Sand dollar

From Wikipedia, the free encyclopedia

sand dollar

Kingdom:	Animalia
Phylum:	Echinodermata
Class:	Echinoidea
Subclass:	Euechinoidea
Superorder:	Gnathostomata
Order:	Clypeasteroida

Suborders and families

See text.

Sand dollars (order Clypeasteroida) are flat, round marine animals related to sea urchins (echinoids), sea stars, and other echinoderms. A sand dollar has a rigid skeleton, or test, and the term sand dollar also refers to this test when it is found as a sand dollar's remains. When sand dollars are living, they have a skin of moveable spines covering the entire test. Like its close relative the sea urchin, the sand dollar has a set of five pores arranged in a petal pattern. The pores are used to move sea water into its internal water-vascular system, which allows the creature to move.

1 Suborders and families
2 Nomenclature
3 Biology
4 Evolutionary history
5 Cloning among larvae
6 External links
7 References

[edit] 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

[edit] Nomenclature

The name "sand dollar" is a reference to their round flat shape, which is similar to a large coin. The term "sand dollar" can also refer to the test left when a sand dollar dies. 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 appearance due to its exposure to the sun.

[edit] Biology

Like other echinoderms, sand dollars have fivefold radial symmetry (pentamerism). Unlike sea urchins, however, the sand dollar has secondary bilateral symmetry, with a front and back as well as a top and bottom. The anus is toward the rear rather than on the top.

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 sand. Fine, hair-like cilia cover the tiny spines. Tubefeet or 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, detritus, diatoms, algae and organic particles that end up in the sandy bottom.^[1]

On the ocean bottom, sand dollars are frequently found together. This is due in part to their preference of 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. The free-swimming larvae metamorphose through several stages before the skeleton or test begins to form, and they become bottom dwellers.

When found alive, some sand dollars are a shade of chartruse green with a velvety feel.

[edit] Evolutionary history

Sand dollars diverged from other echinoids early in the Tertiary (c 65 to 1.8 MYA).

[edit] 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.^[2]^[3]