Free The Big Oyster by Mark Kurlansky

pesticides, sewage, and other pollutants. As it opens and closes its shell to pump in water through its system to absorb nutrients, the oyster is also taking in anything else that is in the water. Oysters under normal conditions feed continually, and between twenty and fifty gallons of seawater are moved through a single oyster’s gills every day. Assuming the water is in fairly good condition, oysters serve a natural function, filtering and cleaning the water. It has been found that a few oysters placed in a tank of algae- and phytoplankton-laden green water will make the water clear in only a few hours. The original oyster population of New York Harbor was capable of filtering all of the water in the harbor in a matter of days. One of the formulas of the balance of nature is that estuaries overproduce plant life and depend on animals to consume it. Oysters and other bivalves play an important role in this process. In his brilliant study of the role of oysters in Chesapeake Bay, published in 1891 before the word
ecology
was in use, William K. Brooks said, “In the oyster we have an animal, most nutritious and palatable, especially adapted for living in the soft mud of bays and estuaries, and for gathering up the microscopic inhabitants and turning them into food for man.”
    Unlike most other bivalves that live around
Crassostreas—
soft-shell clams, mussels, and bay scallops—oysters are able to survive long periods out of the water because of the protection of their thick shells. They can also survive an amazing range of conditions. The American oyster, the same species that prospers along Florida and Louisiana in water that heats up to more than thirty-two degrees centigrade, or ninety degrees Fahrenheit, can live in water that seasonally plunges below freezing. They are at home in almost fresh water and water that has more than 30 percent salinity.

    It took a remarkably
long time, well into the nineteenth century in America, to understand that the best thing to do with oyster shells was not make wampum, burn them for lime, or use them for roadfill, but to dump them back into the oyster beds. This became clear with the study of oyster physiology. The mantle that sends out warnings to the oyster brain also creates the shell. It produces a first pearly layer of shell and throughout the animal’s life constantly creates new shells, each a little larger than the one before. That is why an oyster shell looks like many paper-thin layers pressed together. The oyster shell will conform to the surface it is next to, smooth if attached to a bottle, following the contours of a rock, even molding itself to the shape of a crab claw if attached to it. Oysters can also repair their shells and mend cracks. They are able to do all this by using the lime that they extract from the water. Oysters need lime to grow, and no matter how rich the water is in nutrients, without the presence of lime growth cannot take place. Throwing the shells back into the water not only provides good material for seedlings looking for a surface to attach to, which has come to be known as
cultch,
but also enriches the growing environment of the water. The sea will break down the shell and the oyster will absorb the lime. The more lime that is available, the faster and thicker the shells will grow. The sooner an oyster has a thick shell, the more likely it is that it will survive its many enemies.
    It took considerable observation to grasp this about oyster shells. But elsewhere in nature it is readily apparent. William Brooks pointed out that old dried bones in the woods invariably have wood snails on the underside and rivers that run through limestone beds are generally rich in freshwater mussels. Nature chooses estuaries that are fed by rivers running through limestone to establish oyster beds. Sometimes the ocean beds themselves are rich in lime, which is often a decomposition of coral reefs and shellfish. This is why limestone