Sand dollars, also called cake urchins or sand cakes, are disk-shaped echinoderms that belong to the order Clypeasteroida under Echinoidea, a class comprising sea urchins. They are also commonly called sea cookies or snapper biscuits in Brazil and New Zealand, while in South Africa, they are known as pansy shells.
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 a range of shades, depending on the species. After death, however, these spines fall off, and the test becomes 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 crawl on the seafloor. When threatened, they quickly burrow themselves under the sand for safety.
Their burrowing lifestyle makes sand dollars excellent sediment aerators, contributing significantly to nutrient recycling.
They are small in size, typically measuring between 3 and 4 in (80 and 100 mm). However, the largest sand dollar discovered to date has a diameter of about 6.49 in (16.4 cm).
Their flattened, bilaterally symmetrical bodies consist of a rigid calcareous skeleton, or the test, made of calcium carbonate. When gently shaken, the dried tests of dead sand dollar tests produce a faint rattling sound caused by the loose internal skeletal parts. In living sand dollars, the test is covered with countless velvety spines, which are further coated with tiny hairs called cilia. Depending on the species, these spines appear in shades of green, blue, violet, or purple.
Unlike other echinoderms, like starfish, sand dollars lack the five distinct arms, but their test is composed of plates arranged in a petal-like pentaradial pattern. Some species, such as the six-holed keyhole urchin (Leodia sexiesperforata), possess oval openings in the test known as lunules. These openings reduce lift by allowing water to pass through the body, helping the sand dollar stay anchored in the sediment, even in strong water currents.
The petal-like pattern on the test is perforated with five double (paired) rows of pores, which allow the extension of tube feet or podia and help in gas exchange. The mouth is located on the underside, at the center of the body, 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 the gonads.
Since the sexual structures are internal, there is no visible sexual dimorphism.
According to the World Register of Marine Species, all sand dollar species are divided into 2 suborders and 11 extant families. Additionally, there are 12 extinct families of sand dollars.
Palaeontological studies suggest that the first true sand dollar, Togocyamus seefriedi, appears abruptly in Paleocene deposits (approximately 10 to 15 million years after the end of the Cretaceous Period) of West Africa and India, emerging directly from echinoid precursors, like cassiduloids. By the Early Eocene, more flat-bodied genera such as Sismondia had emerged, exhibiting broader petals. By the Middle Eocene, within less than 10 million years, modern sand dollars had evolved, characterized by a highly flattened test and prominent petals.
These exclusively marine echinoderms are found in tropical and temperate coastal regions. Most sand dollars are found below the low tide line (subtidal zone) to depths of about 430 ft (130 m). However, some species, like the common sand dollar (Echinarachnius parma), are occasionally recorded at depths of about 5,000 ft (1,500 m).
Members of the family Mellitidae, the keyhole sand dollars, are found along the coasts of the Caribbean, Gulf of Mexico, and Atlantic shores of North and South America.
Sand dollars are primary consumers that typically feed on phytoplankton such as diatoms and algae. They may also opportunistically consume zooplankton, like crustaceans and fish larvae. When there is a rise in dead or decaying organic matter on the ocean floor, sand dollars also feed on this detritus.
These echinoderms are not active predators and usually do not consume prey larger than themselves.
Most commonly observed species are known to live between 6 and 10 years. Like the growth rings in trees, the age of a sand dollar can be determined by counting the rings along the edge of its test.
Sand dollars are gonochoric, meaning they have separate male and female sexes. They gather on the ocean floor and release their gametes into the water column (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, sand dollar larvae can reproduce asexually by splitting into two or more genetically identical individuals (clones) within 24 hours.
Since these echinoderms have a hard exoskeleton, only a few predators target them. Nonetheless, they are preyed upon by crabs and some fish, such as ocean pouts, California sheepheads, and starry flounders. Occasionally, some predatory sea stars may also feed on juvenile sand dollars.
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