May 2012 - MicroCT Scan of a Sand Dollar
3D Volume Rendering of a Sand Dollar
With the thought of summer approaching around the corner, this 3D rendering of a sand dollar will hopefully get you dreaming about warm sunshine, refreshing ocean breezes, and long walks on the beach. This little sea creature has several names including “sand dollar”, “sea cookie”, “snapper biscuit”, or “pansy shell”, but its scientific name is Dendraster Excentricans and is part of the Echinoidea class. The term “sand dollar” refers to the likeness to old-style dollar coins (38-40mm) and the sandy color which is the appearance the skeleton gets from being bleach by the sun on the beach. Close relatives to the sand dollar are the sea urchin, sea cucumber, and the starfish.
Sand dollars possess rigid, flat, internal skeleton known as a “test” and consists of calcium carbonate which makes for great contrast in a microCT scan. On the top, five petaloids can be seen giving it the appearance of a leaf or flower, these are called ambulacra. The under part shows the peristomial aperture or mouth located at the center and a smaller hole called lunule.
When examining the inside of the shell, small particles of sand can be seen stuck in the inner perimeter as well as the peristomal aperture protruding to the inside of the shell. Sand dollars are bottom feeders and process the sand to feed on detritus in the sediment. A dense layer of tiny spines filter out large particles but let the finer grains in to be processed. Tiny cilia move these particles to the food grooves and then to the mouth.