Sand dollars are closely related to sea urchins, sea cucumbers and starfish. They are also called "sea biscuits" or "sea cookies" depending on where in the world you are from. Sand dollars, while alive, are covered with cilia, or small hairs that give them a velvet-like appearance. These hairs actually move and help them dig in the sand and glide across the ocean floor. They also help them eat! The sand dollar's mouth is in the center of it's characteristic star on the bottom. Once the sand dollar finds a good meal, it holds onto it with it's cilia and moves it towards it's mouth. Sand dollars will eat plankton as well as other microscopic organisms and even fish eggs!
Sand dollars have a unique talent when they are young. If they feel threatened by a predator they are able to clone themselves! This doubles their odds of escaping and also halves their size. Once they are adults though, they lose this ability however they also don't have near as many predators as adults.
When they are living they are darker in color, sometimes even purplish. When they die, they wash up on shore, dry out and become bleached by the sun. What you are seeing is their exoskeleton, or "test", which is all that remains. Sand dollars don't have a lot of predators in the wild. Sadly, their number 1 predator is people. Unknowledgeable people will take them from the ocean, and they cannot survive outside of the water for very long. Unfortunately, the people who take them don't have any intentions of returning them. In some places, including Hilton Head Island, it is illegal to harass the sand dollars, which includes removing them from the ocean as well as picking them up and throwing them back. Sand dollars are unable to flip themselves over so if they don't land on their bottoms, they will not be able to survive and will eventually die. Fines for harassing any wildlife, including sand dollars, can be up to $1,000. That is an expensive souvenir!
On the top surface of a sand dollar, you will notice a flower or star shaped pattern of pores that create a circle around a perforated plate. Water is able to flow through this plate into a canal system that ends in many tiny projections, called tube feet. These emerge from the pores in the "petals". Through the thin walls of the feet are how the sand dollars absorb oxygen from their surroundings.
On the bottom of a sand dollar, are more tube feet which help to funnel microscopic organisms along a network of mucous-filled grooves towards the mouth in the disc's center. The food and debris are strained and chewed here via a complex feeding apparatus of muscles, plates and sharp teeth.
Sand Dollars may pack together in large numbers, sometimes standing on their edges in shallow water. Females will release eggs and males emit sperm through tiny pores on the top surface. Fertilization occurs in the surrounding water.
A sand dollar begins its life as tiny larvae that are free swimming. They look nothing like their adult counterparts! Eventually, they form a test and begin to look more like a sand dollar. Some species of sand dollar have slit-like holes in their test. These are called lunules. The function of these lunules in unknown. The species of Sand Dollar found on Hilton Head, like Sammy, are called Keyhole Sand Dollars, which have 5 lunules.
Sand Dollars will die if they are out of water. As stated previously, they get their oxygen from the surrounding water, so like fish, they need water to breathe.
*Some information from the Island Packet in an article written by Vicky McMillan