Sand dollars are flattened, disk-shaped marine invertebrates often found washed up on beaches, their white skeletons masking a complex living creature. Related to sea stars and sea urchins, sand dollars do possess tube feet, which are a defining characteristic of their biological group. However, the function of these structures is highly specialized, differing significantly from the roles they play in other seafloor dwellers.
Defining the Sand Dollar and its Relatives
Sand dollars belong to the echinoderm group, which includes sea cucumbers and brittle stars. All echinoderms share a unique internal hydraulic mechanism, the water vascular system. This system is a network of fluid-filled canals that generates the pressure required to extend and move the tube feet.
The body of a sand dollar is encased in a rigid, flattened skeleton called a test, which is what is commonly found on the shore. Because of their disc-like shape, sand dollars are classified as irregular echinoids, a subgroup of the sea urchins. The test is covered in a dense, velvet-like coating of minute, movable spines when the animal is alive.
The test displays a distinct five-part radial symmetry, most notably on the upper surface where a structure resembling a flower petal is visible. This petal-like pattern, scientifically known as the petaloid ambulacra, is a specialized area of the test. The pores within this area are where the sand dollar’s tube feet emerge, connecting them directly to the water vascular system within the body.
The Role of Tube Feet in Sand Dollar Biology
Unlike the prominent, suction-cupped tube feet used by sea stars for movement and prey capture, the sand dollar’s tube feet are tiny and hair-like. These small, flexible appendages are not designed for strong adhesion or locomotion. Their primary role supports the sand dollar’s sedentary, burrowing lifestyle in soft sediment.
A major function of the tube feet, especially those within the petaloid pattern on the upper surface, is gas exchange, or respiration. These specialized feet act like gills, drawing oxygen from the surrounding water and releasing carbon dioxide through their thin walls. This respiratory function is important because the sand dollar often lives partially or fully buried in the sand.
The tube feet also serve a sensory role, helping the animal perceive its immediate environment. They detect changes in water chemistry and subtle physical contact, which assists the sand dollar in positioning itself optimally.
Specialized Movement and Feeding
Since the tube feet are specialized for respiration and sensing, the sand dollar relies on other structures for movement and feeding. The animal’s subtle locomotion and ability to burrow are managed by the dense carpet of short spines and microscopic hairs, called cilia, that cover its entire body. Coordinated movements of these spines allow the sand dollar to slowly creep across the sandy bottom and burrow efficiently into the sediment. This burrowing action, often driving edgewise, helps the animal hide from predators or anchor against strong currents.
When feeding, the sand dollar utilizes its spines and cilia, not its tube feet, to capture food particles. They are primarily detritus feeders, consuming small organic matter, algae, and larval organisms found in the water or sand. Mucus secreted across the body surface traps these fine food particles, which are then ferried by the action of the cilia toward the mouth, located on the underside.
Some species, such as the eccentric sand dollar, will partially bury themselves vertically in the sand to suspension feed. In this upright position, specialized spines and the tube feet may work together to filter plankton from the water current flowing past them.