Sand dollar

Sand dollars (order Clypeasteroida) are flat, round marine animals related to sea urchins, sea stars, and other echinoderms. The most common sand dollar, Echinarachnius parma, is widespread in circumpolar ocean waters of the Northern Hemisphere, from the intertidal zone to considerable depths.

A sand dollar has a rigid skeleton known as a test. When sand dollars are living, they have a skin of motile spines covering the test. Movement is accomplished by the coordinated action of the spines. Like sea urchins, sand dollars have five paired rows of pores. In sand dollars they are arranged in a petal-like pattern. These pores are perforations in the endoskeleton through which the podia, used in gas exchange, project from the body.

Suborders and families

Clypeasterina
Arachnoididae
Clypeasteridae L. Agassiz, 1835
Laganina
Fibulariidae Gray, 1855
Laganidae
Rotulina
Rotulidae
Scutellina
Astriclypeidae
Dendrasteridae Lambert, 1889
Echinarachniidae Lambert, 1914
Mellitidae Stefanini, 1911

Nomenclature

The term sand dollar refers to the round flat shape of the test which is similar to a large coin. By the time the test washes up on the beach, it is usually missing its velvety covering of minute spines and has a somewhat bleached and often slightly greenish appearance due to its exposure to sunlight. Because the bare test is often the only part of the animal which is seen, it is commonly referred to in isolation as a sand dollar. Clypeaster rosaceus is a sand dollar that is much thicker than most sand dollars, and is often referred to as a "sea biscuit".

Anatomy

Like other echinoderms, sand dollars have fivefold radial symmetry (pentamerism). Unlike other sea urchins the sand dollar has developed a secondary bilateral symmetry, with a front and back as well as a top and bottom: this is a result of the flattening of the test in the sand dollar's evolutionary transition from a lifestyle on top of the substrate (epibenthos) to that of a burrowing animal (hyperbenthos). The anus is toward the rear rather than on the top, as normal with other sea urchins.

Lifestyle and habitat

Sand dollars live beyond mean low water on top of or just beneath the surface of sandy or muddy areas. The spines on the somewhat flattened underside of the animal allow it to burrow or to slowly creep through the sediment. Fine, hair-like cilia cover the tiny spines. Podia that line the food grooves move food to the mouth opening, which is in the center of the star-shaped grooves on the underside of the animal (called the oral surface). Its food consists of crustacean larvae, small copepods, diatoms, algae and detritus.

On the ocean bottom, sand dollars are frequently found together. This is due 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. They are conceived by external fertilization as with most echinoids. The nektonic larvae metamorphose through several stages before the skeleton or test begins to form, at which point they become benthos.

Live sand dollars can be greenish, bluish or purple. Their highly modified spines and podia give them a velvet-like texture and appearance.

Evolution

The ancestors of the sand dollars diverged from the other irregular echinoids, namely the cassiduloids, during the early Jurassic, with the first true sand dollar genus, Togocyamus, arising during the Paleocene. Soon after Togocyamus, more modern-looking groups emerged during the Eocene..

Cloning among larvae

In 2008, scientists showed that sand dollar larvae can clone themselves as a mechanism of self defense. Larvae exposed to mucus from predatory fish cloned themselves, effectively halving their size. The smaller larvae are believed to better escape detection from fish predators, but may increase the danger of predation from smaller animals, such as crustaceans.

Translation of "Sand dollar"

German: Sanddollars, Japanese: スカシカシパン, Persian: سکه دریایی, French: Clypeasteroida, Dutch: Zanddollar, Portuguese: Bolacha-da-praia, Russian: Плоский морской ёж, Swedish: Clypeasteroida, Chinese: 沙錢, Hungarian: Homokdollárok.