The eye of the casual explorer in the bay or on the beach is first taken by the numerous shelled animals, the Mollusks, like crown conchs and lightening whelks, or the Arthropods, like horseshoe, spider and blue crabs. But on closer inspection, one finds the remains or living specimens of another major group – the Echinoderms. Echinoderms include starfish, sand dollars, brittle stars, sea cucumbers, and sea urchins and are by far some of the most extraordinary animals alive. First, they are all marine. Think about it. You find freshwater clams, freshwater prawns and shrimp, etc., yet there are no freshwater starfish or freshwater urchins. The mollusks and the crustaceans invaded freshwater, but to be such a major group, the Echinoderms never managed to adapt to freshwater habitats. You won’t find any land starfish either. There are no terrestrial echinoderms, yet we have plenty of terrestrial mollusks (snails) and arthropods (think insects, etc.).
The most distinctive feature of Echinoderms is their radial symmetry. Fish and insects, dogs and cats, and you and I have bilateral symmetry. Our major sense organs are on our heads or the direction of movement. Echinoderms, with their radial symmetry, meet the world 360 degrees around. This can be a major advantage with sense organs facing all directions. There is no “head” and motion can be in any direction. The other extraordinary feature is their power to regenerate limbs and organs. If a starfish loses one of its legs, a new one will regrow. This ability has made Echinoderms the focus of a lot of biomedical research. Imagine if you lost a finger or arm in an accident that you could regrow it, or if we could regenerate lost or damaged nerves. Unlocking Echinoderm’s ability to regenerate tissues could be a huge boon to mankind.
Ever notice the banner of this newspaper? Go ahead; turn back to the front page. It features what appears to be a keyhole sand dollar, an appropriate logo with its five pointed radial star pattern. The keyhole sand dollar, Mellita tenuis, inhabits the Florida Gulf coast. It’s the species we find here. Another species, M. quinquiesperforata, ranges from the mouth of the Mississippi, along the Texas coast and down as far as South America. Yet another species, M. isometra, inhabits our Atlantic coasts. Some researchers believe these are all varieties ofMellita quiquiesperforata.
While there are subtle differences, they all share certain features. They all grow up to about three inches in diameter and in life are brown, greyish tan, and if there are algae growing among the cilia, a greyish green. Most people are familiar with only the bleached skeletal remains found on the beach. If you have been fortunate to handle a living sand dollar you know that the brown outer covering is a moving mosaic of tiny cilia. The calcareous skeleton (called a test) is, in life, covered by a thin skin and muscles. On this living surface are spines, tube feet, and cilia. The cilia help to move food particles to the mouth, which is in the center of the flat underside. Tube feet help provide locomotion and spines help in burrowing. The sand dollars burrow in sandy bottoms for protection from wave action and from predators, mainly bottom feeding fish like flounder. They feed on tiny planktonic organisms or other organic matter suspended in the water or in the sandy substrate as they burrow.
Sand dollars are really just flattened forms of sea urchins and are sometimes called keyhole urchins. On the upper surface can be seen the five features that form the star-shaped pattern. These are called petalloids, and are used as gills and have specialized tube feet. The five oval holes in the test are called lunules, and they are paired except for the larger long hole that looks like an old-fashion keyhole. As you turn a living sand dollar over, one sees the moving mouth parts with five “teeth” arranged in a circle. In death, these pieces become disarticulated and can sometimes be heard rattling around inside the empty test. If you break the test open, these structures look somewhat like a white winged bird in flight. This dove-like shape has led some to ascribe a religious significance to the five mouthparts and various legends have grown up about these formations.
The larvae of keyhole sand dollars are planktonic and have the ability to swim. Once the eggs are fertilized they develop into bilaterally symmetrical larvae and feed on smaller plankton. They will pass through several life stages and in four to six weeks they will develop into radially symmetrical adults and spend the rest of their lives burrowing in or living on the sea floor.
The remains of sand dollars and sea urchins are avidly collected by beachcombers. You probably have a few white sand dollar tests in your house. They are incorporated in numerous art works and craft pieces. Yet this silent little animal is not only of major importance to the ecology of the bay, but holds secrets within its cells that could benefit all of us in the future.
Tom Baird has been a fisheries biologist, high school and community college teacher (oceanography and microbiology), director of a science and environmental center, teacher of science and principal in Pinellas County as well as an educational consultant. He retired from the Florida Department of Education and he and his wife divide their time between Tallahassee and Cape San Blas.