- Published: 31 March 2008
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Joe Reid, age 12, of Albany, New York, for his question:
How and where do clams grow?
The average clam is a soft bodied animal, living inside a pair of clam tight half shells. Other species are not much bigger than buttons, while the giant clam is a quarter ton monster with shells four feet wide big enough to make a couple of fancy bathtubs. We find the various clams in warm, mild and even coldish regions, usually buried or partly buried in muddy mire or soggy sand.
We share our watery world with 11,000 different clams and no two species are exactly alike. Most of them belong to the sea, usually along the tidal shores. But 1,000 species are fresh water clams. And of these, 500 can be found in and by the streams and lakes of North America. We can dig for them in the soft mud and silty sand beside the water. Most of them are tasty and good to eat and the shiny linings of their shells are used to make glossy buttons.
The secretive clam has a tongue shaped foot which he uses to travel from here to there. In some species, he has a tubular neck with syphons to draw in water and expel it. The incoming current brings food and oxygen to be sifted by his gills. The outgoing current expels his wastes. He has a heart and a simple nervous system, a mouth but no head.
The life story of the clam varies somewhat from species to species. But in all cases the adult retires from strenuous activity early in life and leads a very sedentary existence. However, if he had a head, he might be tempted to day dream about the frisky risky days of his youth. The details may differ, but almost all young clams spend their kindergarten days swimming freely in the hazardous water.
Where you find one adult clam, there are bound to be others nearby ¬males and females. Usually the young must travel to find less crowded retirement homes. A new life cycle begins when cells from male and female parents merge to form fertilized eggs. The eggs hatch in about 12 hours. The mini larvae look like two halves of a pinhead, hinged together. It takes about 300 of them to measure an inch.
In freshwater species and also in some saltwater species, both parents release their egg making cells into the water. A male and female may release enough cells to form a million eggs. But only a few hundred survivors meet and merge to form fertilized eggs. As a rule, the larvae give up their freedom in the water and settle down when they get to be about a quarter of an inch wide.
In some freshwater species, the newly hatched larvae must attach themselves to the fins and gills of a passing fish. There they live as parasites until they are ready to settle down and retire. In some saltwater species, only the male releases his cells. Some of them enter the gills of a female, where they fertilize her eggs. When the larvae hatch, she launches them into the sea. After two years of life on the bottom, the shells of the average clam are three or four inches wide.