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This material has been added to the existing Eastern Oyster page. Laurasill1 (talk) 17:18, 29 November 2012 (UTC)

Eastern Oyster (Crassostrea virginica)

The Life Cycle
The life cycle of C. virginica is as follows: 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 which varies from north to south, northern oysters spawn at temperatures between 60 and 68 °F (15.5 and 20 °C), while 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 4 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 1 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 6 hours. The trochophore larvae depend on its internal yolk supply for energy. They then develop into a fully shelled veliger larva, also known as the late umbo stage, which have a hinged side and a velum, that is formed within 12 to 24 hours”. During this time the shelled veliger larvae use their ciliated velum to capture food and swim. The larvae remain planktonic for about 2 or 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, and 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 cement themselves to, 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 metamorphoses 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 4 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.

Composition of the Larval Shell
Studies show that 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 post larval adult oyster shell. The epithelium of the oyster’s mantle secretes both the prodissoconch and the post larval shells, but at different times. Tests were ran to try to determine the reason 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 C. virginica 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 shells which are very thin, hyaline, and translucent. The study showed the specific gravity of aragonite is 2.95 and calcite is 2.72, so as far as weight is concerned, there really is not an advantage for a larval oyster to have a shell made of one composition over the other. Then, it was decided to compare oyster larval shells not with adult shells, but instead, with other bivalvia larval shells. The study concluded that all, or almost all, bivalvia have aragonitic larval shells because the majority of them have aragonitic adult shells, and it can be assumed that C. virginica oyster larvae have an aragonitic shell simply to conform to the general pattern in the bivalvia. There is no adaptive need for free-swimming larvae to have shells of a composition other than argonite, they have that composition because their ancestors did. The study then posed the question, “Why do larval oysters suddenly begin depositing calcite after they have attached to a substratum and begun metamorphosis?” It was concluded that adaptations for a thicker shell are required for defense against predators because the oysters are permanently immobilized and therefore live in a different environment than that of the free-swimming larvae.

Effects of the BP Deepwater Horizon Oil Spill
Harvestable size of a C. virginica oyster is 75mm, 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 are from 10 to 30 ppt; 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 reports, “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 that if the water around them was contaminated with oil and also 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 couple of days due to warm temperatures in the Gulf if the shells remains closed . The toxins in the oil and dispersants can also kill the larvae.