Sand dollars are marine invertebrates recognized by their flat, disc-like appearance. These creatures are a type of sea urchin, belonging to the class Echinoidea. Their unique shape and bleached white remains found on beaches often spark curiosity.
The Sand Dollar’s Structure
A living sand dollar possesses a rigid skeleton, known as a test, composed of calcium carbonate plates. This test is typically round or oval and features a five-petal pattern on its top surface, formed by five paired rows of pores. These pores allow for the extension of tube feet, which aid in gas exchange and respiration.
The living animal is covered in tiny, flexible, velvet-textured spines, which vary in color from green, blue, violet, or purple, depending on the species. These spines, along with microscopic hair-like structures called cilia, are essential for movement and feeding. The sand dollar’s mouth is located on its underside, at the center of its body, while the anus is positioned near the rear edge of the underside.
Life Beneath the Waves
Sand dollars typically inhabit sandy or muddy bottoms in shallow coastal waters, though some species can be found in deeper ocean environments. They primarily use their spines and cilia to burrow just beneath the surface of the sand, providing protection from strong currents and predators. When waters are calm, they may stand on end, partially buried, but will lie flat or burrow completely when conditions become rough.
Their diet consists mainly of detritus, algae, diatoms, and small organic particles found on the seafloor. They also consume zooplankton, such as crustacean larvae. Sand dollars use their specialized tube feet and cilia to transport these food particles along their body surfaces to their central mouth.
From Ocean Floor to Keepsake
When a sand dollar dies, its soft tissues decompose, leaving behind the durable calcium carbonate skeleton, or test. This test, initially dark like the living animal, loses its spines and becomes smooth. Ocean currents and waves then wash these bleached white tests ashore, transforming them into familiar souvenirs found by beachcombers.
The five-petal pattern and sometimes present holes on the test have given rise to various cultural associations. One popular folk tale suggests that the patterns represent a poinsettia flower. If broken open, five small, hard, tooth-like structures, resembling doves, are found inside. This “five doves” myth connects the sand dollar to symbols of peace and goodwill.
Remarkable Adaptations
The sand dollar’s flattened body shape allows it to efficiently burrow into sandy substrates. This streamlined form helps it avoid strong currents and hide from predators. Its dense covering of tiny spines and cilia are used for movement and feeding, enabling the sand dollar to move food particles along its body to its mouth.
The internal skeletal structure, or test, provides protection and stability. Some species have specialized openings, called lunules, which help reduce lift and prevent the sand dollar from being swept away by strong waves by allowing water to pass through the body. Young sand dollars may ingest sand grains to increase their weight, aiding in anchorage in turbulent waters.