The approximately 40 species of eel-like fishes that comprise the family Petromyzontidae are known as lampreys and lack the jaws, scales, paired fins, and bone that are present in most of the vertebrates we know as "fishes". This is one of the two extant groups of jawless vertebrates (the other being the hagfishes, Myxini) (Janvier 2010 and references therein). Nearly all lamprey species are found outside the tropics in the northern hemisphere, but a few are in the southern hemisphere.
Most lampreys excavate pits in stream riffles (more rarely in wave-swept areas of lakes) to be used as spawning sites by moving stones with their suction-disc mouths and fanning out fine particles with body vibrations. Eggs hatch into blind larvae known as ammocoetes. Lampreys may live for 3 to 8 years (or even longer) as larvae, filtering microorganisms from the water in mud- or sand-bottomed pools, before metamorphosing into the radically different adult form. Adults of some parasitic species are anadramous, migrating to the ocean to feed, but returning to freshwater to spawn. Some other parasitic species are restricted to freshwater. Around half of the lamprey species are non-parasitic, feeding only as larvae and spawning the spring following metamorphosis. Parasitic lampreys typically feed by attaching to a large fish and rasping a hole in its side (some species feed predominantly on the blood of their hosts, whereas others ingest mainly flesh, and yet others feed on a combination of blood and flesh).
Adult lampreys have a skeleton made of cartilage rather than bone and an oral disc with rasping teeth on the tongue. The type and arrangement of teeth are important characters in distinguishing adult lampreys of different species (distinguishing ammocoetes of different species is often more difficult).
(Potter and Hilliard 1987, as cited in Gill et al. 2003; Page and Burr 2011)
Most lamprey genera include some closely related forms (sometimes treated as distinct species) with differing life history traits (e.g, parasitic and anadramous versus nonparasitic and freshwater resident, with the nonparasitic forms often reproducing at an earlier age and not feeding or growing after metamorphosis) (Espanhol et al. 2007)
According to Renaud (1997), of the 34 nominal lamprey species in the Northern Hemisphere ten are endangered, nine are vulnerable in at least part of their range, and one is extinct. The major threat is habitat degradation through pollution and stream regulation.
Renaud (2011) provides identification keys and reviews the biology of all 39 species that he recognizes.