Sand dollars are marine invertebrates, recognized by their distinctive, flattened, disc-like appearance. They are echinoids, a class of marine animals that also includes sea stars and sea urchins. Living on the sandy ocean floor, they are often reddish-brown to deep purple and covered in flexible, hair-like spines.
How Long Do They Live?
Sand dollars typically live for approximately 6 to 10 years in their natural habitat. Scientists determine a sand dollar’s age by examining growth rings on the plates of their exoskeleton, called a test. This method is similar to counting tree rings, though sand dollar rings are much more subtle and require microscopic analysis.
These growth rings form from calcium carbonate deposition during periods of growth. Environmental conditions can influence the clarity of these rings. While this technique provides a reliable estimate, it may not pinpoint the exact age.
What Affects Their Longevity?
Several environmental and biological factors influence a sand dollar’s lifespan. Habitat quality, including water temperature, salinity, and the stability of the sandy substrate, plays a role. Sand dollars prefer sandy or muddy bottoms, often found from the intertidal zone to depths greater than 7,000 feet. They can burrow into the sand for protection from currents and predators.
Food availability also impacts their longevity. Sand dollars are filter feeders, primarily consuming microscopic organisms such as plankton, algae, diatoms, and detritus (decaying organic matter) found on the seafloor. Their tiny spines and cilia help move food particles towards their mouth, located on their underside. A steady supply of these food sources supports their growth and health.
Predators also affect sand dollar populations. Common predators include sea stars, crabs, and certain fish species like starry flounders. Sea stars use their strong tube feet to pry sand dollars from the ocean floor. Human activities also pose threats, including pollution from agricultural runoff and industrial discharge, and habitat destruction from coastal development and dredging. Over-collection of live specimens can also disrupt local populations.
Life Beyond the Lifespan: What Happens Next?
When a sand dollar dies, its body transforms. The living sand dollar, which is typically brown, red, or purple and covered in velvety spines, loses its organic tissues and spines. The hard, calcium carbonate skeleton, known as a test, remains. This test is then bleached by the sun and waves, causing it to turn the familiar white or silvery-white color often found on beaches.
Decomposition and sun exposure remove pigments and organic material, revealing the naturally white calcium carbonate structure. These white tests are what beachcombers find. Even after death, sand dollar tests have an ecological role. Their calcium carbonate composition contributes to the sand and can help buffer ocean acidity. They can also provide a substrate for other small organisms.