Petromyzontidae are the lampreys. A lamprey (sometimes also called lamprey eel) is a jawless fish with a toothed, funnel-like sucking mouth, with which most species bore into the flesh of other fish to suck their blood. In zoology, lampreys are often not considered to be true fish because of their vastly different morphology and physiology. Lampreys live mostly in coastal and fresh waters, although at least one species, Geotria australis, probably travels significant distances in the open ocean, as is 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 Africa. Their larvae have a low tolerance for high water temperatures, which is probably the reason that 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. Lampreys have one or two dorsal fins, large eyes, one nostril on the top of their head, and seven gills on each side. The unique morphological characteristics of lampreys, such as their cartilaginous skeleton, means that they are the sister taxon (see cladistics) of all living jawed vertebrates (gnathostomes) and are not classified within the Vertebrata itself. The hagfish, which superficially resembles the lamprey, is the sister taxon of the lampreys and gnathostomes (a clade termed the Craniata). Lampreys begin life as burrowing freshwater larvae (ammocoetes). At this stage, they are toothless, have rudimentary eyes, and feed on microorganisms. This larval stage can last five to seven years and hence was originally thought to be an independent organism. After these five to seven years, they transform into adults in a metamorphosis which is at least as radical as that seen in amphibians, and which involves a radical rearrangement of internal organs, development of eyes and transformation from a mud-dwelling filter feeder into an efficient swimming predator, which typically moves into the sea to begin a predatory/parasitic life, attaching to a fish by their mouths, secreting an anticoagulant to the host, and feeding on the blood and tissues of the host. In most species this phase lasts about 18 months. Whether lampreys are predators or parasites is a blurred question.
Not all lampreys can be found in the sea. Some lampreys are landlocked and remain in fresh water, and some of these stop feeding altogether as soon as they have left the larval stage. The landlocked species are usually rather small.
To reproduce, lampreys return to fresh water (if they left it), build a nest, then spawn, that is, lay their eggs or excrete their semen, and then invariably die. In Geotria australis, the time between ceasing to feed at sea and spawning can be up to 18 months long.
Recent studies reported in Nature suggest that lampreys have evolved a unique type of immune system with parts that are unrelated to the antibodies found in mammals. They also have a very high tolerance to iron overload, and have evolved biochemical defenses to detoxify this metal.
Lampreys have long been used as food for humans. During the Middle Ages, they were widely eaten by the upper classes throughout Europe, especially during fasting periods, since their taste is much meatier than that of most true fish. King Henry I of England is said to have died from eating "a surfeit of lampreys" .
Especially in Southwestern Europe (Portugal, Spain, France) they are still a highly prized delicacy. Overfishing has reduced their number in those parts. Lampreys are also consumed in South Korea.
On the other hand, lampreys have become a major plague in the North American Great Lakes after artificial canals allowed their entry during the early 20th century. They are considered an invasive species, have no natural enemies in the lakes and prey on many species of commercial value, such as lake trout. Since the majority of North American consumers, unlike Europeans, refuse to accept lampreys as food fish, the Great Lakes fishery has been very adversely affected by their invasion. They are now fought mostly in the streams that feed the lakes, with special barriers and poisons called lampricides, which are harmless to most other aquatic species. However those programs are complicated and expensive, and they do not eradicate the lampreys from the lakes but merely keep them in check. New programs are being developed including the use of sterilization of male lamprey by trapping of prespawn adults. Research is currently under way on the use of pheromones and how they may be used to disrupt the life cycle (Sorensen, et al., 2005). Control of sea lampreys in the Great Lakes is conducted by the U.S. Fish and Wildlife Service and the Canadian Department of Fisheries and Oceans. The work is coordinated by the Great Lakes Fishery Commission.
