Starfish are marine invertebrates in the phylum Echinodermata, which also includes sea urchins, brittle stars, sea lilies, and sea cucumbers. Starfish belong to the class Asteroidea, which includes 1,500 species inhabiting the shallow margins of all of the world's oceans. Starfish vary widely in appearance. Some species grow up to 3 ft (1 m) in diameter; others are barely 0.5 in (1.3 cm) across. Starfish come in a rainbow of colors including bright red, cobalt blue, yellows, and the familiar orange-brown.
Starfish are radially symmetrical with from 5 to 50 arms radiating from a central disk. The skin of starfish is thick with bony plates (ossicles), spines, tiny pincers on stalks (the pedicillerae which keep the animal's skin clean of debris), and bumps, between which are tiny folds of skin which function as the starfish's gills.
The nervous system of starfish consists of three main networks: the ectoneural (oral), the hyponeural (deep oral), and the entoneuoral (aboral) systems. There is no central ganglion, but this rather simple arrangement effectively allows the starfish to move (including the ability to right itself should it be turned over) and sense the world around it.
The eyes of starfish are extremely simple, are located at the tip of each arm, and are primarily light-sensing dots. Starfish can tell light from dark, but are unlikely to see much more than that. The sense of smell, however, is quite sensitive. Chemoreceptors on the starfish's skin can detect the faintest smell of its prey (clams), and even determine the direction from which it is coming. The starfish then sets off to catch its prey, slowly and deliberately, at the rate of 6 in (15.25 cm) per minute. As it moves it does not pinwheel, but follows one arm. The underside of each arm is lined with hundreds of tiny tube feet. Each tube foot ends in a suction cup, and is the terminal point of an elaborate hydraulic system within the animal.
This hydraulic system has as its starting point a small reddish spot on the top of the central disk, the madreporite. The madreporite is comparable to the drain of a sink, as it serves as the entry for water into the stone canal, which joins the ring canal, off which radiate the tubes that run down the starfish's arms and branch off into the tube feet. Movement is an elaborate process for a creature with so many feet, which are extended and placed on the substratum by filling each tube foot with water. To attach the tube foot, the starfish creates suction by drawing water out again and closing a tiny valve on the ampulla, a bulb at the top of the tube. The animal then contracts a muscle and draws itself forward on its tube feet, which are tremendously strong and able to keep a starfish clinging to rocks in all but the heaviest storms.
Starfish also use their tube feet to prey on bivalve molluscs. When a starfish encounters a clam, it attaches its tube feet and begins to pull. It can pull for hours, or even days. Eventually, the clam's adductor muscle that keeps the shell closed tires under this relentless tug, and the clam's shell opens a bit. The starfish does not need much of an opening-just enough to get part of its digestive system in. Starfish have two stomachs, one that remains inside the body and another than can be protruded through the starfish's mouth on the underside of the body. The starfish inserts this into the clam's shell, and releases digestive enzymes.
Because starfish lack teeth, they must convert their food to liquid form before they can ingest it. Among the other prey items this bottom-dwelling predator eats are sea urchins, other starfish, small fish, sponges, and carrion. Some species draw in mud as they crawl along the bottom and extract organic material from the mud. One genus, Acanthaster, (the crown-of-thorns starfish) has become famous for the damage it does to coral reefs, moving over the reef and stripping it clean of coral polyps. (The overabundance of the crown-of-thorns starfish can be partly attributed to the reduction in the population of its major predator, the giant triton, by humans.) Starfish have long been the bane of shellfishermen. In an effort to kill starfish, shellfishermen hack them to pieces and throw the pieces back into the sea. Unknown to humans, all that was being done was the creation of more starfish, since starfish have remarkable regenerative abilities; all species can regenerate lost arms, and some can produce a whole new starfish from an arm with a piece of the central disk attached.
Although such regeneration is a form of reproduction, starfish generally reproduce by shedding eggs and sperm into the water. Once one female releases her eggs (up to 2.5 million at a time), other starfish nearby join in a kind of breeding frenzy, all releasing their sperm or eggs. The eggs float free with the plankton and develop into bipinnaria larvae, which remain free floating for another three weeks. They then settle to the bottom and metamorphose into the familiar star shapes.
See also Brittle star; Sea lily.
Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinaur Associates, 1990.
Whiteman, Kate. World Encyclopedia of Fish & Shellfish. New York: Lorenz Books, 2000.
F. C. Nicholson
Science EncyclopediaScience & Philosophy: Spectroscopy to Stoma (pl. stomata)