You probably have seen sand dollars on the beach before and thought it was a cool sort of shell or rock. However, you may not know that the sand dollar you just picked up was once a living animal.
Sand dollars are invertebrates related to sea urchins and starfish. Their resemblance to starfish can be seen in their symmetrical five star shape. When they are alive, they are a gray or purple color, and this color comes from cilia (hairs) on their calcium carbonate skeleton, called the test. The sand dollars you find on a beach are white because the sun has bleached their color. The cilia of a sand dollar can also be used as gills and can also catch prey, such as algae, diatoms, and crustacean larvae. After filtering their prey out of the water, the cilia pass it along to the other side of the sand dollar, where its mouth is located. The mouth is arranged in 5 sections of teeth. Sometimes, a sand dollar can chew for up to 15 minutes because of its inefficient chewing.
Below: a live sand dollar
Sand dollars tend to live in very crowded areas beyond the low water line. Because they are always on the bottom of the ocean floor, they must adapt to the harsh currents and the sand always hitting them. One way they are able to resist the current is to bury themselves into the sand. Using the cilia (hairs) on the skeleton, they are able to slowly shift the sand above them and wedge themselves into the sand. They can also align themselves so that they are parallel to the current, or push themselves flat on the sand, both of which could not be done if it weren’t for their flat shape. In fact, they can be viewed as just a flat sea urchin.
A video of a sea urchin burying itself is here.
Sand dollars that eat sand
Believe it or not, a sand dollars’ diet consists of sand when it is young. Because they live in turbulent waters, they must make themselves heavy so that they don’t get carried away. One way to do this is to eat magnetite particles in the sand. Young sand dollars are able to use their cilia to find the heaviest particles (which are magnetite particles), and they eat them up to store in a place called the diverticula. Scientists have been able to x-ray sand dollars and have seen a significant amount of magnetite in adult sand dollars.
The white areas shown are the magnetite that the sand dollars has swallowed over the course of its life, about 6-10 years.
Sand dollars live nearby each other so it is easier for them to reproduce. They reproduce by releasing their eggs and sperm into the water sometime around May, and the egg and sperm join to form a larvae. The larvae float freely in water and swim until they develop and their calcium carbonate skeleton grows, and they sink to the bottom of the ocean.
Sand dollars in the wild live around 6-10 years. Their predators are sea snails and sea stars.