The sand dollar, a flattened member of the echinoid family, is a common find for beachcombers. The white, disk-shaped object discovered is the creature’s exoskeleton, known as the test, which remains after the animal has died. This bony structure records the animal’s life history. Determining a sand dollar’s age is possible by examining the annual growth rings preserved within this skeletal material. Careful preparation and magnification reveal these rings, which typically indicate a lifespan of six to ten years for many species.
Where Sand Dollar Growth Rings Form
A sand dollar’s test is composed of numerous interlocking plates made of calcium carbonate. Growth rings are formed by the periodic addition of new skeletal material at the plate margins, similar to how a tree adds a new ring each year. These growth lines are most clearly visible on the large, flat plates that make up the aboral surface, or top side, of the test.
Researchers focus on the plates located in the interambulacral areas, which are the sections between the five petal-like patterns. The rings are clearest near the outer edge, or ambitus, though they can become compressed and difficult to distinguish in older specimens. Each ring represents a cycle of growth, typically corresponding to one year, seen as alternating dark and light bands of calcium carbonate density.
Step-by-Step Counting Technique
To begin counting growth rings, the sand dollar test must first be cleaned and dried to remove organic matter and moisture. The next step involves preparing the plates to make the faint growth lines visible, which is often the most challenging part of the procedure. One common method involves carefully sectioning the test, or taking a thin sliver of the plate, and then treating it with specialized chemicals to enhance ring clarity.
An alternative, less destructive technique involves charring the entire test, or a section of it, in a muffle furnace around 350 degrees Celsius. This heat treatment causes a chemical change, resulting in concentric dark and light bands that become apparent when the plate is cleared with a solvent like xylene. Once the bands are visible, a low-power microscope or strong hand lens is necessary to accurately count the alternating light and dark rings. A complete annual growth line is counted as one dark band and one adjacent light band.
Environmental Influences on Ring Visibility
The interpretation of sand dollar growth rings is tied directly to the surrounding marine environment. Growth rate and ring clarity are driven by seasonal changes in water temperature and the availability of food. Periods of abundant food or warmer water lead to faster skeletal growth, forming a wider, lighter band, while lean periods or colder temperatures result in a denser, darker, and narrower band.
Inconsistent environmental conditions can lead to the formation of “false rings,” which are growth markers that do not represent a full year’s cycle. These smaller, less distinct lines make accurate age estimation difficult. As a sand dollar approaches its maximum size, the growth lines near the edge of the test become increasingly compressed, making them nearly impossible to distinguish.