The common name oyster is used for a number of different groups of bivalve mollusks, most of which live in marine habitats or brackish water. The shell consists of two usually highly calcified valves which surround a soft body. Gills filter plankton from the water, and strong adductor muscles are used to hold the shell closed.
Some of the groups known as oysters, also known as mullusks (true oysters), are highly prized as food, both raw and cooked. Some other groups also called oysters, such as pearl oysters, are not widely eaten, at least not since recent times.
True (edible) oysters are incapable of making gem-quality pearls, although the opposite idea is a commonly-encountered misapprehension, often seen in illustrations or photographs where an edible oyster shell is mistakenly paired with a gem-quality pearl.
The "true oysters" are members of the family Ostreidae. This family includes the edible oysters, which mainly belong to the genera Ostrea, Crassostrea, Ostreola or Saccostrea. Examples are the Edible Oyster, Ostrea edulis, Eastern oyster Crassostrea virginica, Olympia Oyster Ostreola conchaphila, Pacific Oyster Crassostrea gigas, Sydney rock oyster Saccostrea glomerata, and the Wellfleet oyster (a variety of C. virginica).
Physical characteristics
Oysters are filter-feeders: they draw water in over their gills through the beating of cilia. Suspended food plankton and particles are trapped in the mucus of a gill, and from there are transported to the mouth, where they are eaten, digested and expelled as feces or pseudofeces. Feeding activity is greatest in oysters when the water temperatures are above 50°F (10°C). Healthy oysters consume algae and other water-borne nutrients, each one filtering up to five litres of water per hour. Scientists believe that the [Chesapeake Bay's] once-flourishing oyster populations historically filtered the estuary's entire water volume of excess nutrients every three or four days. Today that process would take almost a year, and sediment, nutrients, and algae can cause problems in local waters. Oysters filter these pollutants, and either eat them or shape them into small packets that are deposited on the bottom where they are harmless.
Oysters breathe much like fish, using both gills and mantle. The mantle is lined with many small, thin-walled blood vessels which extract oxygen from the water and expel carbon dioxide. A small, three-chambered heart, lying under the abductor muscle, pumps colorless blood, with its supply of oxygen, to all parts of the body. At the same time two kidneys located on the underside of the muscle purify the blood of any waste products they have collected.
There is no way of determining male oysters from females by examining their shells. While oysters have separate sexes, they may change sex one or more times during their life span. The gonads, organs responsible for producing both eggs and sperm, surround the digestive organs and are made up of sex cells, branching tubules and connective tissue.
Habitat and life habits
As a keystone species, oysters provide habitat for an extensive array of marine life. There are three main groups of oysters, the Ostrea species, the Crassostrea" species and the Saccostrea species.
Crassostrea and Saccostrea species live mainly in the intertidal zone while Ostrea species are subtidal. The hard surfaces of oyster shells and the nooks between the shells provide places where a host of small animals can live. Hundreds of animals such as anemones, barnacles, and hooked mussels use oyster reefs as habitat. Many of these animals serve as food for larger animals, including fish such as striped bass, black drum and croakers.
An oyster reef can encompass 50 times the surface area of an equally extensive flat bottom. The oyster contributes to improved water quality through its filter feeding capacity. An oyster's mature shape often depends on the type of bottom to which it is originally attached, but it always orients itself with its outer, flared shell tilted upward. One valve is cupped and the other is flat. The submerged shell opens periodically to permit the oyster to feed.
Oysters usually mature by one year of age. They are protandric, which means that during their first year they spawn as males (releasing sperm into the water). As they grow larger over the next two or three years and develop greater energy reserves, they release eggs, as females. Bay oysters are usually prepared to spawn by the end of June. An increase in water temperature prompts a few initial oysters to spawn. This triggers a spawning 'chain reaction', which clouds the water with millions of eggs and sperm. A single female oyster can produce up to 100 million eggs annually. The eggs become fertilized in the water and develop into larvae, which eventually find suitable sites on which to settle, such as another oyster's shell. Attached oyster larvae are called 'spat'. Spat are oysters 25 mm or less in length. Many species of bivalve, oysters included, seem to be stimulated to settle by the proximity of adults of their species.
Some tropical oysters in a different family, the family Isognomonidae, grow best on mangrove roots, and are exposed at low tide, making them easy to collect. In Trinidad in the West Indies tourists are often astounded when they are told that in the Caribbean, "oysters grow on trees."
The oyster's greatest predators include crabs, sea birds, sea stars, and humans. Some oysters contain live crabs, known as an Oyster crab.