Discovering a sand dollar on a beach is a common find. A frequent question arises: are sand dollars alive? Many people only encounter their bleached, hard remains, leading to curiosity about their living form.
What is a Sand Dollar?
Sand dollars are living marine invertebrates belonging to the class Echinoidea, which also includes sea urchins. They are part of the phylum Echinodermata, characterized by spiny skin, radial symmetry, and tube feet for movement. The sand dollar’s skeleton is called a “test,” a rigid structure left after death. This test consists of calcium carbonate plates arranged in a five-fold symmetric pattern, often displaying a petal-like design on its upper surface.
A live sand dollar’s test is covered by a dense layer of tiny, flexible spines, giving it a velvety appearance. These spines and microscopic hairs (cilia) aid movement, helping the sand dollar burrow and creep across the seafloor. They also aid in feeding, transporting food particles towards the central mouth on the underside. Live sand dollars display various colors, including green, blue, violet, purple, gray, or reddish-brown, depending on the species.
Life and Habitat of Sand Dollars
Sand dollars inhabit shallow, sandy, coastal waters, often living just beneath the surface of the sediment. They prefer areas with soft, fine sand, which allows them to easily burrow for protection and feeding. These animals frequently gather in large groups on the ocean floor, sometimes with hundreds of individuals in a single square yard. This close proximity can aid in their reproduction and foraging.
Sand dollars are filter feeders, consuming small organic matter, plankton, algae, and detritus from the water or sand. They use their tube feet and cilia to capture these food particles, which are then transported along their bodies to their mouth. Their mouth contains a specialized jaw structure called Aristotle’s lantern, equipped with five tooth-like sections to grind their food. Digestion can take up to two days.
Movement involves the coordinated action of its spines and cilia, allowing it to slowly burrow or creep across the seabed. In calm waters, they may stand on end, partially buried in the sand. During rougher conditions, sand dollars lie flat or burrow deeper to maintain their position, sometimes ingesting sand to weigh themselves down. Sand dollars reproduce through broadcast spawning, releasing eggs and sperm into the water, which then fertilize and develop into larval stages before settling on the seafloor. They live for about 6 to 10 years.
Live vs. Washed-Up Sand Dollars
The familiar white, hard “shell” found on beaches is the sand dollar’s skeleton, or test, remaining after death and decomposition. This test is bleached white by sunlight over time, and all the tiny spines fall off. Observing the differences between a live sand dollar and a washed-up test helps determine if the creature is still alive.
A live sand dollar is darker in color, ranging from brown, gray, or purplish hues, and is covered in fine, velvety spines. If gently held, a live sand dollar’s spines may show movement. Another indicator of a live specimen is the release of a harmless yellowish substance called echinochrome, which can stain the skin. In contrast, a dead sand dollar’s test is smooth, brittle, and bleached white, lacking any spines or color. Live sand dollars cannot survive long out of water, so it is important to return any living specimens to their marine environment if found on the beach.