Sand Dollars or Sea Biscuits are some of the weirdest creatures in the ocean. They are basically living fossils; their bodies consist of velvet-textured spines inside of a skeleton of calcium carbonate. Sand Dollars are actually a brown, sandy color in their natural conditions. When they die and wash to shore they are bleached white by the sun. They move across the ocean by coordinating the movements of their spines.
So you’re probably wondering what and how the intricate shape on their shell forms? Well the development of these creatures is even more intriguing than the final product. The petal is actually like their lungs, it consists of tiny pores in the exoskeleton that exchange oxygen and carbon dioxide. The beautiful markings on the back of sand dollar are actually excretions from its anal glands. The hole in the bottom of the dollar is its mouth with which it consumes crustacean larvae, algae and detritus. So what brings nutrients in is right to next to what sends them out. But these creatures get even more interesting.
Sand dollars group together in the ocean so they can reproduce easily. Fertilization is external, so gametes are released from the male and the female, eggs are fertilized, and little free-swimming larvae result, without any skeleton. Natural selection drives predatory defense adaptations, and for the sand dollar, it decided to be very creative. When larvae are exposed to mucus from predatory fish, they simply clone themselves, halving their size. So when you see smaller sand dollars on the beach, they are result of this defense mechanism. Once Sand Dollars mature, they have almost no known predators.
More Photos of Sand dollars can be found at http://ryancorrigan.shutterfly.com/stockprints/634