Sand dollars aren’t just something beautiful to find on the beach, and they aren’t seashells. Sand dollars (also known as sand cakes, sea cookies, pansy shells, or sea biscuits) are living animals, sea urchins, which burrow in the sand and come in a variety of different shapes and sizes.
The closest relatives of sand dollars are other sea urchins, sea cucumbers, and, more distantly, sea stars. Sand dollars are “irregular” sea urchins because they aren’t radially symmetrical. The average sand dollar is about two to four inches across but is not a perfect circle.
It is possible to determine the age of a sand dollar by counting its rings, like growth rings on a tree’s trunk. The average lifespan for a sand dollar is between five and 10 years.
When they’re alive, sand dollars are reddish-brown to purple and bristly. The white sand dollars found on the beach are sand dollar skeletons (called tests). Their spines, which disappear after death, allow the sand dollars to eat. The spines move food along their bodies and into their mouths, located on the bottom of their bodies. They also move themselves with the spines located on the underside of their bodies.
A sand dollar’s diet consists mostly of algae, but they do eat other meat particles which land on their spines. For this reason, they are classed as carnivores. Sand dollars’ mouths are on the bottom and have five jaws. When you find a sand dollar test and shake it, what you hear rattling inside in part of their mouth.
The distinctive flower or star design on the top of sand dollars isn’t just for design. The five petals are called lunules. They process gas and water through the sand dollar’s body and add weight to the sand dollar to prevent them from being washed away. In rough weather, the sand dollar will often lie flat on the sand rather than on one edge.
While sand dollars have only a few predators, namely crabs, seals, and some sea stars, they are in danger from humans. Bottom trawling can decimate populations of sand dollars, as they live in close proximity to one another. A drop in salinity of their environment negatively impacts their fertilization, as well. Luckily, many states have outlawed trawling.
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