What are sand dollars? Sand dollars are animals! Seeing one for the first time, it may be hard to believe that these are living creatures. They are so flat and appear lifeless. Sand dollars belong to Phylum Echinodermata and Class Echinoidea which includes sea urchins and heart urchins.
Features: They got their name because they resemble a one-dollar coin. Like other echinoderms, sand dollars are symmetrical along five axes, and have tube feet and spines. Their flat disk-like shape is an adaptation for life on the sea bottom where they gather detritus.
Coat of spines: Instead of the ferocious, long spines of their spherical sea urchin cousins, sand dollars have tiny, soft spines. These spines are moveable and used like tiny spades to dig into the sand or to move around. The dense layer of spines also keeps sediments off so there is a flow of oxygenated water across the body. Like the sea urchins, sand dollars also have tiny structures called pedicellariae which look like jaws on stalks. The main function of these is to keep the body of the sand dollar free of debris and parasites. They may also be used to collect tiny food particles.
Flat skeleton: Sand dollars have an internal skeleton (called the test) formed out of large ossicles (pieces made of calcium carbonate) fused together into plates in multiples of five. The test is a rigid, hollow, flattened disk. To grow larger, each ossicle is enlarged, and new ossicles added near the anus. In some sand dollars, there is internal buttressing to support the test. Nevertheless, sand dollars are still quite fragile so please handle live ones with care.
Breathing petals: The petal design on the upperside of a sand dollar is called a petaloid. The petaloid is a series of tiny holes in the skeleton. Tube feet emerge through these holes and the sand dollar breathes through these feet! These breathing tube feet are short and flattened.
A sand dollar’s mouth is on its underside, facing the ground. Its anus is on the its underside as well, usually, this is located off-center. Some, but not all, sand dollars have jaws made of a circle of five plates that meet in the middle. Unlike those of the sea urchin, however, the sand dollar’s jaws cannot be extended outwards.
Picky eaters: Most sand dollars are deposit feeders and process sand to feed on detritus in the sediments. They don’t just process any sand. The dense layer of tiny spines keep out larger particles and only let in fine ones. Tiny tube feet and cilia (minute beating hairs) move these fine particles to the food grooves and along these grooves to the mouth in the center.
Dead or alive? Sand dollars may appear dead, but they are very much alive. A living sand dollar is covered with fine spines and appears velvety. A dead one is smooth, without any spines, and the details of skeleton can be seen more clearly. The skeleton is fragile and will shatter at the slightest pressure.
I remember to see them once in a while come ashore at nights and buried themselves in the sand when i was a kid. We called them soft urchins
I’ve never seen a live one, but when I was a kid visiting my grandparents in Florida we’d find them washed up on the beach