Taxonomy o Petromyzontidae [Petromyzontiformes sensu stricto] (modern lampreys; nykynahkiaiset) [N: 3/41] [FB: 8/38]
|-- Geotria australis Gray, 1851 [Exomegas] [Geotriinae] (australiannahkiainen; Pouched lamprey) |--o Mordacia [Mordaciinae] (etelätyynenmerennahkiaiset)  | |-- M. lapicida (Gray, 1851) (Chilean lamprey) | |-- M. mordax (Richardson, 1846) (Australian lamprey) | `-- M. praecox Potter, 1968 (Non-parasitic lamprey) `--o Petromyzoninae ()? |--o Petromyzontini ()? [3/8] | |-- Petromyzon marinus Linnaeus, 1758 (merinahkiainen; Sea lamprey) | `--o Ichthyomyzon ()?  | |-- I. bdellium (Jordan, 1885) (Ohio lamprey) | |-- I. castaneus Girard, 1858 (Chestnut lamprey) | |-- I. fossor Reighard & Cummins, 1916 (Northern brook lamprey) | |-- I. gagei Hubbs & Trautman, 1937 (Southern brook lamprey) | |-- I. greeleyi Hubbs & Trautman, 1937 (Mountain brook lamprey) | `-- I. unicuspis Hubbs & Trautman, 1937 (Silver lamprey) `--o Lampetrini [Lampetra sensu lato] ()? [N: 1/29] [FB: 3/26] |-- Caspiomyzon wagneri (Kessler, 1870) (kaspianmerennahkiainen; Caspian lamprey) `--+-- Tetrapleurodon spadicea Bean, 1887 (Mexican lamprey) (meksikonnahkiaiset)  [ssp. => Lampetra?] `--+--o Entosphenus (tyynenmerennahkiaiset) [†1,7] [ssp. => Lampetra?] | |--+-- E. lethophaga (Hubbs, 1971) (Pit-Klamath brook lamprey) | | `-- E. minima (Bond & Kan, 1973) (Miller Lake lamprey) | `--+-- E. tridentata (Richardson, 1836) (Pacific lamprey) | `--+-- E. similis (Vladykov & Kott, 1979) (Klamath river lamprey) | `-- E. macrostoma Beamish, 1982 (Vancouver lamprey) `--+--o Lethenteron (jäämerennahkiaiset) [N: 6] [FB: 4] | |-- L. appendix (DeKay, 1842) (American brook lamprey) | |-- L. camtschaticum (Tilesius, 1811) (Arctic lamprey) | |-- L. kessleri (Anikin, 1905) (Siberian brook lamprey) | `-- L. zanandreai (Vladykov, 1955) (Po brook lamprey) `--+--o Eudontomyzon (mustanmerennahkiaiset) [N: 6] [FB: 4] | |-- E. sp.1 | |-- E. morii Berg, 1931 (Korean lamprey) | |-- E. danfordi Regan, 1911 (Carpathian lamprey) | |-- E. hellenicus Vladykov, Renaud, Kott & Economidis, 1982 (Greek brook lamprey) | |-- E. mariae (Berg, 1931) (Ukrainian brook lamprey) | `-- E. vladykovi Oliva & Zanandrea, 1959 (Vladykov's lamprey) `--o Lampetra [incl. Okkelbergia] ()? [N: 7] [FB: 18] |-- L. aepyptera (Abbott, 1860) (Least brook lamprey) |-- L. ayresi (Günther, 1870) (River lamprey) |-- L. fluviatilis (Linnaeus, 1758) (tavallinen nahkiainen, nahkiainen; European river lamprey) |-- L. folletti (Vladykov & Kott, 1976) (Modoc Brook lamprey) |-- L. geminis (Alvarez, 1964) (Mexican brook lamprey) |-- L. hubbsi (Vladykov & Kott, 1976) (Kern brook lamprey) |-- L. lanceolata Kux & Steiner, 1972 (Turkish brook lamprey) |-- L. pacifica Vladykov, 1973 (Pacific brook lamprey) |-- L. planeri (Bloch, 1784) (pikkunahkiainen; European brook lamprey) |-- L. reissneri (Dybowski, 1869) (Far Eastern brook lamprey) `-- L. richardsoni Vladykov & Follett, 1965 (Western brook lamprey)