The Zoology/Biology of the Edible Oyster

In the natural virginica oyster, the eggs are fertilized in the water. At a single spawning, from a single oyster, anywhere from 15 to 115 million eggs are produced. A Japanese gigas oyster may produce as many as 60 million eggs. The European edulis oyster fertilizes its eggs within its bivalve body. It is astonishing that an animal so prolific could ever face extinction.

In contrast to other moluscs, the spat have a single sticky foot that attaches itself to its environment very early in life. It is the attachment of the oyster to its environment that guarantees its asymmetrical shape. When an oyster is farmed, the farmer must often separate the attached shell from its environment early in its life. The farmer must also decide whether to float the oyster growing container high up near the surface of the water, lower near the sandy bottom, or mount it on a frame that is exposed to view when the tide goes out. This decision affects the ultimate thickness and appearance of the oyster's shell. If the oyster shell is exposed to sunlight by the outgoing tide during its daily cycle, it will also be less prone to certain shell diseases. The position of the oyster farm itself will determine what each oyster eats and the eventual taste of its flesh.

At the perimeter of the fleshy body of each oyster is a visibly darkened rim called the mantle. The mantle secretes a fluid called nacre. It is primarily through the secretions of nacre that the oyster is able to produce the pearlescent interior finish of the shell. Eventually the secretions build up to become the limestone shell. In the Spring, the new layers can be seen extending beyond the limits of the shell like finger nails.

The oyster also uses the nacre from the mantle to protect itself from intruding particles. When the particle enters the shell of the oyster, the oyster smothers it with nacre. In pearl oysters, a tiny piece of shell is inserted into the oyster for coating. It is also thought that fine hairlike extensions at the edge of the mantle aid the oyster in detecting danger and send signals that close the shell.

The oyster has no brain. It is speculated that the oyster may once have had a brain located somewhere near the top or “head” of the shell where the hinge or ligament now exists. Today’s oyster has a "ganglia" instead of a brain. The ganglia does not think but instead can receive signals from its environment and automatically respond to them. When the ganglia alerts the adductor muscles attached to both shells, the muscles close the shell.

The oyster does have a mouth, a stomach, an anus, and a visible heart. When the cupside of the shell is down, the mouth of the oyster is located usually to the right of the ligament. When the shells open, they suck water into the body of the oyster and cause the water to flow past a series of filters, similar in appearance to the gills of a fish. These filters are visible in every oyster when the mantle edge is folded back onto the fleshy body. Even without a brain, the oyster is able to select appropriate food and pass it from the "gills" to its mouth and into its intestines.

As mentioned earlier, the main diet of oysters is algae. In order to live, it must inhale a significant amount of algae-laden water. Depending on its size, the oyster can clean 30-50 gallons of water daily. In order to understand the scale of this endeavor, it is important to understand that each adult human uses about 80 gallons of water per day – that includes uses for hygiene, and eating.

The precise temperature and salinity of the water is crucial to an oyster. It is the warmer temperature of the water that causes the oyster to spawn in the Spring. Too much fresh water will kill an oyster. That is why great care must be taken in storing oysters in iced fresh water. Nevertheless oysters prefer to grow where fresh water mixes with the sea and the salinity of the sea can be diluted. Diseases that affect oysters are sometimes more numerous in warmer waters. Oysters that are grown in the southern States are often restricted by health officials and cannot be served raw by restaurants in the hotter summer months.

The oyster’s fleshy body is attached to the top and bottom shell by means of an adductor muscle. At the place of attachment, there is often a “scar” on the inside surface of each shell. It is often dark blue but can also be brownish. As the oyster grows, the place where the muscle attaches to the shell migrates outward from the ligament. So the scar moves too, as the oyster grows. If you hold a cupped oyster shell so that light falls across it, you can see the shallow grooved trail of movement of the adductor muscle. The blue trail is covered over by nacre as the oyster continues to grow.

An edible virginica oyster must be at least three inches long to be sold in Massachusetts and Connecticut. In Rhode Island the limit is reduced to  2 1/2 inches. In Maine, there is no legal size limit for the virginica oyster. In nature, they grow about one inch per year depending on their environmental conditions. Canadian oysters do not have a length restriction but are sorted into various sizes for sales purposes. Kumamoto oysters rarely grow to such a length. The exterior of the edible oyster shell does telegraph its prior life experiences and differences in coloration often allow the three years of life to be visible – like looking at the growth rings of a tree.