|Oyster bed on Cockspur Island, Georgia, USA|
( Gmelin, 1791)
The eastern oyster (Crassostrea virginica)—also called Wellfleet oyster,  Atlantic oyster, Virginia oyster, or American oyster—is a species of true oyster native to the eastern seaboard and Gulf of Mexico coast of North America. It is also farmed in Puget Sound, Washington, where it is known as the Totten Inlet Virginica.  Eastern oysters are and have been very popular commercially. Today, less than 1% of the population when the first 17th-century colonists arrived is thought to remain in the Chesapeake Bay and its tributaries. 
This particular type of oyster has an important environmental value. Like all oysters, C. virginica is a filter feeder. They suck in water and filter out the plankton and detritus to swallow, then spit the water back out, thus cleaning the water around them. One oyster can filter more than 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.   Similar to coral reefs, oyster beds 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 value; the pearl oyster, from which commercial pearls are harvested, is of a different family.
The lifecycle of C. virginica is: spawning, floating fertilized egg, trochophore, swimming straight-hinge veliger, swimming late veliger, swimming and crawling pediveliger, early spat, later spat, and adult oysters.  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 also occur throughout the warm months. 
Eastern oysters can reach sexual maturity at four months old.  The eastern oyster has a complex reproductive cycle. The 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.  The gametes begin to mature in late spring and then, from June to August they are spawned into the water column, where fertilization occurs.  Each female produces from 75 to 150 million eggs, but only one in 1000 survive.  Then, fertilized eggs develop into planktonic, free-swimming, trochophore larvae, also known as the early umbo stage, which have cilia and a small shell, in about six hours.  The trochophore larvae depend on their internal yolk supply for energy.  They then develop into a fully shelled veliger larvae, also known as the late umbo stage, which have a hinged side and a velum, formed within 12 to 24 hours.  During this time, the shelled veliger larvae use their ciliated vela to capture food and swim.  The larvae remain planktonic for about 2 to 3 weeks, depending on food and temperature conditions, and towards the end of this period, they develop into pediveliger larvae, also known as eyed larvae, which have an umbo, an eyespot, and a foot.  During this time. the pediveliger larvae "settle to the bottom of the water column where they seek a hard substrate".  Ideally, the pediveliger larvae try to locate an adult oyster shell to which they attach, but other hard surfaces will suffice. Lastly, "after careful selection of the proper attachment site, in or during relatively still water, the pediveliger larvae cement themselves to a firm clean surface and metamorphose to the adult form; these newly attached oysters are known as 'spat'".   Upon being stimulated to settle, a larva cements its left valve to the substrate and metamorphoses into an oyster spat by discarding its velum, reabsorbing its foot, and enlarging its gills.  During the first year of life, C. virginica oysters are protandric. Most spat are male, but once they reach sexual maturity, within four months in southern waters, some males change to females after the first or second spawning.  Then, some females can even change back to males again. 
The prodissoconch, the shell of the free-swimming veliger larval stage of C. virginica, is composed of aragonite, as opposed to the calcite composition of a postlarval adult oyster shell.  The epithelium of the oyster's mantle secretes both the prodissoconch and the postlarval shells, but at different times.  Tests were conducted to try to determine why larval and adult shells have different compositions. At the Biological Laboratory of the U.S. Bureau of Commercial Fisheries in Milford, Connecticut, larvae from the eastern oyster were reared in breeding tanks and were then collected, washed with distilled water, and dried as they died.  The sample included a variety of larval stages, starting with the straight-hinge veliger larva with its shell, the protostracum, to the last stage of the umbo larva with its shell, the prodissoconch. Both of these larval stages have very thin, hyaline, and translucent shells.  The study showed the specific gravity of aragonite is 2.95 and calcite is 2.72, so as far as weight is concerned, no advantage results for a larval oyster to have a shell made of one composition over the other.  Then, oyster larval shells were compared not with adult shells, but instead with other bivalve larval shells.  All, or almost all, bivalves have aragonitic larval shells because the majority of them have aragonitic adult shells, and C. virginica oyster larvae are assumed to have an aragonitic shell simply to conform to the general pattern in the bivalves.  No adaptive need is seen for free-swimming larvae to have shells of a composition other than aragonite; they have that composition because their ancestors did.   Adaptations for a thicker shell are required for defense against predators because the oysters are permanently immobilized and therefore live in different environments than those of the free-swimming larvae. 
