The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified.
Some kinds of oysters are commonly consumed, cooked or raw, by humans as a delicacy. Other kinds, such as pearl oysters, generally not eaten by humans, are harvested for the pearl produced within the mantle.
Etymology
First attested in English 17th century, the word "oyster" comes from Old French oistre, in turn from Latin ostrea, the feminine form of ostreum, which is the latinisation of the Greek ὄστρεον (ostreon), "oyster". Compare ὀστέον (osteon), "bone".
Anatomy
Oysters are filter feeders, drawing water in over their gills through the beating of cilia. Suspended 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 pseudofaeces. Oysters feed most actively at temperatures above 10°C (50°F). An oyster can filter up to 5 l (1.3 US gal) of water per hour. Chesapeake Bay's once-flourishing oyster population historically filtered excess nutrients from the estuary's entire water volume every three to four days. Today, that would take nearly a year. Excess sediment, nutrients, and algae can result in the eutrophication of a body of water. Oyster filtration can mitigate these pollutants.
In addition to their gills, oysters can also exchange gases across their mantles, which are lined with many small, thin-walled blood vessels. A small, three-chambered heart, lying under the adductor muscle, pumps colorless blood to all parts of the body. At the same time, two kidneys, located on the underside of the muscle, remove waste products from the blood.
While some oysters have two sexes (European oyster and Olympia oyster), their reproductive organs contain both eggs and sperm. Because of this, it is technically possible for an oyster to fertilize its own egg. The gonads surround the digestive organs, and are made up of sex cells, branching tubules, and connective tissue.
Once the female is fertilized, she discharges millions of eggs into the water. The larvae develop in about six hours and swim around for about two to three weeks. After that, they settle on a bed and mature within a year.
