Sand dollars, often mistaken for decorative shells, are in fact living marine animals. Their distinct, flattened, disc-like structures are the bleached remains of once vibrant creatures. These organisms live beneath the ocean’s surface, playing a role in their marine ecosystems.
Understanding Sand Dollar Classification
Sand dollars are living invertebrates belonging to the phylum Echinodermata. This phylum includes marine animals such as sea stars, sea urchins, and sea cucumbers, all characterized by a unique five-part radial symmetry. More specifically, sand dollars are a type of flattened sea urchin, classified under the class Echinoidea and order Clypeasteroida. The term “echinoderm” means “spiny skin,” referencing the spiny surfaces found across this group. Like other echinoderms, sand dollars possess a water vascular system, a network of fluid-filled canals that aids in various bodily functions.
Physical Features of Sand Dollars
A live sand dollar differs considerably from the familiar white “test” or skeleton found on beaches. Living specimens are covered in dense, tiny, velvety spines, giving them a soft, often purplish, brownish, or gray appearance. These spines, alongside fine hair-like cilia, are essential for movement, feeding, and respiration.
The rigid skeleton, known as a test, measures between 2 to 4 inches in diameter. On the upper surface of this test, a distinct five-part, petal-like pattern is visible, formed by paired rows of pores. These pores allow specialized tube feet, used for gas exchange, to extend from the body. The sand dollar’s mouth is centrally located on its underside, while the anus is positioned towards the posterior edge.
Behavior and Environment
Sand dollars inhabit sandy or muddy seafloors, typically in shallow coastal waters, though some species live at greater depths. They move slowly across the seabed using their spines in a rowing-like manner, progressing at about 13.7 mm per minute. To change direction, they rotate their bodies around their central mouth. These animals are filter feeders, consuming microscopic organisms such as plankton, algae, detritus, and crustacean larvae found in the water column or within the sand. They use their spines and cilia to transport food particles along their body surfaces to their mouths.
Sand dollars burrow into the sand for protection from predators and strong currents. In calm conditions, they may stand upright and partially buried, but lie flat or completely burrow when waters become rough. Young sand dollars can ingest heavy sand grains to weigh themselves down and prevent being swept away. Sand dollars often live in dense aggregations, with densities reaching up to 625 individuals per square yard, a strategy that may aid in reproduction and collective defense.