Sand dollars are intriguing marine creatures often found washed ashore, known for their distinct, flattened, coin-like appearance. These animals are far more complex and active when alive than their familiar bleached skeletons suggest. Their unique biology and behaviors in their natural ocean habitats offer a glimpse into marine life.
What is a Sand Dollar?
A sand dollar is a type of marine invertebrate classified as an echinoderm, a group that also includes sea urchins, starfish, and sea cucumbers. They are essentially flattened sea urchins, adapted for life on sandy seafloors.
The rigid skeleton of a sand dollar, known as a test, is made of calcium carbonate and typically measures between 2 to 4 inches in diameter. This test features a distinctive five-petal pattern on its surface. While the dry, dead test is smooth and white, a living sand dollar is covered in numerous tiny, flexible spines and fine, hair-like cilia.
Life in the Ocean
Live sand dollars exhibit a range of colors, from gray and brown to purple or reddish hues, a result of the tiny spines covering their bodies. These spines, along with microscopic cilia, are crucial for their movement and feeding. Sand dollars move slowly across the seafloor or burrow into the sand using a rowing motion with their spines.
They primarily inhabit sandy or muddy areas in shallow coastal waters, though some species can be found in deeper ocean habitats. Sand dollars are filter feeders, using their spines and cilia to capture small food particles like algae, diatoms, detritus, and plankton from the water column or sift them from the sand. Their mouth, located on their underside, is equipped with five tooth-like sections for grinding food.
In rough waters, younger sand dollars may even ingest sand grains to weigh themselves down and avoid being swept away. These creatures often live in dense groups, which assists in reproduction and food gathering. Sand dollars contribute to the marine ecosystem by recycling nutrients and serving as a food source for some fish and sea stars.
Finding Sand Dollars
Beachgoers commonly encounter the bleached, white tests of dead sand dollars. These are the skeletons remaining after the animal has died and its soft tissues and spines have deteriorated. In contrast, a live sand dollar appears darker and is covered in a velvety layer of tiny, moving spines.
To determine if a sand dollar is alive before collecting it, gently hold it. If its tiny spines are moving, or if it has a dark color (brown, gray, or purple), it is likely alive. Live sand dollars cannot survive long out of water, so any live specimen found should be gently returned to the ocean. Collecting live sand dollars is prohibited in many areas to protect marine ecosystems.