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Range and Habitat: Sea stars are found in sea waters throughout
the world, from the Arctic and the Antarctic to the tropics; in
depths ranging from shallow tidal pools to water 6,000 meters
(3.75 miles) deep.
Description: The body of a common sea star is a hollow
cavity (coelom) composed of five arms joined in a central disk.
They range in size from that of a small bug to the giant Oreastar,
50 cm (20 in) across. Some species may have up to 40 arms. Colors
vary from brilliant scarlets, oranges and purples to mottled rusts
and creamy yellows. Hard, blunt spines, projections of the internal
skeleton, cover the upper body surface. Surrounding the spines
are soft gills and minute pincers (pedicellariae) projecting from
the body cavity. Each pincer will snap shut when touched and serves
to keep the body free of debris and may help to capture food.
The mouth occupies the center of the disk on the underside of
the body. Water vessels within the body radiate from the oral
cavity into two or more rows of "tube feet" on each
arm. Each tube ends in a muscular sucker (podia) operated by water
pressure, which is used for locomotion and food gathering.
Habits and Adaptations: The sea star is an extremely aggressive
nocturnal carnivore. With few exceptions, it is a slow mover,
about 15 cm (six in) a minute and remains in one position on the
sea bottom for extended periods of time. It has a unique adaptation
for consuming shellfish. It positions its mouth on the tight seam
between the shells; then pulls the shells apart with suckers on
the undersides of its arms. When the shell opens (the clam has
very strong muscles but can't maintain resistance to the steady
long pull of the sea star), the sea star inserts its stomach into
the clam and digests the fleshy body parts inside the clam's own
shell. The sea star then returns its stomach through its mouth.
Each arm contains a branch of the digestive tract, the reproductive
organs and a primitive eye at the end of the arm. The sea star
can regenerate lost limbs.
Diet: Oysters, clams, scallops, snails, worms, anemones,
sea urchins, fish, shrimps, and decaying animal matter.
Breeding and Maturation: Sexes are separate, but there
is no external distinction. Eggs are deposited in the water and
fertilized by the male. Free-swimming larva develop and later
mature to adult.
Miscellaneous: Because of their partiality to shellfish,
stars have decimated commercial fishing beds from time to time.
The crown-of-thorns sea star has also damaged coral reefs by denuding
them of all living polyps. Enemies of the sea star include man,
fish and eels who eat the larva, and gulls and crows who eat the
sea stars in shallow water.
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