If your knowledge about sand dollars is restricted to shells that you came across while walking on a beach, here's a compilation of facts about this species will give you an insight of their lives.
Sandy ocean floor often comes across as 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 derive their name.
Not many people are aware of their existence, 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 Facts about Sand Dollars
Sand dollars belong to Clypeasteroida order of Echinodermata family in kingdom Animalia. On an average, they have a lifespan of around eight to ten years. Their age can be determined by counting the growth rings on the plates of their exoskeleton.
Etymology
Sand dollars (written as sand-dollars at times) are also referred to as 'sand cakes' and 'cake urchins', owing to their peculiar shape. Their washed up tests, i.e., the hard outer covering of sea urchins, are often seen stranded on shores.
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 and thus, the common name.
Habitat
Sand dollars are found at the ocean floor, wherein the surface is sandy or muddy and facilitates easy penetration for them. 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. Their geographical distribution is most often traced to the ocean floors of temperate and tropical zones.
Appearance
The fact that sand dollars generally inhabit deep oceans means you are less likely to come across live specimen. If at all you do come across one, you will notice that these colorful creatures have a semi-pliable shell covering their tiny body.
Usually they are 2 to 4 inches in length and covered with many short spines. Beneath the shell lies the five sets of pores―a typical echinoderm characteristic, which helps these creatures move around on the ocean floor.
Anatomy
Sand dollars are typically characterized by their fivefold radial symmetry like various other echinoderms. Its secondary bilateral symmetry differentiates it from the other species, 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 do.
Instead, they 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 they feed on. Normally, it takes a sand dollar around two days to digest its food.
Interestingly, when threatened by predators, like sheepsheads, pink sea stars, etc., sand dollars quickly burrow in sand and crop up only when the lurking danger is gone.