The sand dollar is a marine animal recognized by its smooth, bleached remains found along coastlines. This flattened, disc-shaped creature is a type of sea urchin, classifying it as an echinoderm, a relative of sea stars and sea cucumbers. The familiar white disc is the animal’s internal skeleton, or test, which remains after the organism dies and its soft tissues decay. This skeletal structure gave the animal its common name because of its resemblance to large, old Spanish or American coins.
Biological Classification and Anatomy
The sand dollar belongs to the Phylum Echinodermata and the Class Echinoidea, grouping it with spiny-skinned animals like sea urchins and sea biscuits. It is categorized into the Order Clypeasteroida, and is considered an irregular echinoid due to its distinct flattened body shape, unlike the globular form of its regular sea urchin cousins. Its body structure features a five-part radial symmetry, a characteristic shared by all echinoderms.
The animal’s rigid, internal skeleton, called the test, is composed of fused plates of calcium carbonate. On the dorsal surface, the test features a distinct petaloid pattern of five paired rows of tiny pores. These pores are openings for specialized tube feet that function primarily in respiration, drawing oxygen from the surrounding water. In life, the test is covered by a dense coating of short, velvety spines and finer, hair-like cilia.
These tiny spines and cilia are continuously moving, facilitating locomotion and feeding. The sand dollar uses these spines to move across the seabed rather than relying on its tube feet for primary movement. The mouth is centrally located on the underside of the test, containing a complex jaw structure known as Aristotle’s lantern. This specialized apparatus contains five tooth-like parts used to grind up small food particles.
Habitat and Behavior
Sand dollars prefer shallow, temperate, and tropical marine waters, typically dwelling on soft, sandy, or muddy bottoms below the low tide line. They often aggregate in large groups on the seafloor, forming vast beds where food sources are abundant. Their flattened shape is an adaptation that allows them to live buried just beneath the surface of the sand, offering protection from strong currents and predators.
The animal uses its short, velvety spines to burrow into the sediment, driving itself edgewise into the sand to hide. When the water is calm, some species position themselves vertically to filter food from passing water currents. In areas with stronger currents, juvenile sand dollars may ingest grains of sand to weigh down their bodies, preventing them from being swept away. Their primary diet consists of small organic particles, including detritus, plankton, and algae, which are captured by the spines and cilia and transported to the mouth.
The Difference Between Live Sand Dollars and the Test
The appearance of a live sand dollar is markedly different from the smooth, white skeletal souvenir collected by beachgoers. A living sand dollar is covered in a dense layer of tiny, movable spines, giving it a velvety texture and a color that ranges from dark purple, reddish-brown, or gray. This dark coloration helps the animal camouflage itself in the sandy sediment of its habitat.
The white, smooth disc found on the beach is the test, the animal’s skeleton, which remains after the sand dollar dies and the spines and soft tissue fall away. This post-mortem test is then bleached white by the sun and surf, leading to its clean, pale appearance. Beachcombers can determine if a sand dollar is alive by observing its color and movement. If it is dark-colored and its tiny spines are moving, or if it leaves a yellowish stain (echinochrome) when handled, it is alive and should be returned to the water.
Live sand dollars cannot survive out of the water, and collecting a live specimen is often prohibited to support conservation efforts. The white, spine-less tests are the acceptable souvenirs to gather, as they signify the animal has completed its life cycle. A common piece of folklore associated with the test is the legend of the five “doves” found inside, which are actually the five jaw components of the Aristotle’s lantern structure. When the test is broken open, these five individual pieces are said to resemble tiny white doves.