Sand dollars are marine animals with flattened, coin-like skeletons found along the shore. These commonly collected souvenirs are merely the bleached remains of once-living creatures. While the dead shells are widely recognized, a live sand dollar presents a different appearance, offering a glimpse into its fascinating existence as a burrowing sea urchin. They are related to other echinoderms like sea stars and sea cucumbers, sharing a similar five-part radial symmetry.
Recognizing a Live Sand Dollar
A live sand dollar looks different from the white tests found on beaches. Its coloration is dark, ranging from purplish, brown, reddish-brown, gray, or even charcoal, which helps it blend into its sandy environment. This contrasts with the bleached white or tan appearance of a dead sand dollar, where the soft tissues have disintegrated and the skeleton is exposed to sunlight. The surface of a living sand dollar is covered in tiny, movable spines, giving it a velvety or fuzzy texture. These spines, which are absent on dead shells, are small, hair-like cilia that aid in movement and feeding.
Observing a live sand dollar reveals subtle movements of these spines. If gently held, one might feel the coordinated motion of these tiny structures, indicating the animal is alive. Some species may also leave a yellowish stain, called echinochrome, on the skin if handled, which is another indicator of life. The underside of the sand dollar, which faces the seafloor, contains the mouth located at the center of its five-part pattern. This side also features tube feet that assist in movement and gathering food particles.
Their Natural Environment and Habits
Sand dollars primarily inhabit sandy or muddy seafloors in shallow coastal waters, where they can be found just beneath the surface. These marine invertebrates prefer soft, fine sand, which facilitates their burrowing activities. They often live in large aggregations, with hundreds of individuals in a single area.
Their spines are used to burrow into the sand, allowing them to remain hidden from predators and access food sources. In calm waters, sand dollars may stand on end, partially buried, but in rougher conditions, they flatten themselves or burrow completely to maintain their position. Younger sand dollars can ingest sand grains to weigh themselves down, preventing them from being swept away by currents.
Sand dollars are filter feeders, using their spines and tube feet to collect organic particles from the sand. Some species can tilt on their side to capture floating organic matter from the water column. The collected food is then transported to their central mouth, with digestion sometimes taking up to two days.