Sand dollars are disk-shaped echinoderms that belong to the order Clypeasteroida under the class Echinoidea, which consists of sea urchins. Their rigid exoskeleton, known as a test, often features a distinctive petal-like design marked by numerous tiny pores. The test is coated with velvety spines that display shades of green, blue, violet, or purple. After death, however, these spines fall off, and the test becomes sun-bleached, taking on a silvery-white appearance similar to the old Spanish dollar, which inspired its name.
These invertebrates use their spines to feed on plankton, other tiny invertebrates, such as crustaceans, and even detritus from the sediment. They also use these spines to glide along the seafloor. When threatened, they burrow themselves under the sand for safety.
Their burrowing lifestyle makes sand dollars excellent soil aerators, contributing significantly to nutrient recycling. As primary consumers, they help regulate plankton populations and play a crucial role in cleansing ocean floors.
They are small in size, typically measuring between 3 to 4 inches (80 to 100 mm). However, the largest sand dollar discovered to date has a diameter of about 6.49 in (16.4 cm).
Sand dollars exhibit the pentaradial symmetry characteristic of most sea urchins but are distinguished by an additional front-to-back bilateral symmetry. Their flattened bodies consist of a rigid calcareous skeleton, or the test, made of calcium carbonate. In living sand dollars, the test is covered with countless velvety spines, which are further coated with tiny hairs called cilia. These spines, varying in color depending on the species, can appear in shades of green, blue, violet, or purple.
While these echinoderms lack the five distinct arms seen in other echinoderms, their test is composed of plates arranged in a petal-like pentaradial pattern. In certain species, such as the six-holed keyhole urchin (Leodia sexiesperforata), the test features oval openings known as lunules. These structures aid the sand dollar in anchoring itself to the sediment, even in strong water currents.
The petal-like pattern on the test is perforated with five paired rows of pores, allowing the extension of tube feet or podia and helping in gas exchange. The mouth is situated on the underside, centrally within the petal-like pattern, while the anus, in contrast to most sea urchins, is positioned on the upper surface. Within the body of a sand dollar lies a central nerve ring and gonads.
Males and females are not visually distinct, as they lack noticeable morphological differences.
In New Zealand and Brazil, they are called sea cookies or snapper biscuits, while in South Africa, they are known as pansy shells. They are also sometimes referred to as cake urchins or sand cakes.
According to the World Register of Marine Species, all sand dollar species are divided into 2 suborders and 11 extant families.
The lineage of sand dollars diverged from that of other irregular echinoids, such as the cassiduloids, during the Early Jurassic Period. Their first true genus, Togocyamus, appeared in the Paleocene Epoch. Eventually, more modern groups arose in the Eocene Epoch.
These exclusively marine echinoderms are spread across all continents in the tropical and temperate regions, where they inhabit waters below the mean low tide line. They are also found beneath the sand and mud along coastlines.
The common sand dollar (Echinarachnius parma) is native to the Northern Hemisphere, ranging from the calm, intertidal zone to depths of about 5,000 ft (1,500 m) in the ocean. In contrast, the keyhole sand dollars (family Mellitidae) are confined to the coasts in and around the Caribbean Sea.
Sand dollars are primary consumers, feeding on phytoplankton such as diatoms and algae, as well as zooplankton like crustaceans and fish larvae found on the ocean floor. Unlike their relatives, such as sea stars, they are not active predators and are unable to consume prey larger than themselves. Also, they act as scavengers, feeding on dead and decaying organic matter.
These animals typically live between 6 to 10 years. Their age is determined by counting the number of growth rings on the plates of their exoskeleton.
Sand dollars are gonochoric, meaning they have separate male and female sexes. For reproduction, they congregate on the ocean floor and release their gametes (sperm and eggs) into the water column in a process known as broadcast spawning. Fertilization occurs externally, resulting in the formation of a free-swimming (nektonic) larva. This larva undergoes several stages of metamorphosis, during which it begins to develop its skeleton or test. Eventually, it settles on the ocean floor (benthic) and transitions into a juvenile before growing into an adult.
Under adverse conditions, such as increased predation or even in favorable environments with ideal temperatures and plentiful resources, sand dollar larvae can reproduce asexually by splitting into two or more genetically identical individuals (clones) within a span of 24 hours.
Since sand dollars have a very hard exoskeleton, not many animals make the effort to eat them. However, they are often preyed upon by crabs, ocean pouts, California sheepheads, starry flounders, and even other echinoderms, such as sea stars.
When shaken, sand dollar fossils make a slight rattling noise caused by the dried jaws and other skeletal elements of the animal.
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