Before Columbus and the rise of industrial oyster operations, oysters abounded in the bay. Oysters first arrived in the Chesapeake 5,000 years ago,[ citation needed] and shortly after, local Indians began eating them. Archaeologists found evidence the local Native Americans returned to the same place to collect oysters for 3,000 years. John Smith, on a voyage up the Chesapeake, stated oysters, "lay as thick as stones."  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 local 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.  Not only were they being taken for food, but also oyster reefs, where oysters had built hills of their dead shells over thousands of generations, were being dredged out. Surplus oyster shells had many uses then. They were ground into mortar, used as filler in roads, and as a source of lime in agricultural fertilizer. By the 1920s, harvests would be down to just 3–5 million bushels per year because of overharvesting.
Overharvesting would eventually deplete the remaining oyster population in the bay to just 1% of its historical level, where it stands today. Oyster harvests began to decline in the 1890s. They were being taken much faster than they could reproduce. Also, many of the shells and reefs were being taken and not being replaced. Oyster spat need a hard surface on which to attach, and these were vanishing because of the destruction of oyster reefs. By the 1920s, harvests were down to 3–5 million bushels per year,  stabilized for a time by returning oyster shells back to the bay. But in the 1950s, the weakened oyster population had to deal with the diseases "dermo" and MSX. These decimated the remaining oyster population. The parasites which carried the disease are alien to eastern waters, and they were thought to have been brought to the Chesapeake by Asian oysters. Currently, oyster harvests average less than 200,000 bushels a year.
The eastern oyster used to be of great commercial value. Due to the steep decline in the number of oysters in various traditionally harvested areas, primarily because of overfishing and diseases,  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 and Wellfleet, Massachusetts, on Cape Cod.
Effects of the BP Deepwater Horizon oil spill
Harvestable size of a C. virginica oyster is 75 mm, 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.  Typically, when salinity levels are less than 6 ppt, larvae will not settle and metamorphose into spat.  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.  At the mouth of the canals, salinity fell to almost zero, which was probably why most of the oysters died.  Sujata Gupta ventured into the marshlands and Gulf of Mexico with Brad Robin, a man from a line of generations of oystermen in southeastern Louisiana. Robin and his crew threw a net over the side to haul in a catch.  There were dozens of palm-sized oysters, but 75% of them were "boxes" or empty shells.  However, as they traveled further towards the Gulf of Mexico, where the water was less salinity-stressed by the flush, only 20% of the haul came back as boxes, a promising sign the oysters are trying to come back.  Gupta reported, "Now since there are so many empty shells scattered on the sea floor, the larvae have more to latch onto, improving their odds".  However, salinity levels are not the only concern. Eastern oysters are filter feeders, so they are greatly affected by their surroundings since they are sessile organisms. This means if the water around them was contaminated with oil and the dispersant used to get rid of the oil, then these chemicals were collected by the oysters as they filtered the water.  This is cause for great concern that the oysters are being killed by the toxins in the dispersant, as well.  An added dilemma is oysters are in their weakest state after spawning season, which may have caused some of them to close their shells, resulting in death by suffocation within just a few days due to warm temperatures in the Gulf if the shells remain closed.  The toxins in the oil and dispersants can also kill the larvae.  To highlight the recovery of the state's oyster industry, the shell of C. virginica cut into cabochons was made Louisiana's official state gem in 2011.  
"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.
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