starfish

sea star, also called starfish, echinoderm of the class Asteroidae, common in tide pools. Sea stars vary in size from under ¹/₂ in. (1.3 cm) to over 3 ft (90 cm) in diameter. They are commonly dull shades of yellow or orange, but there are many brightly colored ones as well. There are about 2,000 species distributed throughout the world, mostly in shallow water along rocky coasts.

The body of most species consists of a central disk from which radiate a number of tapering arms-usually five, but up to 25 in some species. Some sea stars are pentagonal, the points of the disk not extending into arms. Each arm contains an extension of the body cavity and body organs. A network of calcareous plates located beneath the skin forms an external skeleton; the plates are joined by connective tissue and muscle, giving the apparently rigid sea star considerable flexibility. Calcareous spines, some of them movable, project from the skin.

Tube Feet

The underside of the body bears a mouth at the center and a groove running along each arm. The grooves contain rows of tiny, flexible appendages called tube feet. Sea stars move by means of the tube feet, which are operated by a hydraulic, or water-vascular, system unique to echinoderms. Seawater, circulated through the radiating canals of this system, enters and extends the tube feet. Each tube foot can be withdrawn by its attached muscles. The tube feet are equipped with suction cups, and the animal moves in any direction by gripping with some of its tube feet and pulling itself forward. A sea star that is turned upside down can right itself by turning an arm under and walking with the tube feet.

Sensory Tentacles

Each arm has a short sensory tentacle at its end that responds to chemicals and vibrations in the water, and a red photosensitive eyespot. A sea star often lifts the end of an arm to perceive light and movement.

Predation

Sea stars are carnivorous. Members of many species have protrusible stomachs and prey largely on bivalves, such as clams and oysters; they are extremely destructive to commercial oyster beds. The sea star wraps its arms around the bivalve, grips the shell with its tube feet, and opens it by sustained powerful suction. The shell needs to open only about 1/100 in. (0.25 mm). The sea star then extrudes its stomach through its mouth and inserts it inside the shell of the prey, where it digests and absorbs the soft inner tissues.

Reproduction

Sea stars shed their eggs and sperm into the water, and fertilization occurs externally, producing a swimming, bilaterally symmetrical larva. The larva settles and undergoes a sessile (attached) period while metamorphosing into the free-living, radially symmetrical adult form. A single female may produce over 2 million eggs in one spawn, but the eggs and larvae form part of the plankton on which many marine animals feed, and few survive.

Classification

The brittlestars, of a different echinoderm class, have long, slender, jointed arms and are found in deeper waters. Sea stars are classified in the phylum Echinodermata, class Asteroidae.