sand dollar

 

common name for a marine animal in the same phylum as the starfish (see sea star). The sand dollar has a rigid, flattened, disk-shaped test, or shell, made of firmly united plates lying just beneath the thin skin. Small spines that densely cover the test enable the animal to burrow in sand just below the surface. Like other members of its class, the sand dollar is radially symmetrical. It also shows evidence of a secondary bilateral symmetry, i.e., the mouth is centered on the oral (under) surface, but the anus lies near the rear edge of the test. Tube feet are similar to those in other echinoderms and are used for locomotion and to convey small food particles, mostly organic matter found in sand, to the mouth. Tube feet on the upper surface are used for respiration. Sand dollars differ from the closely related heart urchins by their shorter spines and more flattened shape. More convex, short-spined sand dollars are called sea biscuits. Sand dollars are abundant on the sandy bottom of deeper waters on both the Atlantic and Pacific coasts. They are classified in the phylum Echinodermata, class Echinoidea, order Clypeastroida.

sand dollar
Scientific classification
Kingdom: Animalia Phylum: Echinodermata Class: Echinoidea Subclass: Euechinoidea Superorder: Gnathostomata Order: Clypeasteroida
Suborders and families
See text.

Sand dollars are in the Echinoid (Echinoderms) class of marine animals. When they are living, they are covered with a suit of moveable spines that encompass the entire shell. Like its close relative the sea urchin, the sand dollar has five sets of 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.

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 plankton and organic particles that end up in the sandy bottom.

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.

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.

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

Underside of same live sand dollar

A number of sand dollars on a seabed

Keyhole sand dollar, Mellita quinquiesperforata

External links

• Clypeasteroida (TSN 157984). Integrated Taxonomic Information System.
• Ask a Scientist
• The Common Sand Dollar by Cheryl Page

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