Oysters on a tray with selective focusing.
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 oyster are commonly consumed by humans, cooked or raw. Other kinds, such as pearl oysters, are not.
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 faeces or pseudofaeces. Oysters feed most actively at temperatures above 10 °C (50 °F). An oyster can filter up to 5 litres (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 mantle, which is 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 oysters have two sexes, they may change sex one or more times during their life span. 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.