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A Natural History of Oysters
The oysters we eat belong to two genera, Ostrea and Crassostrea, which differ not only in appearance
(Ostrea are rounder, more like scallop shells, and Crassostrea are elongated and asymmetrical) but also in their styles of hermaphrodism.
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Oyster Reproduction: Au Naturel
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Members of the genus Ostrea are bisexual, that is, they alternate between being male and female during the course of a single breeding season. During a female phase, the oyster deposits eggs within the shell, and these eggs are fertilized by sperm
released when the same oyster switches to a male phase. After a 12-day period of incubation, the larval oysters, or spat, swim away from the parent in search of their own place to settle.
Members of genus Crassostrea are intersexual. They begin life as males, and then change to females the following season. After this, they remain primarily female but revert from time to time into males. Reproduction is quite a bit more haphazard
for this genus, because the eggs and sperm are released directly into the water, and fertilization takes place when a pair happens to cross paths. Successfully fertilized eggs, should they survive, rapidly grow into spat, and they, too, swim off in search
of a home.
After fourteen days, the spat, whether Ostrea or Crassostrea, must attach themselves to a stationary object - a rock, a mangrove tree, the post of a pier, even another oyster. There they will remain for their entire lives -- unless they
happen to be cultured oysters, in which case, the spat has settled on a collector planted there by an oyster farmer.
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Oyster Reproduction on the Farm
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Various methods are used to farm oysters. In France, special lime-coated tiles are used as collectors.
Once encrusted with small seed oysters, the tiles are moved to special basins, or parks, where the oysters are scraped off and placed in a sheltered habitat, to be cosseted until they are ready to be taken to market, usually at three to four years old --
their flavor begins to diminish once they reach the age of five. (By contrast, a really lucky oyster -- one who manages to survive unmolested by humans, starfish, or the oyster drill snail, one who escapes infestation -- could live up to 50 years.)
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Preferred Oysters
- The most sought-after, most expensive oysters, those classified as fines de claires, are produced by raising the native European oyster (Ostrea edulis) in claires, specially converted salt marshes, where the water is frequently
changed and is higher in mineral content. The special ingredient in this water is the tiny organism, Navicula ostrearia, which, because it contains chlorophyll, imparts a distinctive -- and much admired -- green tinge to the oysters.
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Things That Go Bump on the Oyster
- The oyster, in spite of its tough-looking shell, is vulnerable to quite a few marine creatures. Oyster drill snails and sea urchins get at the soft meat by boring through the shell; starfish achieve the same by wrapping themselves around the shells
and wrenching them open; octopuses can crack the shells with their beaks. Oysters are also vulnerable to parasites, which often thrive best in the sheltered environments of the oyster farms.
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The Pearl
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Many members of the oyster family will produce a pearl when a foreign object gets inside the shell and irritates the sensitive flesh. But the pearl that threatens to crack the tooth of the patron of a raw bar will, at best, resemble a tiny bit of gravel.
Luminous, iridescent pearls -- the valuable ones -- are produced by a different kind of mollusk.
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