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Eastern Oyster

Crassostrea virginica

An oyster's bottom shell is cupped and has a purple muscle scar on the inside. (Fisheries and Oceans Canada/World Register of Marine Species)
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An oyster's bottom shell is cupped and has a purple muscle scar on the inside. (Fisheries and Oceans Canada/World Register of Marine Species)

Also known as the American or Virginia oyster, the eastern oyster is a bivalve with two rough, whitish shells. It forms reefs in brackish and salty waters throughout the Chesapeake Bay.

Appearance:

  • Two rough shells enclose a soft body and vary in color from white to gray to tan  
  • Right or top shell is flat 
  • Left or bottom shell is cupped, with a purple muscle scar on the inside
  • Grows three to five inches long. Can reach a length of eight inches 

Habitat:

  • Live in brackish and salty waters from 8 to 35 feet deep
  • Often concentrated on oyster bars, beds or rocks, which are located in waterways with firm bottom areas 
  • Attach to one another, forming dense reefs that provide habitat for many fish and invertebrates 

Range:

  • Found in the middle and lower Chesapeake Bay and its rivers
  • Range extends along eastern North America from the Gulf of St. Lawrence to the Gulf of Mexico 

Feeding:

  • Filter feeders, oysters feed on plankton by opening their shells and pumping water through their gills. This action traps particles of food  

Predators:

  • Prone to infection by parasites that cause the aquatic diseases MSX and Dermo

Reproduction and Life Cycle:

  • Spawns in early summer when water temperatures rise 
  • Adults release eggs and sperm into the water. Females can produce about 100 million eggs each year 
  • After spawning, oysters are thin because they have used up their stored food reserves. Adults grow larger and stronger as the weather cools 
  • In less than 24 hours, fertilized eggs develop into free-swimming larvae. Over the next two to three weeks, oyster larvae grow a "foot," which is used to crawl over and explore a surface before settling 
  • Once larvae find a suitable surface to settle on, they secrete a cement-like substance, which fixes the left valve into place. Settled juvenile oysters are called "spat"
  • Many oysters change sex over the course of their lifetime. Most oysters less than one year old are male, while most older oysters are female 

Other Facts:

  • The cavity within an oyster’s shell is always filled with water. This allows oysters to survive for a long time without having to open their shells to feed. During cooler months, oysters can live out of the water for extended periods of time.
  • Oysters are one of the most important commercial catches in the Chesapeake Bay. 
  • Historically, oysters were only eaten during months whose names contain an “R.” This is because oyster quality is poor right after they have spawned, and because oysters would spoil without refrigeration during warmer months.

Bay 101: Oysters from Chesapeake Bay Program on Vimeo.

Sources and Additional Information:


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