Sand dollars are flattened, disc-shaped marine invertebrates that belong to the order Clypeasteroida, making them relatives of sea urchins and starfish. While many recognize their bleached skeletons found on beaches, live sand dollars in their natural habitat appear quite different. They are covered in a layer of fine, velvet-like spines and cilia, often exhibiting colors like gray, brown, red, or purple.
Their Diverse Diet
Sand dollars primarily obtain their nutrition as detritivores and filter feeders, consuming a variety of microscopic particles. They feed on decaying organic matter, known as detritus, which includes fragments of dead algae, plant material, and tiny animal remains that settle onto the seafloor. In addition to detritus, sand dollars also consume microscopic algae, such as diatoms, which are single-celled organisms found either suspended in the water column or attached to sand grains. They also filter plankton from the water, encompassing both phytoplankton (microscopic plants) and zooplankton (tiny animals like crustacean larvae). Sand dollars use specialized body structures to capture these minute food sources, rather than actively preying on larger organisms.
The Mechanics of Feeding
Sand dollars possess unique adaptations to efficiently collect and process tiny food particles. Their underside is covered with thousands of fine, hair-like structures called cilia, which coordinate to sweep food along intricate grooves on their body surface. These cilia work in conjunction with mucus, which traps food particles, adhering them together for transport.
Specialized tube feet further assist in gathering food from the surrounding sand or water. Once collected, the food particles are moved towards the sand dollar’s central mouth. Inside the mouth lies a complex chewing apparatus known as Aristotle’s Lantern, equipped with five tooth-like sections that grind the collected food before it enters the digestive system. This grinding can take several minutes before the food is swallowed.
Where They Find Their Food
Sand dollar habitats are directly linked to their feeding habits, as their environment provides necessary food sources. These invertebrates inhabit shallow coastal waters, residing on or partially buried within soft, sandy, or muddy seabeds. Their flattened body shape and ability to burrow allow them to live just beneath the surface of the sand, positioning themselves for feeding. They thrive in areas with relatively calm currents, which permits detritus and plankton to settle effectively onto the seafloor or remain suspended within their reach. These sheltered sandy environments are rich in the microscopic organisms and decaying organic matter that constitute their diet.