Lampreys are one of two groups of jawless fish (the other being hagfish), which are the most primitive true vertebrates. They are members of the Petromyzontidae family. Jawless fish are fishlike vertebrates that resemble eels in form, with a cartilaginous or fibrous skeleton that has no bones. They have no paired limbs and no developed jaws or bony teeth. Their extremely slimy skin lacks scales. Fossils of lampreys have been dated back 280 million years.

The jawless, eel-like lampreys are just as ugly as their hagfish cousins in form and feeding habits; they differ in other respects, however. Hagfish are strictly marine, whereas lampreys are either totally freshwater inhabitants or, if they live in the sea, they return to freshwater rivers to spawn.

Lampreys have a large sucking disk for a mouth and a well-developed olfactory system. The mouth is filled with horny, sharp teeth that surround a filelike tongue. A lamprey’s body has smooth, scaleless skin; two dorsal fins; no lateral line; no vertebrae; no swim bladder; and no paired fins. The lamprey has no prominent barbels on its snout; its eyes are well developed in the adult and visible externally; there are seven external gill openings on each side; and the nasal opening is on the upper part of the head.

Lampreys are usually parasitic. The lamprey attaches itself to the side of a live fish by using its suctorial mouth; then, by means of its horny teeth, it rasps through the victim’s skin and scales and sucks the blood and body juices.

Lampreys spawn in the spring. They ascend streams where the bottom is stony or pebbly and build shallow depressions by moving stones with the aid of their suctorial mouths. Usually, the male and the female cooperate in constructing the nest. When ready to spawn, the pair stirs up the sand with vigorous body movements as the milt and the eggs are deposited at the same time.

The eggs stick to particles of sand and sink to the bottom of the nest. The pair then separates and begins another nest directly above the first, thereby loosening more sand and pebbles, which flow down with the current and cover the eggs. The procedure is repeated at short intervals until spawning is completed. Adults die after spawning.

After several days the young appear and drift downstream until they are deposited in a quiet stretch of water, where they settle down and burrow into the bottom to spend several years as larvae (called ammocetes). When they reach a few inches in length (this varies with the species), the ammocetes transform during the late summer or the fall into adultlike lampreys, complete with sucking disks and circular rows of horny teeth.

The sea lamprey is most notorious as a despoiler of valued sport and commercial fish. It ranges the western Atlantic from southern Greenland, Labrador, and the Gulf of St. Lawrence south to the Gulf of Mexico in Florida. It is landlocked in the Great Lakes, the Finger Lakes, Oneida Lake, and Lake Champlain. It breeds exclusively in freshwater.

Young lampreys, when in saltwater or en route to saltwater, are white underneath and blackish blue, silvery, or lead-colored above. Large specimens approaching maturity are usually mottled brown or dressed in different shades of yellow brown and various hues of green, red, or blue. Sometimes they appear black when the dark patches blend with each other. The ventral surface may be white, grayish, or a lighter shade of the ground color of the dorsal surface. Colors intensify during the breeding season.

Mature sea lampreys are from 2 to 2.5 feet long. The maximum recorded length is nearly 4 feet, and the maximum weight 5.4 pounds.

Commonly, but erroneously, lampreys are known or referred to as “lamprey eels.” They are not true eels (see Eel, American) of the family Anguillidae. For easy differentiation, eels possess jaws and pectoral fins; these are lacking in the lamprey.