lamprey, name for several primitive marine and freshwater fishes of the order Cyclostomata, or jawless fishes (see cyclostome). As in the other member of the order, the hagfish, the adult lamprey retains the notochord, the supporting structure that in higher vertebrates is found only in the embryo. An ancient fish that still resembles fossils that are 360 million years old, the lamprey lacks a sympathetic nervous system, a spleen, and scales. Most adult lampreys are parasitic, sucking the blood of other fishes. The horny teeth, set in the circular, jawless mouth, attach to the prey and the lamprey feeds as it is carried along. Lampreys have an anticoagulant in the saliva that keeps the blood of the victim fluid. Some freshwater lampreys eat flesh as well as blood.

Lampreys resemble eels in external appearance and, although not related to the true eels, are sometimes called lamprey eels. When not attached to prey, they swim with undulating movements. The marine lampreys normally migrate into freshwater to spawn, and some populations have become landlocked in freshwater.

The sexes are separate in lampreys and fertilization is external. The parents die shortly after the eggs are deposited in a nest. The larvae, called ammocoetes, are about 1/4 in. (6 mm) long. They are transparent, eyeless filter-feeders and live in muddy river bottoms, eating particles of organic matter. Ammocoetes are used in zoology courses to demonstrate a theoretically primitive vertebrate construction. At about five years of age they metamorphose into the adult, parasitic form. In some species the adult does not feed and remains the size of the larva.

There are 7 genera and about 25 species of lampreys, with 13 species in the United States. The Atlantic lamprey, Petromyzon marinus, found on both sides of the Atlantic, has become well established in the Great Lakes, where it is considered a serious pest by the fishing industry. Lampreys are classified in the phylum Chordata, subphylum Vertebrata, class Agnatha, order Cyclostomata, family Petromyzontidae.

Any of about 22 species of primitive, jawless fishes (with hagfishes in class Agnatha). Lampreys live in coastal and freshwater in temperate regions worldwide except Africa. Eel-like, scaleless animals, they are 6–40 in. (15–100 cm) long. Lampreys have well-developed eyes, a single nostril on top of the head, a cartilaginous skeleton, and a sucking mouth with horny teeth surrounding the round opening. They spend years as burrowing larvae; adults of most species move into the sea. They attach to fish with their mouth and feed on their host's blood and tissues. Some species will remain in freshwater, notably the sea lamprey, which entered the Great Lakes and nearly eliminated lake trout and other commercially important fishes there.

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