Шығыс ұлуы: сипаттамасы және қорғау мәртебесі

Species of bivalve

The eastern oyster (Crassostrea virginica)—also called the Atlantic oyster, American oyster, or East Coast oyster—is a species of true oyster native to eastern North and South America. Other names in local or culinary use include the Wellfleet oyster, Virginia oyster, Malpeque oyster, Blue Point oyster, Chesapeake Bay oyster, and Apalachicola oyster. C. virginica ranges from northern New Brunswick south through parts of the West Indies to Venezuela. It is farmed in all of the Maritime provinces of Canada and all Eastern Seaboard and Gulf states of the United States, as well as Puget Sound, Washington, where it is known as the Totten Inlet Virginica. It was introduced to the Hawaiian Islands in the 19th century and is common in Pearl Harbor. The eastern oyster is an important commercial species. Its distribution has been affected by habitat change; less than 1% of the population present when the first European colonists arrived is thought to remain in the Chesapeake Bay and its tributaries. As of 2014, the global conservation status of Crassostrea virginica, as assessed by NatureServe, is "vulnerable," as the oyster's populations are threatened by overharvest and water pollution.

Like all oysters, C. virginica is a bivalve mollusk with a hard calcium carbonaceous shell that protects it from predation. This particular type of oyster is important to its ecosystem. Like all oysters, C. virginica is a filter feeder. It sucks in water and filters out the plankton and detritus to swallow, then spits the water back out, thus cleaning the water around it. One oyster can filter up to 50 gallons of water in 24 hours. Eastern oysters also provide a key structural element within their ecosystem, making them a foundation species in many environments, and they serve as ecosystem engineers in western Atlantic estuaries. Like coral reefs, oyster beds, also known as oyster reefs, provide key habitat for a variety of different species by creating hard substrate for attachment and habitation. Oyster beds have an estimated 50 times the surface area of an equally sized flat bottom. The beds also attract a high concentration of larger predators looking for food. The eastern oyster, like all members of the family Ostreidae, can make small pearls to surround particles that enter the shell. These pearls, however, are insignificant in size and of no monetary value; the pearl oyster, from which commercial pearls are harvested, is of a different family. Unlike most bivalves, whose shells are aragonite, adult eastern oysters have calcite shells. The larvae, however, retain the aragonite shell of their ancestors. The specific gravity of the two types of shell is similar, so neither would confer a weight advantage over the other for a freely swimming larva. The transition to the thicker calcite shell in the adult of this species is thought to be an adaptation for defense against predators because the oysters are immobilized in exposed locations. Spawning of C. virginica is controlled by water temperatures and varies from north to south; northern oysters spawn at temperatures between 60 and 68 °F (15.5 and 20 °C), whereas southern oysters spawn at temperatures above 68 °F (20 °C). Spawning can occur throughout the warm months. The eastern oyster's reproductive cycle begins during late summer and autumn months with the storage of glycogen energy reserves. This glycogen is then used to support gametogenesis during the next winter and early spring when food intake is at a minimum. Fertilized eggs develop in about six hours into planktonic, free swimming, trochophore larvae, also known as the early umbo stage, which have cilia and a small shell. They then develop within 12 to 24 hours into a fully shelled veliger larvae, also known as the late umbo stage, which has a hinged side and a velum. These studies indicated that heat shock driven oxidative stress induces increased oxidative damage in DNA (e. g. measured as 8 OHdG formation) and cellular apoptosis (programmed cell death) in the gills of American oysters. In fact, the word Chesapeake derives from an Algonquian word meaning 'Great Shellfish Bay'. Because of the abundance of oysters filtering the waters of the Chesapeake, the water was much clearer than it is now. Visibility would sometimes reach 20 feet. When the English began settling the area, they evidently had a localized impact of the oyster population. One archaeological site measured oyster sizes near Maryland's old capital St. Mary's city from 1640 to 1710. In 1640, when the city was still small, oysters measured 80 mm, and in the city's maximum population in 1690, they measured to 40 mm. When the capital moved to Annapolis, the population moved with it, and by 1710, the oysters were back up to 80 mm. However, the effect of overharvesting would remain local until after the Civil War, when a combination of new technologies led to the removal of nearly all the bay oysters.

During the industrial revolution, several new technologies were introduced to the Chesapeake Bay area which allowed for more intensive oyster harvesting. First was the invention of canning. This allowed oysters to be preserved much longer, and created demand for oysters across the world. Secondly, the invention of the dredge enabled oyster harvesters to reach untouched depths of the Chesapeake. And finally, the proliferation of steam powered ships and railroads made transportation more reliable, enabling merchants to sell oysters far and wide. Estimates for the harvest in 1839 give a figure of 700,000 bushels. After the Civil War, dredges were legalized, and harvesting exploded to 5 million bushels that year. By 1875, 17 million bushels were taken from the bay. The harvesting would reach its peak in the 1880s, with 20 million bushels being harvested from the bay each year. the annual catch has declined significantly. In Maryland, the 2006–2007 catch was 165,059 bushels (about 7600 m³) of oysters. Other regions of the East Coast of the United States have successful oyster farms, including most notably Cotuit , Martha's Vineyard and Wellfleet, Massachusetts, on Cape Cod.

Harvestable size of a C. virginica oyster is , which can take from 12 to 36 months, depending on temperature, salinity of the water, and food supply. Salinity is a very important climatological variable that affects spatfall. Oysters do best where salinities range from 10 to 30 ppt; the range of 15 to 18 ppt is considered optimal. In 2010, 665 miles of coastline were affected by the Deepwater Horizon oil spill. To keep the oil at bay and to spare the oystermen, the authorities of Louisiana made an unprecedented decision to maximize the fresh water flow through the region's canals to three times usual levels. This is cause for great concern that the oysters are being killed by the toxins in the dispersant, as well.

"Dermo" (Perkinsus marinus) is a marine disease of oysters, caused by a protozoan parasite. It is a prevalent pathogen of oysters, causing massive mortality in oyster populations, and poses a significant economic threat to the oyster industry. Multinucleated sphere X (MSX) (Haplosporidium nelsoni), another protozoan, was first described along the mid Atlantic coast in 1957. Mortalities can reach 90% to 95% of the oyster population within 2 to 3 years of being seeded. MSX slows the feeding rates of infected oysters, leading to a reduction in the amount of stored carbohydrates, which in turn inhibits normal gametogenesis during spawning, resulting in reduced fecundity.

The eastern oyster is the state shellfish of Connecticut, its shell is the state shell of Virginia and Mississippi, and its shell in cabochon form is the state gem of Louisiana.

Who Killed Crassostrea virginica? The Fall and Rise of Chesapeake Bay Oysters (2011), Maryland Sea Grant College (60 min. film)
Galtsoff, Paul Simon. The American Oyster: Crassostrea Virginica Gmelin. US Government Printing Office, 1964.