Sand dollars are marine invertebrates known for their distinctive flat, disk-like shape. Related to sea urchins and sea stars, they are often found along coastlines. Their intricate patterns make them a cherished find.
The Iconic White and Gray
The appearance most people associate with a sand dollar is its bleached white, off-white, or light gray form. This smooth, hard disc is the skeleton, or test, of a sand dollar after it has died. Once the animal perishes, its living tissues and tiny spines decompose and detach. These familiar white tests are typically collected as souvenirs.
The Living Sand Dollar’s True Colors
A living sand dollar presents a striking contrast to the familiar white discs found on beaches. When alive, these creatures are typically much darker, displaying hues that can range from purplish or reddish-brown to dark green or gray. Their entire body is covered in a dense layer of tiny, velvety, hair-like spines, also known as cilia.
These minute spines are essential for the sand dollar’s movement, its ability to burrow into the sand, and even for gathering food. The vibrant, darker coloration of a living sand dollar is a result of its active living tissues and the various pigments contained within its body. If a live sand dollar is handled, it may release a harmless pigment called echinochrome, which can temporarily stain the skin yellow.
Factors Influencing Sand Dollar Shades
Beyond the distinction between living and deceased specimens, several factors contribute to the diverse shades observed in sand dollars. Different species of sand dollars can inherently possess slightly varied base colors, whether they are alive or have recently died. For instance, some species naturally exhibit gray, purple, or blackish spines and tests.
The surrounding environment also plays a role in influencing the final color of a sand dollar’s test. Minerals present in the sand or other organic matter within its habitat can cause staining or alter the test’s appearance. Additionally, the presence of algae can impart a pale green tint to the surface of the test.
The transformation from a living, dark sand dollar to the iconic white test involves a natural bleaching process that occurs after death. Once the sand dollar dies, its soft tissues begin to decay, and the countless tiny spines that covered its surface fall off. The remaining hard, calcium carbonate test is then exposed to the persistent effects of sunlight and saltwater.
The ultraviolet (UV) radiation from the sun, combined with the chemical action of saltwater, acts as a natural bleaching agent. This gradual process strips away any residual organic pigments, eventually revealing the familiar white or light gray skeleton. The duration and intensity of this exposure to the sun and water largely determine the ultimate bleached shade of the sand dollar.