Eastern Oyster

Eastern Oyster
Crassostrea virginica
(A.K.A. American oyster)

Key Distinguishing Markings:

  • Eastern Oysters have two shells, hence the oyster is a bivalve and is related to other bivalves, such a clams and mussels.
  •  The soft body of the oyster is inside the shells, which help protect the oyster from predators and harsh environments.
  •  The shells fit tightly together forming a water tight seal when fully closed. 
  • The shells are generally greyish/brown in color and rough, except for the inside which is smooth and white.
  • How each oyster looks is based greatly on where it has set and what the environmental conditions are. For example, mud bottom oysters tend to be long, while hard bottom oysters tend to be round.


  • Adults can grow to about 8", but 3" to 5" is more typical in the Chesapeake Bay.
  • Newly set oysters called “spat”, range from pin-head size to about 1”.  


  • The range of the American or eastern oyster extends well beyond the Chesapeake Bay, encompassing the east coast of North America from the Gulf of St. Lawrence in Canada to Key Biscayne, Florida, and continuing south through the Caribbean to the Yucatan Peninsula of Mexico and to Venezuela.
  • Oysters are found throughout the Chesapeake Bay on firm bottom areas called oyster bars.

Food Preference:

  • Oysters feed on algae in the Chesapeake Bay by filtering it from the water with their gills.
  • A large oyster (about 3”) can filter about 50 gallons of water per day in the warm months. During the winter, filtering rates drop dramatically and even cease.
  • Oysters also remove silt particles from the water as they filter their food.

Eastern oysters photograph courtesy of Chris Judy

Spawning and Habitat:

  • Oysters reproduce by spawning into the water column. This generally takes place between June and September. Male and female oysters cast their gametes into the water, where fertilization occurs by chance.
  • Six to eight hours after fertilization the new larvae has formed the first part of its shell. Cilia have also developed which allow it to swim.
  • The larvae swim for two to three weeks before heading to the bottom to find a place to set (attach).
  • At this stage the oyster has a foot which it uses to find a suitable place to set.
  • Upon setting, a gland in its foot secretes a drop of liquid cement, which attaches the oyster to a suitable substrate. At this point it is now called a spat.
  • Oysters prefer to set on the shells of other oysters but other hard, clean substrates are also used. They can’t set in mud or sand bottom – they need a hard surface.
  • Oysters will grow to about three inches in three years, unless the water is of low salinity. Then it may take 4 to 5 years to reach three inches.
  • Oysters under a year old are about 90% male. As they grow many will change sex, and older oysters are about 80% female.
  • Oysters prefer salty water with an optimum salinity range of 10ppt (parts per thousand) to 28 ppt.
  • Extended periods under 5ppt salinity can cause significant mortality.

Oystering Tips:

Fun Fact:

  • Oysters have been around for 15 million years and in some places their shell deposits are 50 feet thick.
  • Many people like to eat oysters. For that reason, they are a very important seafood resource for the Chesapeake Bay.
  • Oyster beds or reefs form a suitable habitat for other living creatures.
  • Oysters at one time were very plentiful. However, over the years oyster diseases, MSX and Dermo have killed many of them.
  • In the mid to late 1800's conflicts between oystermen who used different gear types and between those in Maryland and Virginia, escalated to deadly levels in a time known as the Oyster Wars.
  • In 1868, Maryland formed the Oyster Navy to police the waters of the State to enforce oystering regulations and help deter the violence.
  • Parts of the city of Crisfield are built on a foundation of oyster shells.

In Focus: Oyster Restoration in the Chesapeake Bay

Family: Ostreidae

Order: Ostreoida

Class: Bivalvia

For more information on eastern oysters and their management, please contact Chris Judy or Eric Campbell.

Illustration is from a 1902 Colorlitho by S.F. Denton.
Photograph by Christopher Judy

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