Lampreys live mostly in coastal and fresh waters, although at least one species, Geotria australis, probably travels significant distances in the open ocean, as evidenced by the lack of reproductive isolation between Australian and New Zealand populations, and the capture of a specimen in the Southern Ocean between Australia and Antarctica. They are found in most temperate regions except those in Africa. Their larvae have a low tolerance for high water temperatures, which is probably why they are not found in the tropics.
Outwardly resembling eels, in that they have no scales, an adult lamprey can range anywhere from 13 to 100 centimetres (5 to 40 inches) long. Lacking paired fins, Lampreys have large eyes, one nostril on the top of the head, and seven gills on each side. The unique morphological characteristics of lampreys, such as their cartilaginous skeleton, mean that they are the sister taxon (see cladistics) of all living jawed vertebrates (gnathostomes), and are usually considered the most basal group of the Vertebrata. They attack prey by attaching their mouthparts to the animals body, using their teeth to cut through skin and scales to get to the blood and body fluid. They will usually not attack humans but they will if starved. Hagfish, which superficially resemble lampreys, are the sister taxon of the true vertebrates (lampreys and gnathostomes).
Lampreys provide a valuable insight into the evolution of the adaptive immune system, possessing a type of primordial adaptive immunity lacking canonical T and B cells as in higher vertebrates. Lamprey leukocytes express surface Variable Lympocyte Receptors (VLRs) generated from somatic recombination of leucine-rich repeats gene segments in a recombination activating gene-independent manner, in contrast to T and B cells of classic adaptive immune systems.
They also have a very high tolerance to iron overload, and have biochemical defenses to detoxify this metal.