sand dollar

sand dollar Fossil range: 56 –0 Ma PreЄ Є O S D C P T J K Pg N Late Paleocene to Recent[1]
A live individual of Mellita quinquiesperforata from the Pacific coast of Costa Rica
Scientific classification
Domain:	Eukarya
Kingdom:	Animalia
Phylum:	Echinodermata
Class:	Echinoidea
Subclass:	Euechinoidea
Superorder:	Gnathostomata
Order:	Clypeasteroida
Suborders and families
See text.

Sand dollars are any species of flattened, burrowing sea urchin belonging to the order Clypeasteroida. 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

Common name

The Morgan silver dollar

The name sand dollar comes from the shape and color of the test after it washes up on the beach. At that point the test is usually missing its velvety covering of minute spines and is often bleached white by the sunlight. In many species the test is quite similar in shape and size to a large coin, and the whiteness makes it resemble a large silver coin, for example the old American silver dollar coin, which is 38 mm across.

In Spanish, such as in Costa Rica, a sand dollar is known as a "galleta de mar" or sea cookie.

Clypeaster rosaceus however 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.^[2]

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,^[3] with the first true sand dollar genus, Togocyamus, arising during the Paleocene. Soon after Togocyamus, more modern-looking groups emerged during the Eocene.^[1].

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.^[4][5]

Gallery

Underside of the same live Mellita quinquiesperforata shown at the top of the article

A number of sand dollars on a seabed

Sand Dollar beneath the sand at low tide on Hilton Head Island

A group of Sand dollars in an aquarium.

References

External links

Wikimedia Commons has media related to: Clypeasteroida

• Clypeasteroida (TSN 157984). Integrated Taxonomic Information System.
• The Common Sand Dollar by Cheryl Page
• The Biogeography of the Western Sand Dollar - Dendraster excentricus
• Video showing the life cycle of Clypeaster subdepressus