Sand dollars are marine invertebrates, recognized by their flattened, disc-shaped appearance. These creatures are a type of burrowing sea urchin, belonging to the class Echinoidea, which also includes sea stars and sea cucumbers. Often discovered washed ashore, their distinctive skeletal remains are a common sight for beachcombers.
Anatomy of a Living Sand Dollar
A living sand dollar presents a different appearance than the bleached white disc found on beaches. When alive, its body is covered in a dense layer of short, velvety spines, which can be various colors depending on the species. These spines, along with tiny hair-like structures called cilia, facilitate movement across the seabed and aid in burrowing into the sediment.
The underside of a sand dollar, known as the oral surface, features a central mouth used for feeding. The anus is located on the upper surface. Small, flexible tube feet, part of a water vascular system, extend through pores on the body, assisting with movement, gripping surfaces, and gas exchange.
The Test and Its Features
The familiar disc-shaped object found on beaches is the sand dollar’s “test,” which is its rigid internal skeleton. This calcified structure is primarily composed of calcium carbonate plates that are fused together, offering both strength and support to the animal’s delicate internal organs. The test typically measures between 5 to 10 centimeters (2 to 4 inches) in diameter for most species.
A prominent feature of the test is its five-part radial symmetry, visible as a distinct petal-like pattern on the upper surface. These “petals” are ambulacral areas, which contain paired rows of tiny pores. In a living sand dollar, these perforations allow the tube feet to extend through for functions like gas exchange. Some species also possess distinctive slits, called lunules, that help the animal remain embedded in the sand and prevent it from being swept away by currents.
The “Doves” Inside
A common discovery when a sand dollar test breaks open are small, white, bird-like structures, often referred to as “doves.” These are not actual birds but rather ossicles, which are components of the sand dollar’s feeding apparatus. Specifically, they are the five jaw sections of what is known as Aristotle’s lantern. This structure, also found in sea urchins, functions for grinding food.
In a living sand dollar, Aristotle’s lantern consists of five tooth-like sections supported by numerous calcified elements and muscles. These components work together to scrape and grind microscopic food particles from the seafloor. Once the sand dollar dies and its soft tissues decompose, these durable ossicles detach and can be found loosely rattling inside the empty test. Their distinct shape has led to the popular folklore and symbolism associated with “sand dollar doves.”
Habitat and Lifestyle
Sand dollars primarily inhabit sandy or muddy seafloors in shallow coastal waters, though some species extend into deeper oceanic zones. Their specialized spines allow them to burrow efficiently, seeking refuge from predators and strong currents. In calm waters, sand dollars may orient themselves upright and partially bury into the sand, while in rougher conditions, they lie flat or burrow completely. Young sand dollars can even ingest sand grains to increase their weight and stability.
Their diet consists mainly of microscopic organisms and organic debris found in the sediment. They are filter feeders, consuming diatoms, algae, and detritus. The spines and cilia on their body surface help to capture and transport these food particles towards the central mouth.
Sand dollars exhibit slow movement, using their spines and tube feet to crawl across the seafloor. They often aggregate in groups, with hundreds of individuals found within a single square meter, which can aid in reproduction. Most sand dollar species have a lifespan ranging from six to ten years.