The flat, disc-shaped objects often discovered along the shore are the skeletons of marine animals related to sea urchins and sea stars. Many beachcombers are surprised to learn the familiar white souvenir looks completely different when it is still a living creature in the ocean. The dramatic contrast between the live organism and the washed-up structure is determined by the presence or absence of soft tissues and the effects of natural processes.
Distinct Features of a Living Sand Dollar
A living sand dollar is easily distinguished by its color and texture, which are far from the pale, smooth shell typically found on sand. The organism is covered in a dense layer of tiny, movable spines, or cilia, which give it a velvety or fuzzy feel. These flexible structures are typically a dark shade, ranging from purplish-black to reddish-brown or gray, providing camouflage on the ocean floor.
These spines serve a dual purpose for movement and feeding. They help the sand dollar slowly burrow into the sandy substrate and push food particles toward a central mouth located on the underside. Specialized respiratory tube feet also protrude through the skeletal covering to allow the animal to draw oxygen from the surrounding seawater. If a live specimen is handled, it may release a harmless yellow pigment called echinochrome, which is a clear sign the animal is still alive.
The Anatomy of a Found Sand Dollar Test
The object most people associate with the name “sand dollar” is the test, the deceased animal’s internal skeleton, or endoskeleton. This structure is composed of rigid plates of calcium carbonate fused together to create the recognizable, flattened disc shape. When all the soft tissues and spines have fallen away, the test reveals a surface that is smooth, brittle, and typically bleached white or light gray.
The most prominent feature is the five-part radial symmetry on the top surface, known as the petaloid pattern, resembling a flower or star. These petaloids are rows of paired pores that, in the living animal, accommodated the respiratory tube feet. The underside features two distinct openings: the larger central hole (the peristome), which was the mouth, and a smaller hole near the edge (the periproct), which served as the anus. If a preserved test is gently shaken, one might hear a soft rattling sound caused by the dried remnants of the complex jaw structure, known as Aristotle’s lantern, loose inside.
Why the Appearance Changes and Collection Ethics
The transformation from the dark, fuzzy living creature to the white, smooth test is a natural biological process occurring after death. Once the sand dollar dies, the soft organic tissues, including the dark epidermis, spines, and internal organs, decompose and fall away. The remaining calcium carbonate skeleton is then exposed to the elements.
The continuous action of the sun’s rays and tumbling in the water strip the test of pigment and also bleach it pure white. This bleaching and drying process causes the structure to lose strength, making the found test much more fragile and prone to breaking than the live animal’s structure. Responsible beachcombing requires ensuring that any collected sand dollar is definitively dead—white, smooth, and completely devoid of movable spines. In many coastal regions, collecting a live sand dollar is discouraged or prohibited by law to protect marine populations.