Eastern Oyster (Crassostrea virginica)

WHAT IS AN OYSTER?  An oyster is a shellfish with 2 rough white shells that hook together at one end.  The oyster has strong muscles which hold the shell shut.  It is very difficult for predators (like people) to pry the shell open.  An Eastern oyster is usually 2 to 6 inches long.  The inside of the shell has a purple mark where the oyster was attached.  The shell is sharp and can cut you!

HOW DO THEY GROW THEIR SHELLS?  Oyster shells are made of calcium carbonate (lime).  The oysters get the chemicals to make their shells from the water they live in.  They also have a sort of skin, called a mantle, which spreads this calcium carbonate on the outside of their bodies to form a protective shell.  Oysters must live in water that is temperate (warm all year) and not too cloudy.  They grow only in areas where salt and fresh water mix together, like salt marshes.  Oysters are born as free-swimming plankton (tiny microscopic organisms).   When they grow up, they find a place (on mud, coral, trash, or other oyster shells) to attach and grow.  Once they grow their shells, they can’t move around anymore.  When the tide is high, oysters are covered by water, but when the tide goes out, they are left sticking up into the dry air.  Their shells close tightly together so the animal inside will not dehydrate (dry out) before the tide comes back in.

HOW DO OYSTERS EAT?  Oysters are filter-feeders.  They suck in water and filter out the plankton and detritus to swallow.  Then they spit the water back out.  (Detritus is dead plant and animal matter.)  They also accidentally filter out and collect poisons and bacteria that might be in the water.   This cleans the water, but is not healthy for the people who eat those oysters.  Because these poisons are more likely to be in coastal waters in the summer months, it is usually not safe to eat oysters in the summer time.  Oysters have gills and get their oxygen from the water. 

WHAT ABOUT PEARLS?  The oyster’s mantle (skin) makes both an outer white crusty shell, and a smooth inner shell.  The smooth inner part is called “nacre” or “Mother of Pearl.”  Sometimes a bit of sand gets inside the oyster’s shell.  This is very irritating to the oyster, like getting an eyelash in your eye.  So the oyster covers this bit of dirt with shiny smooth Mother of Pearl.  It keeps covering the dirt and rolling it around until it doesn’t cause any more irritation.  This makes a pearl.  The oysters that people eat in north Florida (Eastern oysters) hardly ever make pretty pearls.  But there are other kinds of oysters, clams, mussels, conchs, whelks, and even abalone that do make nice pearls. In addition to being round and white, pearls can be yellow or black in many different shapes.

Arms, Karen. Biology, Third Ed. Philadelphia: Saunders College Publishing, 1987.
Kunz, George F. Gems and Precious Stones of North America. New York: Dover Publications, Inc., 1968.
Smith, G. F. Herbert. Gemstones. New York: Pitman Publishing Company, 1972.
Wernert, Susan J. ed. Readers Digest North American Wildlife. United States: Reader’s Digest Association, Inc., 1982.

Provided by the Pelotes Island Nature Preserve
http://pelotes.jea.com