Genetic evidence suggests that lampreys are more closely related to hagfish, the only other living group of jawless fish, than they are to jawed vertebrates, forming the superclass Cyclostomi.

Some species (e.g. Geotria australis, Petromyzon marinus, and Entosphenus tridentatus) travel significant distances in the open ocean,[13] as evidenced by their lack of reproductive isolation between populations.

[16][17] Distribution of lampreys may also be adversely affected by dams and other construction projects due to disruption of migration routes and obstruction of access to spawning grounds.

Conversely, the construction of artificial channels has exposed new habitats for colonisation, notably in North America where sea lampreys have become a significant introduced pest in the Great Lakes.

[19] As a result, the flesh-feeders have smaller buccal glands as they do not require the production of anticoagulant continuously and mechanisms for preventing solid material entering the branchial pouches, which could otherwise potentially clog the gills.

[34] Close to the jaws of juvenile lampreys, a muscular flap-like structure called the velum is present, which serves to generate a water current towards the mouth opening, which enables feeding and respiration.

[35][8] The unique morphological characteristics of lampreys, such as their cartilaginous skeleton, suggest they are the sister taxon (see cladistics) of all living jawed vertebrates (gnathostomes).

The mouth and suction capabilities of the lamprey not only allow it to cling to a fish as a parasite,[41] but provide it with limited climbing ability so that it can travel upstream and up ramps or rocks to breed.

[42][41] This ability has been studied in an attempt to better understand how lampreys battle the current and move forward despite only being able to hold onto the rock at a single point.

Some species can be distinguished by their unique markings – for example, Geotria australis individuals display two bluish stripes running the length of its body as an adult.

[44] These markings can also sometimes be used to determine what stage of the life cycle the lamprey is in; G. australis individuals lose these stripes when they approach the reproductive phase and begin to travel upstream.

Generated from a somatic recombination of leucine-rich repeat gene segments, lamprey leukocytes express surface variable lymphocyte receptors (VLRs).

[47] Pouched lamprey (Geotria australis) larvae also have a very high tolerance for free iron in their bodies, and have well-developed biochemical systems for detoxification of the large quantities of these metal ions.

While the majority of (invertebrate) suspension feeders thrive in waters containing under 1 mg suspended organic solids per litre (<1 mg/L), ammocoetes demand minimum 4 mg/L, with concentrations in their habitats having been measured up to 40 mg/L.

[63] Some species, including those that are not carnivorous and do not feed even following metamorphosis,[60] live in freshwater for their entire lifecycle, spawning and dying shortly after metamorphosing.

[64] In contrast, many species are anadromous and migrate to the sea,[60] beginning to prey on other animals while still swimming downstream after their metamorphosis provides them with eyes, teeth, and a sucking mouth.

[68] Research on sea lampreys has revealed that sexually mature males use a specialized heat-producing tissue in the form of a ridge of fat cells near the anterior dorsal fin to stimulate females.

[69] Taxonomists place lampreys and hagfish in the subphylum Vertebrata of the phylum Chordata, which also includes the invertebrate subphyla Tunicata (sea-squirts) and the fish-like Cephalochordata (lancelets or Amphioxus).

Recent molecular and morphological phylogenetic studies place lampreys and hagfish in the infraphylum Agnatha or Agnathostomata (both meaning without jaws).

The other vertebrate infraphylum is Gnathostomata (jawed mouths) and includes the classes Chondrichthyes (sharks), Osteichthyes (bony fishes), Amphibia, Reptilia, Aves, and Mammalia.

[78] These Paleozoic stem-lampreys are small relative to modern lampreys, and while they had well developed oral discs with a small number of radially arranged teeth, they lacked the specialised, heavily toothed discs with plate-like laminae present in modern lampreys, and it is possible that they fed by scraping algae off of animals, rather than feeding by predation/parasitism.

Organisms possessing a notochord, dorsal hollow nerve cord, pharyngeal slits, pituitary gland/endostyle, and a post anal tail during the process of their development are considered to be chordates.

Similarities between adult amphioxus and lamprey larvae include a pharynx with pharyngeal slits, a notochord, a dorsal hollow nerve cord and a series of somites that extend anterior to the otic vesicle.

[85] In a series of studies by Rovainen and his student James Buchanan, the cells that formed the neural circuits within the spinal cord capable of generating the rhythmic motor patterns that underlie swimming were examined.

Another trait is the ability to delete several genes from their somatic cell lineages, about 20% of their DNA, which are vital during development of the embryo, but which in humans can cause problems such as cancer later in life, after they have served their purpose.

During the Middle Ages they were widely eaten by the upper classes throughout Europe, especially during Lent, when eating meat was prohibited, due to their meaty taste and texture.

Sixty years later, the city of Gloucester had to use fish from North America for her Diamond Jubilee, because few lampreys could be found in the River Severn.

In Finland (county of Nakkila),[citation needed] and Latvia (Carnikava Municipality), the river lamprey is the local symbol, found on their coats of arms.

[118] Kurt Vonnegut, in his late short story "The Big Space Fuck", posits a future America so heavily polluted – "Everything had turned to shit and beer cans", in his words – that the Great Lakes have been infested with a species of massive, man-eating ambulatory lampreys.

[120] In season 9, episode 16, of "Bones" (American TV series from 2005-2017), Agent Booth grabs a lamprey that is escaping from a bag holding a dead body which has been found in a pond.