Clypeasteroida
3 species
Sand dollars (also known as sea cookies or snapper biscuits in New Zealand and Brazil, or pansy shells in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits. Sand dollars can also be called "sand cakes" or "cake urchins".
Sand dollars can be found in temperate and tropical zones along all continents. Sand dollars live in waters below the mean low water line, on or just beneath the surface of sandy and muddy areas. The common sand dollar, Echinarachnius parma, can be found in the Northern Hemisphere from the intertidal zone to the depths of the ocean, while the keyhole sand dollars (three species of the genus Mellita) can be found on many a wide range of coasts in and around the Caribbean Sea.
The spines on the somewhat flattened topside and underside of the animal allow it to burrow or creep through the sediment when looking for shelter or food. Fine, hair-like cilia cover these tiny spines. Sand dollars usually eat algae and organic matter found along the ocean floor, though some species will tip on their side to catch organic matter floating in ocean currents.
Sand dollars frequently gather on the ocean floor, in part to their preference for soft bottom areas, which are convenient for their reproduction. The sexes are separate and, as with most echinoids, gametes are released into the water column and go through external fertilization. The nektonic larvae metamorphose through several stages before the skeleton or test begins to form, at which point they become benthic.
In 2008, biologists discovered that sand dollar larvae will clone themselves for a few different reasons. When a predator is near, certain species of sand dollar larvae will split themselves in half in a process they use to asexually clone themselves when sensing danger. The cloning process can take up to 24 hours and creates larvae that are 2/3 smaller than their original size which can help conceal them from the predator. The larvae of these sand dollars clone themselves when they sense dissolved mucus from a predatory fish. The larvae exposed to this mucus from the predatory fish respond to the threat by cloning themselves. This process doubles their population and halves their size which allows them to better escape detection by the predatory fish but may make them more vulnerable to attacks from smaller predators like crustaceans. Sand dollars will also clone themselves during normal asexual reproduction. Larvae will undergo this process when food is plentiful or temperature conditions are optimal. Cloning may also occur to make use of the tissues that are normally lost during metamorphosis.
The flattened test of the sand dollar allows it to burrow into the sand and remain hidden from sight from potential predators. Predators of the sand dollar are the fish species cod, flounder, sheepshead and haddock. These fish will prey on sand dollars even through their tough exterior.
Sand dollars have spines on their bodies that help them to move around the ocean floor. When a sand dollar dies, it loses the spines and becomes smooth as the exoskeleton is then exposed.
Sand dollars (also known as sea cookies or snapper biscuits in New Zealand and Brazil, or pansy shells in South Africa) are species of flat, burrowing sea urchins belonging to the order Clypeasteroida. Some species within the order, not quite as flat, are known as sea biscuits. Sand dollars can also be called "sand cakes" or "cake urchins".
Sand dollars can be found in temperate and tropical zones along all continents. Sand dollars live in waters below the mean low water line, on or just beneath the surface of sandy and muddy areas. The common sand dollar, Echinarachnius parma, can be found in the Northern Hemisphere from the intertidal zone to the depths of the ocean, while the keyhole sand dollars (three species of the genus Mellita) can be found on many a wide range of coasts in and around the Caribbean Sea.
The spines on the somewhat flattened topside and underside of the animal allow it to burrow or creep through the sediment when looking for shelter or food. Fine, hair-like cilia cover these tiny spines. Sand dollars usually eat algae and organic matter found along the ocean floor, though some species will tip on their side to catch organic matter floating in ocean currents.
Sand dollars frequently gather on the ocean floor, in part to their preference for soft bottom areas, which are convenient for their reproduction. The sexes are separate and, as with most echinoids, gametes are released into the water column and go through external fertilization. The nektonic larvae metamorphose through several stages before the skeleton or test begins to form, at which point they become benthic.
In 2008, biologists discovered that sand dollar larvae will clone themselves for a few different reasons. When a predator is near, certain species of sand dollar larvae will split themselves in half in a process they use to asexually clone themselves when sensing danger. The cloning process can take up to 24 hours and creates larvae that are 2/3 smaller than their original size which can help conceal them from the predator. The larvae of these sand dollars clone themselves when they sense dissolved mucus from a predatory fish. The larvae exposed to this mucus from the predatory fish respond to the threat by cloning themselves. This process doubles their population and halves their size which allows them to better escape detection by the predatory fish but may make them more vulnerable to attacks from smaller predators like crustaceans. Sand dollars will also clone themselves during normal asexual reproduction. Larvae will undergo this process when food is plentiful or temperature conditions are optimal. Cloning may also occur to make use of the tissues that are normally lost during metamorphosis.
The flattened test of the sand dollar allows it to burrow into the sand and remain hidden from sight from potential predators. Predators of the sand dollar are the fish species cod, flounder, sheepshead and haddock. These fish will prey on sand dollars even through their tough exterior.
Sand dollars have spines on their bodies that help them to move around the ocean floor. When a sand dollar dies, it loses the spines and becomes smooth as the exoskeleton is then exposed.
