1 What is a sand dollar?
It refers to the extraordinarily flattened and burrowing sea urchins that belong to the order Clypeasteroida. They are called by various names such as sea cookies, sand cakes, pansy shells, snapper biscuits, etc. They have rigid skeletons which are called tests that comprise of calcium carbonate arranged in a fivefold radial pattern.
2 Why are they called sand dollars?
Their name does not mean that they have any value in terms of money, but it appears like a dollar. When the dead sand dollars skeleton washes ashore, they become bright white because as they get bleached by the sun rays. Long ago, when people found these dead sand dollar, they seemed to them like the old American dollar coins; hence they kept its name sand dollars.
3 When they are alive, they are not white
It's a strange thing about sand dollars as when sand dollars are alive, they appear purple. They are covered with a dense and velvety layer of short spines. When it dies, its skin gets bleached by the sun, which turns it white, and its spines fade away.
4 They cannot survive long without water
If you see a sand dollar on the sand do not try and take it home. The reason behind it is they can survive without water for just a couple of minutes, so the best thing is to put back in the water gently. There are various places where it is illegal to take a live sand dollar away. As they live in shallow coastal waters along the sandy ocean floor.
5 They eat with their hair
Sand dollars rely on their small spines and cilia to eat. Their mouth has a jaw in which they have five teeth like sections to grind the food. It takes around 15 minutes for the sand dollar to grind the food, and after that, it swallows it. Also, it can take around 2 days for it to digest the food.
6 What do they eat?
They eat a mix of a lot of things such as microscopic algae, copepods, crustacean larvae and other small animals. (See What is Blobfish (Psychrolutes marcidus)?)
7 They have fewer predators
The good thing for sand dollars is that there are very few animals that harm them or eat them because they have a hard skeleton and its very few particles are edible, so other animals do not bother them much. Some of the animals that might prey them are ocean pout, pink sea stars, starry flounders, etc.
8 Their pores propel them
They have five sets of pores in their body which helps in moving water in and out, which helps in allowing the movement. In case if the water is still, they can stand with the end one on the sand and the other in the water. But, if the water is fast, then they lie flat or burrow inside the water. (See Are Sea Anemones Plants or Animals?)
9 How much does it cost?
They have no or very little value. For a living sand dollar, people might pay $5 to $15 for keeping it in the aquarium. The skeletons might cost you $0.10 as they can be used for decorations.