, flattened disc shape, are a common sight on many beaches. Often mistaken for a type of seashell, these objects are actually the skeletons of marine animals. This unique structure, featuring a characteristic five-petal pattern, forms the durable remnant of a living creature.
Biological Classification and Habitat
Sand dollars belong to the phylum Echinodermata, making them relatives of sea urchins, sea stars, and sea cucumbers. They are classified under the order Clypeasteroida. These marine invertebrates inhabit shallow coastal waters, preferring sandy or muddy seabeds where they can burrow or remain partially buried.
They function as detritivores in the marine ecosystem, feeding on organic materials like dead plant matter, microscopic algae, and plankton found along the ocean floor. Sand dollars live in crowded groups, sometimes with hundreds of individuals in a single area. This communal living supports their reproductive strategy.
The Formation of Their Unique Skeleton
The disc-shaped structure of a sand dollar is its rigid internal skeleton, called a test. This test is composed of calcium carbonate, a hard, chalk-like material. As the sand dollar grows, specialized cells secrete these calcium carbonate plates, which fuse together to form its characteristic flattened shape.
Skeletal growth includes the development of a five-petal pattern on its upper surface, known as petaloid ambulacra. These areas are perforations in the endoskeleton through which tube feet, used for gas exchange, extend. The flattened form of the test is an adaptation that helps the sand dollar maintain a stable position against wave action and facilitates burrowing into the sand.
Life Cycle and Transformation
The life cycle of a sand dollar begins with eggs and larvae, released into the water during broadcast spawning. These planktonic larvae, known as pluteus larvae, develop arms and float in the open water. After several weeks, typically around six, these larvae undergo metamorphosis, settling onto the seabed and developing into their juvenile form.
Once a sand dollar dies, its soft tissues decompose, a process aided by bacteria and other microorganisms. This decomposition leaves behind the durable calcium carbonate test. The test then becomes bleached white by the sun and surf, transforming into the “sand dollar” object found on beaches.
Distinguishing Live from Dead Sand Dollars
Identifying a live sand dollar from a dead one is straightforward. Live sand dollars are dark in color, ranging from gray, brown, or reddish to deep purple. They are covered in thousands of tiny, velvety spines, which give them a fuzzy appearance and allow them to move slowly or burrow into the sand. If gently held, a live sand dollar’s spines will move, and it may even leave a harmless yellowish stain, called echinochrome, on the skin.
In contrast, dead sand dollars lack these spines and are smooth, brittle, and bleached white due to sun exposure. They do not exhibit any movement and are found washed ashore. It is recommended to leave live sand dollars undisturbed in their natural habitat.