Sand Dollars: Not Just Shells

                 The very popular shells you may look for on the beach, known as sand dollars, are actually the shells of living creatures. That’s right, sand dollars are not just pretty shells for tourists to shell hunt for but they are living invertebrates. When sand dollars are alive they are actually very colorful creatures whose color can range from gray to dark purple. Sand dollars are very abundant suspension feeders of the ocean floor and are evolutionary equipped to be very efficient at it. First, sand dollars have tiny spine like pincers to capture plankton floating by called pedicellariae. The now captured food in the pedicellariae needs to be transported to the mouth or madreporite. To accomplish this task sand dollars have cilia along their spines, not to be confused with a vertebrate spine, which lead right to the madreporite. These cilia, which are like tiny moving hairs, direct the food along the food grooves on the oral side of the organism to the madreporite where food can be digested. Sand dollars even position themselves to increase their ability of filter feeding. When sand dollars are in calm waters they “standup” by burying a portion of their shell under the sand and allowing the unburied portion to stand up right. If the currents are too high the sand dollar will lay flat or even bury itself to reduce the risk of being taken away by the current. Smaller sand dollars will even ingest sand to act as a weight belt for further stability in rougher waters. Sand dollars are capable of movement from tiny tube like feet that protrude from the ambulacra, the radial areas on the oral side of the shell. So sand dollars are living organisms but whats their importance to their ecosystem? Well, sand dollars are very dense in population on the sea bed, even though they lack species diversity. Sense they are so dense in population they are high contributors in the predation of tiny planktons and crustaceans which some feed on the algae on the sea bed. In short, they keep the populations of these tiny organisms in check. Even more important though is that they are a very large food source for fish and crabs. Fish and crabs are able to crush the shell of the sand dollars and eat the actual organism. Sand dollars are much easier prey than their close relative the sea urchin which are equipped with sharp protruding spikey spines to defend itself, so naturally sand dollars are a better choice. So it’s important to know that the shells we so happily collect were ecologically valuable organisms when they were alive, but that doesn’t mean you shouldn’t collect the shell for a keepsake! 

(Source and further reading on sand dollars)

(Second source)
(Follybeach.com)

Study on the effects of climate change to the sand dollars population