[MARYLAND SEA GRANT - OYSTERS IN THE CLASSROOM
[Oysters in the Environment]

The Eastern Oyster
(Crassostrea virginica)

Classified Information

Kindgom Animalia
   Phylum Mollusca
      Class Pelecypoda or Bivalvia
         Order Lamellibranchia
            Family Filibranchia
               Genus Crassostrea
                  species virginica


Biologically Speaking

Range:
Along the coast of North America from the Gulf of the St. Lawrence to the Gulf of Mexico. It has also been introduced to Hawaii, the West Coast of North America, and other locations worldwide.

Oysters are typically found in estuaries, sounds and bays, from brackish water to very salty lagoons. They are very tolerant organisms, being able to withstand wide variations in temperature, salinity, suspended sediments, and dissolved oxygen. In the Bay, oysters are usually found in areas that have over 5ppt salinity.

Feeding:
Oysters are filter-feeders, drawing water in over its gills through the beating of cilia. Suspended food (plankton) and particles are trapped in the mucus of the gills and transported to the mouth, where they will be eaten, digested and expelled as feces or pseudofeces. Feeding activity is greatest in oysters when water temperatures are above 50°F (~10°C).

Reproduction:
Oysters spawn when water temperatures become greater than 68°F (~20°C). They are broadcast spawners, releasing eggs and sperm into the water column. Fertilized eggs develop into a planktonic or swimming larval form. After about two weeks these larvae will "set", a process of cementing themselves to a hard substrate, and metamorphose. This newly attached oyster is known as a "spat".

Defense:
Mollusks, like many other invertebrates, have an open circulatory system that does not confine hemolymph (blood) to traditional vessels like veins, arteries, and capillaries. Instead, an open circulatory system will circulate hemolymph through a number of cavities and sinuses in various parts of the organism. In an oyster, the hemolymph is circulated in this way and can be readily found in the pericardial cavity that contains the heart. Within the hemolymph of the oyster there are three basic types of hemocytes that perform a wide variety of functions from defense to nutrient transport.

A readily found hemocyte known as a granulocyte can make a very impressive showing under oil immersion light microscopy. These hemocytes appear to be filled with small "grains" and have long pseudopodia that extend from the outer surface of the cell. The pseudopodia are used for mobility and the capture of foreign bodies and disease causing organisms like dermo, Perkinsus marinus.


Challenging Chesapeake

Oysters have been around for millions of years - they were used for food, tools, weapons and decoration. During the early 1600s in the Chesapeake region, oyster bars were so numerous and large, that they were reported as navigation hazards by Captain John Smith. Today, the state of the oyster fishery in the Chesapeake has dwindled to less than one percent of its historical mass. Key reasons for this decline include:

  • Overharvesting
  • Habitat Destruction
  • Disease
  • Sedimentation and pollution


Reef Reality

Oysters are the building blocks of one of the most important benthic communities found in Chesapeake Bay, The Oyster Bar. Formed as a result of years of oyster production and settlement in concentrated areas, bars have become home to a complex assemblage of plants and animals.

Benefits of oyster reefs to Chesapeake Bay include:

  • Providing solid structure within the water column for sessile organism attachment (i.e., barnacles, sea anemones)

  • Creating homes and hiding places for organisms seeking refuge from predation (i.e., polychaete worms, soft-shell blue crabs)

  • Providing spawning substrate (i.e., gobies, blennies, skilletfish)

  • Stablizing bottom sediments for benthic organisms and aquatic plants (i.e., hard clams)

  • Concentrating prey (food) species for larger predator fishes (i.e., striped bass)


Links To Restoration Efforts

Oyster Gardening Program
A cooperative effort of the Oyster Alliance, which includes the Chesapeake Bay Foundation, the Maryland Sea Grant Extension Program, the University of Maryland Center for Environmental Science and the Oyster Recovery Partnership.

VIMS Oyster Restoration Activities
The VIMS Molluscan Ecology group has participates in several oyster reef restoration projects.