Eastern oyster
The eastern oyster —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. Other threats to the eastern oyster include global warming, diseases and parasites, and competition with invasive species.
Description
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.
Life cycle
The life cycle of C. virginica consists of spawn, 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, whereas southern oysters spawn at temperatures above. Spawning can occur throughout the warm months.Eastern oysters can reach sexual maturity at four months old in southern waters. 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. 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 a thousand survives. 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. The trochophore larvae depend on their internal yolk supply for energy. 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. 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, where they seek a hard substrate. Ideally, the pediveliger larvae try to locate an adult oyster shell to which they attach, often as part of an existing oyster reef, but other hard surfaces will suffice. Upon settling, 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, some males change to females after the first or second spawning. Some females may change back to males again.
C. virginica has been used to study oxidative DNA damage caused by environmental heat stress. These studies indicated that heat shock driven oxidative stress induces increased oxidative damage in DNA and cellular apoptosis in the gills of American oysters.