The sand dollar, a flat, disk-shaped marine creature often found washed up on beaches, is a source of fascination. Sand dollars do not possess legs; their movement is accomplished through the coordinated action of thousands of specialized, microscopic structures that cover their body. This unique method of locomotion allows them to glide and burrow efficiently through their preferred sandy environment.
What Exactly is a Sand Dollar?
A sand dollar is an invertebrate belonging to the phylum Echinodermata, which means it is related to sea stars, sea cucumbers, and regular sea urchins. It is a type of irregular sea urchin, characterized by its flattened body shape, an adaptation for living on the seafloor. This animal thrives in shallow, sandy marine environments, where it spends much of its time partially or fully buried beneath the surface.
The live sand dollar is covered in a velvety coating of short, dense spines that may appear green, brown, or purple. These animals are primary consumers and detritivores, feeding on tiny organic particles and microorganisms. They consume plankton, diatoms, algae, and detritus found in the water column or within the sand.
The small organic matter is collected by the spines and tiny hairs and transported toward the central mouth, which is located on the underside of the disk. Sand dollars play an ecological role as filter feeders, helping to process organic material and keep the water clear.
The Locomotion Toolkit: Spines and Tube Feet
The primary method of movement for a sand dollar is the coordinated action of its numerous small, flexible spines. Unlike the long, sharp spines of regular sea urchins, sand dollar spines are short and densely packed, giving the live animal its soft, fuzzy texture. These microscopic spines push against the sandy substrate, allowing the sand dollar to crawl slowly across the bottom.
The spines are also primarily responsible for the animal’s burrowing behavior, which is essential for protection from strong currents and predators. By maneuvering the spines, the sand dollar can drive itself edgewise into the sand until it is partially or completely covered. Microscopic hairs called cilia cover the spines and contribute to the movement of sand and food particles.
Sand dollars also possess tube feet, which are part of their water vascular system, but these structures are not the main source of locomotion. In sand dollars, the tube feet are largely specialized for gas exchange, functioning as gills, and are found projecting through pores on the upper surface. The spines do the bulk of the crawling and burrowing.
The Anatomy of the Test
The rigid, coin-like structure people find on the beach is the sand dollar’s skeleton, known as the test. This test is made of calcium carbonate plates that are tightly fused together, providing structural support and protection for the soft tissues. The test exhibits a five-fold radial symmetry, a characteristic shared with all echinoderms.
On the upper surface of the test, a distinctive five-petal flower pattern, called the petalodium, is clearly visible. This pattern is formed by five paired rows of tiny pores, which are the openings through which the specialized, respiratory tube feet extend. Some species of sand dollars also feature holes called lunules, which pass through the test and help prevent the animal from being swept away by strong currents.
When a dead sand dollar test is broken open, small, hard, triangular pieces are sometimes found inside. These are skeletal support structures known as ossicles, which are remnants of the jaw apparatus, or Aristotle’s lantern. The number of plates in the test increases with the animal’s growth, and scientists can sometimes estimate the sand dollar’s age by counting growth rings on the test plates.