Sand dollars are intriguing marine creatures, often found washed ashore, captivating beachgoers with their distinct, disc-like shape. Many people mistake them for a type of shell or rock, unaware they are remnants of a complex living animal. Their unique form sparks curiosity.
More Than Just a Shell
Sand dollars are not simply shells but are the skeletal remains of living organisms. These animals belong to the phylum Echinodermata, a group that includes sea urchins, starfish, and sea cucumbers. This classification means they share a distant evolutionary relationship with these other spiny-skinned marine invertebrates. Unlike their more globular sea urchin relatives, sand dollars possess a flattened body plan.
Their most recognizable feature is their rigid, disc-shaped endoskeleton, known as a test, which provides structural support. While alive, this test is covered by a dense carpet of tiny, movable spines, giving them a velvety texture. These spines are often purplish, brownish, or reddish-brown and are crucial for movement and feeding. A distinctive petal-like pattern, composed of five radiating rows, is visible on the upper surface of the test. These areas are called ambulacral areas and house specialized tube feet, part of their unique water vascular system.
Life Hidden in the Sand
Sand dollars inhabit shallow coastal waters, preferring sandy or muddy seafloors. They are found in calm, sheltered areas like bays and estuaries, where they burrow into the substrate. They use their short spines and small tube feet to maneuver across or just beneath the sand surface. This burrowing behavior helps them hide from predators and maintain position in shifting sediments.
Their diet primarily consists of detritus, including decaying organic matter, microscopic algae, and diatoms found within the sand. They are filter feeders or detritivores, using mucus-coated tube feet and spines on their underside to collect food particles. These particles are then transported to a central mouth located on their underside. Respiration occurs through specialized thin-walled structures.
Distinguishing Live from Deceased Sand Dollars
When encountering a sand dollar, it is important to differentiate between a living animal and a deceased one. Live sand dollars exhibit a darker coloration, ranging from purplish to reddish-brown or dark brown. They are covered in a dense layer of fine, velvety spines that are often movable if gently touched. These live specimens might also show subtle movements, slowly burrowing into the sand or shifting their position.
In contrast, the white, bleached sand dollars commonly found on beaches are the tests of deceased animals. These tests are rigid, brittle, and completely devoid of spines. They have been cleaned by the ocean’s currents and sun, losing their original color. Observing and respecting live sand dollars in their natural environment is important, as collecting them while alive can disrupt their role in the ecosystem.