Lamprey, any of about 40 species of jawless, eellike fish. The skeleton of a lamprey is made of cartilage. An adult lamprey has a circular mouth, called an oral disc, that exerts strong suction. Teeth are located on the oral disc and also on the tongue.

Lamprey larvae (young) are toothless and blind. They burrow into sediment at the bottom of a stream where they feed on tiny organisms that they filter out of the water. After some three to eight years, the larvae change into adults. The adults die soon after spawning (laying their eggs).

Lampreys are found in cold and temperate waters. Some live exclusively in freshwater. Others live in the ocean but migrate to freshwater to spawn. Some lampreys are parasitic; these species live in freshwater or the ocean. Free-living (nonparasitic) species live in freshwater.

An example of a free-living species is the American brook lamprey, which lives mainly in eastern North American streams. It uses its oral disc to remove stones from the streambed to create a nest for its eggs. Adults live only about a month and do not feed.

An example of a parasitic species is the sea lamprey, which is native to the Atlantic Ocean. Besides using the oral disc to create a nest, the sea lamprey uses it to attach itself to a larger fish. With its tongue, the lamprey bores a hole through a fish's flesh and then sucks the fish's blood. The sea lamprey lives as a parasite on a succession of fish for about two years; then it migrates into streams to spawn.

Sea lampreys entered the Great Lakes during the 1800's. By the 1950's, they had nearly destroyed the Great Lakes whitefish and lake trout fisheries. Since 1962 the chemical TFM has been used to kill lamprey larvae, and the lamprey population has declined. Lampreys remain a problem, however, and additional control methods are being used. One method is to build dams on certain streams to keep lampreys out of their spawning areas.

Lampreys make up the family Petromyzontidae. The American brook lamprey is Lampetra appendix; the sea lamprey, Petromyzon marinus.