Most people probably think of sand dollars as treasures found on a beach. They are usually white with an amazing and intricate five-point shape on the back that looks like petals of a flower (also known as a petaloid ambulacra). These white sand dollars are no longer alive. Which brings up the question, what are they like when they are alive?
Living sand dollars, relatives of sea stars and sea urchins, dwell on the seafloor and are greyish purple. They are covered in tiny hairs and spines called cilia that serve a number of purposes. For starters, sand dollars use the cilia to move along the sea floor – who knew that a sand dollar had a form of locomotion! Not only that, but sand dollars only lie flat on the bottom of the ocean when the water is rough or fast moving. Most of the time they use the cilia to stand themselves up on edge, with one end partly buried in the sand. The spines on the upper half of a sand dollar also serve as gills.
But wait, there’s more. The cilia are used to transport food particles towards the sand dollar’s mouth on the bottom, center of its body. They are considered carnivores because they eat plankton, crab larvae, and small bits of animals, in addition to algae. Inside the sand dollar’s mouth, there’s a jaw with five teethlike sections, used to grind up their food! These creatures chew for up to 15 minutes before swallowing. Food takes up to two days to fully digest.
When you find a sand dollar on the beach, there are probably others nearby because, when alive, sand dollars live in colonies! Whether or not this makes them “social” creatures is up for debate, but they do gather together in groups that can exceed 600 sand dollars in a square yard. These packed crowds may aid in reproduction because sand dollars don’t mate in a traditional sense. Instead, both males and females “broadcast” sperm and eggs into the water. The more closely packed they are, the higher the chance sperm and eggs will meet.
So the next time you find the exoskeleton of a sand dollar, it is indeed a treasure but one with a long and interesting life history before it landed at your feet!