The familiar flat, disc-shaped object found on sandy shores, known as a sand dollar, is a distinctive marine treasure. Its star-like pattern and smooth, white surface often spark curiosity about its origins.
The Sand Dollar: An Organism, Not a Home
The belief that something “lives inside” a sand dollar is a common misconception. The object found on the beach is not a shell, but the skeleton, or “test,” of the sand dollar itself. Sand dollars are living marine invertebrates belonging to the phylum Echinodermata, which includes sea stars, sea urchins, and sea cucumbers. They are flattened, burrowing sea urchins, classified under the order Clypeasteroida. Unlike globular sea urchins, sand dollars have adapted to a life primarily spent within or on sandy seafloors.
Life Beneath the Sand: A Sand Dollar’s World
Living sand dollars inhabit sandy or muddy seafloors in shallow coastal waters. They are often found in large, dense groups, with hundreds of individuals occupying a single square yard. When alive, their bodies are covered in a dense layer of tiny, flexible spines and fine, hair-like structures called cilia, giving them a velvety appearance. These spines and cilia are essential for movement, helping the sand dollar burrow into the sand and creep across the seabed.
Their coloration when alive is not white, but ranges from shades of gray, brown, or reddish-brown to vibrant purple. Sand dollars are filter feeders, consuming microscopic particles like algae, diatoms, detritus, and small zooplankton. They use their specialized spines and tube feet to transport food particles along their bodies to a central mouth located on their underside. In calm waters, they may stand on end, partially buried, to capture food from the water column, but will lie flat or burrow completely when currents are strong. Young sand dollars can even ingest sand grains to increase their weight and better anchor themselves against strong currents.
The Enduring Legacy: What Washes Ashore
When a sand dollar dies, its soft tissues decompose, leaving behind its rigid, calcified skeleton. This skeleton, known as a “test,” is composed of calcium carbonate. Ocean currents, wave action, and sunlight then act upon the test. The sun’s bleaching effect turns the test from its living color to a white or silvery-white hue. The small spines that covered the living animal also detach, leaving the smooth surface of the test.
The five-petal pattern on the top of the sand dollar test corresponds to specialized areas on the living animal called ambulacral areas. These areas contain pores through which tube feet extend, serving functions like gas exchange and helping with feeding. These durable, bleached tests are what beachcombers find.