The sand dollar is not a mollusk shell but the skeleton of a marine invertebrate, a type of flattened sea urchin belonging to the class Echinoidea. Like its relatives, the sea star and the regular sea urchin, the organism possesses a rigid internal structure. Its color depends entirely on whether the creature is alive and active on the seafloor or is the remnant of a deceased animal washed ashore.
The Color of a Live Sand Dollar
A living sand dollar is not the bleached white disk often collected as a souvenir. Instead, it exhibits a dark, velvety coloration that helps it blend into the sandy or muddy seafloor. Common hues include deep shades of brown, reddish-brown, or dark purple, though some species may appear grayish-green or charcoal. This color is contained within the organism’s epidermis and the dense carpet of spines covering its body, not the skeleton itself.
The dark color comes from an external, living layer composed of tiny, movable spines and fine, hair-like structures called cilia. These minute spines are not sharp but give the sand dollar a fuzzy, velvet-like texture. The coordinated movement of these spines facilitates movement, burrowing, and feeding.
Pigments within the epidermal tissue give the spines their dark coloration, obscuring the pale skeleton underneath. The synchronized motion of these functional spines pushes food particles—primarily microscopic algae and organic debris—along channels toward the central mouth on the underside of the body. The dark color confirms the sand dollar is a living, actively feeding member of the benthic community.
Why Beachcombers Find White Shells
The familiar white disk that beachcombers find is the animal’s endoskeleton, known scientifically as the “test.” This porous, rigid structure is composed of calcium carbonate plates fused together in a five-part radial pattern. When the sand dollar is alive, the pale, hard skeleton is entirely covered by the pigmented skin and spines.
After the sand dollar dies, the soft tissues decompose, and the pigmented epidermis and spines slough off. This process exposes the underlying calcium carbonate test, which is naturally white or pale gray. Decay is often accelerated by bacteria, scavengers, and constant tumbling in the surf.
Once the bare test is washed onto the beach, its color becomes even whiter due to natural bleaching. Exposure to strong sunlight and air effectively sterilizes and whitens the calcium carbonate, similar to how bleach works, leaving behind the smooth, pristine white object that is commonly sought after. The bleached test is merely the framework of the organism, devoid of all living material, and is much more fragile than the live animal.
Color Variations Among Species
While a dark color universally signifies a live sand dollar, the exact shade varies significantly depending on the species and its specific environment. Different species are genetically predisposed to produce different baseline pigments in their epidermal layers. For instance, the Pacific Sand Dollar (Dendraster excentricus) often exhibits a color ranging from olive green to purplish-black.
Conversely, species found in tropical and subtropical waters might lean toward reddish-brown or deep violet. These variations provide camouflage against the specific shade of sand or sediment in which the organism lives. A sand dollar residing in dark, volcanic sand may be nearly black, while one on a light-colored seafloor might be a lighter shade of brown or green.
The concentration of these pigments can also fluctuate based on the sand dollar’s diet and age, meaning not every individual of a single species will be identically colored. This spectrum of pigmentation means that any dark color—from gray to purple—indicates the presence of the living, spinous coating and confirms the organism is alive.