Clams and oysters have long been of value to
humans as food, while their shells have been
valued as ornamentation. Paleolithic shell middens,
some twenty-five feet high and seventy feet
in base diameter, consist almost entirely of the discarded
shells of clams and oysters. Oysters were
especially prized because they lay on the surface
of the estuary bottom and were easily gathered.
Clams, buried a few inches deep, required some
effort to gather. To prepare the shellfish for food,
they were often simply placed on a fire and
roasted in the shell. The fleshy contents were removed,
eaten as they were, or cooked in a stew
with some vegetables.
Physical Characteristics of Clams and Oysters
There are more than fifteen thousand different species of clams worldwide. They range in size from the Condylocardia, about 0.004 inches long, to the giant clam (Tridacna gigas) of the southwest Pacific Ocean, which grows up to nearly five feet long and weighs about 550 pounds. Classified as bivalves, the animals have two shells that protect the soft body parts. In clams, two syphons, one incurrent and one excurrent, draw in and discharge seawater. The water supplies the shellfish with oxygen as well as plankton and other organic matter for food. A muscular projection (the foot) enables the clam to burrow into the sandy bottom. The valves are held together by two adductor muscles that slowly contract and relax to pump water into the shell. When the shellfish senses danger, the adductor muscles contract, closing the valves tightly and effectively sealing the soft body parts from a potential predator. Oysters vary in shape depending on the type of bottom on which they lie. The shells usually are elongated with a rough surface. They do not have siphons. Instead, seawater, containing food organisms, is drawn into the body by a pumping action of the valves. Oysters do not have a foot and are sessile.
The Life Cycle of Clams and Oysters
In clams and oysters, the sexes are separate and spawning is triggered by favorable water temperature. The eggs and sperm, several hundred thousand to several million each, are broadcast into the water through the excurrent siphon. Of the millions of eggs, only a very few reach adulthood; many fail to be fertilized, some are eaten by small fishes and other predators,somesuccumb to molds and bacteria, and some adults are eaten by eels, sea stars, and whelks. Each year, more than 112 million pounds of clams and nearly 26 million pounds of oysters are taken for human food. Shellfish that grow in polluted waters may ingest disease organisms which accumulate in their tissues. These can cause disease in humans. Some toxic plankton may cause illnesses, such as paralytic shellfish poisoning in humans who eat the clams or oysters uncooked ("on the half-shell"). Commercially canned shellfish are cooked under pressure and are safe to eat. The incidence of human illness from eating toxic clams is very low. One species of clam, the teredo, or shipworm, burrows into wood and leaves it honeycombed.
Most clams and oysters produce pearls but not all are of gem quality. The pearl is made of the same material as the lining of the shell. It is deposited around some irritant such as a grain of sand. The giant clam produces pearls as large as golf balls.
Class: Bivalva, with ten orders, fifty-seven families
Geographical location: Worldwide
Habitat: The bottom of all marine waters, from coastal subtidal to great depths of the sea Gestational period: Larval stage varies according to species but generally lasts around one month
Life span: For the edible clam (quahog), harvest occurs in three to five years; some researchers estimate a life span of up to three hundred years for this species, if it is not eaten
Special anatomy: Shape of the shell varies only slightly and is generally roundish; lips of the Tridacna shell are wavy; some species have a powerful tongue or foot that enables the clam to dig quickly into the sandy bottom to escape predators
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