The flat, white, coin-like object often found on the beach is the bleached skeleton, or “test,” of a marine creature known as a sand dollar. Sand dollars are living animals belonging to the phylum Echinodermata, making them relatives of sea stars and sea urchins. The test is made of calcium carbonate and remains after the animal has died and its soft tissues have decayed. This disc-shaped creature spends its life partially buried in the sandy bottom of the ocean.
The Anatomy of a Living Sand Dollar
The living sand dollar is a flattened, irregular echinoid covered in a dense layer of minute, flexible spines. These spines give the animal a velvety texture and a coloration that ranges from dark brown and gray to deep purple or reddish-brown, helping it blend into the seafloor. The spines are constantly moving, which allows the animal to achieve locomotion and burrowing.
The hard, internal skeleton, or test, is located just beneath this spiny covering and features a distinctive five-part radial symmetry. On the upper surface of the test is a five-pointed, flower-like pattern called the petaloid ambulacra. This structure is perforated with tiny pores that allow for the extension of specialized tube feet, which are primarily used for gas exchange, acting like gills. Unlike other echinoderms, sand dollars rely on their spines, not their tube feet, for movement.
Life Beneath the Sand: Habitat and Movement
Sand dollars favor life on sandy or muddy bottoms in shallow coastal waters, often congregating in large groups called “beds.” The dense covering of tiny spines and cilia allows the animal to move slowly across the substrate or burrow into the sand for protection. They use their flat, disc-like shape to their advantage, rotating their bodies to align with the current when feeding.
Movement is accomplished by the coordinated action of thousands of spines pushing against the sandy bottom. When strong currents threaten to displace them, young sand dollars may ingest heavy sand grains to act as ballast. They are primarily filter feeders, using the cilia and tube feet on their body surface to capture small organic particles from the water, such as detritus, plankton, and algae. The food is then ferried along mucus-covered channels to the mouth, which is located on the underside of the test at the center of the five-point pattern.
Ethical Collection: Telling Live from Dead
When encountering a sand dollar while beachcombing, knowing how to distinguish a living specimen from a dead test is necessary for responsible coastal ethics. A live sand dollar will have a dark color, typically brown, gray, or purple, and its surface will feel velvety due to the presence of its tiny, moving spines. If gently touched, a live sand dollar may exhibit a slight movement of its spines; some species also secrete a harmless yellow substance called echinochrome, which can stain the skin.
The familiar white, brittle, and smooth object found washed ashore is the dead test, which is safe to collect. This test is white because the spines have fallen off and the sun has bleached the calcium carbonate skeleton. If a sand dollar is still dark-colored and fuzzy, it is alive and should be left in the water, as these animals cannot survive out of their marine habitat. Many coastal regions prohibit the collection of live echinoderms, so it is important to leave moving or dark-colored sand dollars in the ocean.