Sand Dollar Facts

A glance at sandy ocean floor may make you feel that it is totally barren - devoid of life, but a closer look and you will notice faint movement of tiny diskettes all around the place. The hundreds of tiny creatures you see moving around are nothing but sand dollars - a species of burrowing sea urchins typically characterized by their flattened appearance from which they happen to derive their name. Not many people are aware of the basic existence of this creature, forget about being aware of facts which make them so interesting. Our ignorance about the species can be attributed to the fact that they seldom venture towards the coast and the only time a layman is to come in direct contact with them is when some dead sand dollars are washed ashore.

Interesting Sand Dollar Facts

The sand dollars belong to Clypeasteroida order of Echinodermata family in Kingdom Animalia. The lifespan of a sand dollar ranges between eight to ten years. Its age can be determined by counting the growth rings on the plates of its exoskeleton.

Etymology
Sand dollars (sometimes written as sand-dollars) are also referred to as 'sand cakes' and 'cake urchins' owing to their peculiar shape. The name 'sand dollar' is attributed to washed up tests - hard outer covering of sea urchins, which are often seen stranded on the beaches. Over the period, these tests turn white as a result of being continuously exposed to sunlight. This gives them an appearance which resembles the American Dollar to a significant extent, and hence they are known as sand dollars.

Habitat
Sand dollars are found at the ocean floor, wherein the surface is sandy or muddy and facilitates easy penetration for these burrowing sea urchins. They are found below the low tide level at the depth of 5,000 ft - where sunlight penetration is minimal. These marine animals use the spines located on their body for burrowing in these sandy or muddy areas. The geographical distribution of sand dollar species is most often traced to the ocean floors of temperate and tropical zones.

Appearance
The fact that sand dollars generally tend to inhabit deep oceans means you are less likely to come across a live individual. If at all you do come across a live sand dollar, you will notice that these colorful creatures have a semi pliable shell - known as the test, covering their tiny bodies. Usually they are 2 to 4 inches in size and covered with many short spines. Beneath the shell lies the five sets of pores - a typical echinoderm characteristic, which helps these creatures to move around on the ocean floor.

Anatomy
As we mentioned above, sand dollars are typically characterized by their fivefold radial symmetry as with various other echinoderms. Its secondary bilateral symmetry differentiates it from the other species of sea urchins which are defined in marine biology. Even though their mouth is located on the underside, these creatures do not push their stomach out for feeding like starfish species do, but instead use their tube feet (which are also located at the underside) to bring food to their mouth. Their mouth has teeth-like sections which are used to grind the food that they feed on. Normally, it takes a sand dollar around two days to digest its food.

Sand dollar's defense mechanism is one attribute which can grab their attention. When threatened by its predators (which includes sheepsheads, pink sea stars, etc.), they burrow in sand in quick succession and crop up only when the lurking danger is gone.