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=== Anatomy ===
Adults superficially resemble [[eel]]s in that they have [[scale (zoology)|scaleless]], elongated bodies, and can range from {{convert|13|to|100|cm|in|round=0.5|abbr=on}} in length. Lacking [[Fish fin#Evolution of paired fins|paired fins]], adult lampreys have large eyes, one nostril atop the head, and seven [[gill]] pores on each side of the head.
The unique morphological characteristics of lampreys, such as their [[cartilage|cartilaginous]] [[skeleton]], suggest they are the sister taxon (see [[cladistics]]) of all living jawed vertebrates ([[gnathostomes]]). They are usually considered the most basal group of the [[Vertebrate|Vertebrata]]. Instead of true vertebrae, they have a series of cartilaginous structures called arcualia arranged above the notochord. [[Hagfish]], which resemble lampreys, have traditionally been considered the sister taxon of the true vertebrates (lampreys and gnathostomes)<ref name="Mikko" /> but DNA evidence suggests that they are in fact the sister taxon of lampreys.<ref>{{Cite journal |last1=Heimberg |first1=A. M. |last2=Cowper-Sal-Lari |first2=R. |last3=Sémon |first3=M. |last4=Donoghue |first4=P. C. |last5=Peterson |first5=K. J. |year=2010 |title=MicroRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=107 |issue=45 |pages=19379–83 |doi=10.1073/pnas.1010350107 |pmc=2984222 |pmid=20959416
Near the gills are the eyes, which are poorly developed and buried under skin in the larvae. The eyes consummate their development during metamorphosis, and are covered by a thin and transparent layer of skin that becomes opaque in preservatives.<ref>{{Cite book |last1=Iuliis |first1=Gerardo De |url=https://books.google.com/books?id=Zwpjqn8IyRwC&pg=PA19 |title=The Dissection of Vertebrates: A Laboratory Manual
The brain of the lamprey is divided into the [[telencephalon]], [[diencephalon]], [[midbrain]], [[cerebellum]], and [[Medulla oblongata|medulla]].<ref name=":02">{{Cite journal |last1=XU |first1=Yang |last2=ZHU |first2=Si-Wei |last3=LI |first3=Qing-Wei |date=2016-09-18 |title=Lamprey: a model for vertebrate evolutionary research |journal=Zoological Research |volume=37 |issue=5 |pages=263–269 |doi=10.13918/j.issn.2095-8137.2016.5.263 |issn=2095-8137 |pmc=5071338 |pmid=27686784}}</ref>
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The pharynx is subdivided; the ventral part forming a respiratory tube that is isolated from the mouth by a valve called the velum. This is an adaptation to how the adults feed, by preventing the prey's body fluids from escaping through the gills or interfering with [[gas exchange]], which takes place by pumping water in and out of the gill pouches instead of taking it in through the mouth.
One of the key physical components to the lamprey are the [[intestines]], which are located ventral to the [[notochord]]. Intestines aid in [[osmoregulation]] by intaking water from its environment and desalinating the water they intake to an [[Isosmotic|iso-osmotic]] state with respect to blood, and are also responsible for [[digestion]].<ref>{{Cite journal |last1=Barany |first1=A. |last2=Shaughnessy |first2=C. A. |last3=Fuentes |first3=J. |last4=Mancera |first4=J. M. |last5=McCormick |first5=S. D. |date=2020-02-01 |title=Osmoregulatory role of the intestine in the sea lamprey (Petromyzon marinus) |journal=American Journal of Physiology. Regulatory, Integrative and Comparative Physiology |volume=318 |issue=2 |pages=R410–R417 |doi=10.1152/ajpregu.00033.2019 |issn=1522-1490 |pmid=31747320 |doi-access=free}}</ref>[[File:Lamprey skeleton lateral view.jpg|alt=Lamprey skeleton|thumb|The cartilaginous skeleton of a lamprey washed up on a beach in North Carolina.]]Studies have shown that lampreys are among the most energy-efficient swimmers. Their swimming movements generate low-pressure zones around the body, which pull rather than push their bodies through the water.<ref>{{cite news |url=https://www.latimes.com/science/sciencenow/la-sci-sn-jellyfish-lamprey-swimming-pressure-bioinspired-20151103-story.html |title=In swimming, jellyfish and lampreys really pull their weight |work=Los Angeles Times}}</ref>
Lampreys are the only extant vertebrate to have four eyes
▲Near the gills are the eyes, which are poorly developed and buried under skin in the larvae. The eyes consummate their development during metamorphosis, and are covered by a thin and transparent layer of skin that becomes opaque in preservatives.<ref>{{Cite book |url=https://books.google.com/books?id=Zwpjqn8IyRwC&pg=PA19 |title=The Dissection of Vertebrates: A Laboratory Manual |isbn=978-0-08-047735-0 |last1=Iuliis |first1=Gerardo De |last2=Deiuliis |first2=Gerald |last3=Pulera |first3=Dino |date=2006-08-03|publisher=Elsevier }}</ref>
▲The unique morphological characteristics of lampreys, such as their [[cartilage|cartilaginous]] [[skeleton]], suggest they are the sister taxon (see [[cladistics]]) of all living jawed vertebrates ([[gnathostomes]]). They are usually considered the most basal group of the [[Vertebrate|Vertebrata]]. Instead of true vertebrae, they have a series of cartilaginous structures called arcualia arranged above the notochord. [[Hagfish]], which resemble lampreys, have traditionally been considered the sister taxon of the true vertebrates (lampreys and gnathostomes)<ref name="Mikko" /> but DNA evidence suggests that they are in fact the sister taxon of lampreys.<ref>{{Cite journal |title=MicroRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=107 |issue=45 |pages=19379–83 |doi=10.1073/pnas.1010350107 |pmc=2984222 |pmid=20959416 |year=2010 |last1=Heimberg |first1=A. M. |last2=Cowper-Sal-Lari |first2=R. |last3=Sémon |first3=M. |last4=Donoghue |first4=P. C. |last5=Peterson |first5=K. J.|doi-access=free }}</ref>
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. After having attracted a female with pheromones, the heat detected by the female through body contact will encourage spawning.<ref>{{Cite news |url=https://www.nbcnews.com/science/science-news/only-hot-sea-lamprey-guys-get-sex-thermally-f6C10506147 |title=Only the hot sea lamprey guys get sex – thermally, that is |work=NBC News |date=2015-11-02 |last=Poppick |first=Laura}}</ref>▼
Due to certain peculiarities in their [[adaptive immune system]], the study of lampreys provides valuable insight into the evolution of vertebrate adaptive immunity. Generated from a somatic recombination of [[leucine-rich repeat]] gene segments, lamprey leukocytes express surface [[variable lymphocyte receptors]] (VLRs).<ref name="Nagawa et al.">{{cite journal |first1=Fumikiyo |last1=Nagawa |first2=Natsuko |last2=Kishishita |first3=Kazumichi |last3=Shimizu |first4=Satoshi |last4=Hirose |first5=Masato |last5=Miyoshi |first6=Junnya |last6=Nezu |first7=Toshinobu |last7=Nishimura |first8=Hirofumi |last8=Nishizumi |first9=Yoshimasa |last9=Takahashi |first10=Shu-ichi |last10=Hashimoto |first11=Masaki |last11=Takeuchi |first12=Atsushi |last12=Miyajima |display-authors=9 |title=Antigen-receptor genes of the agnathan lamprey are assembled by a process involving copy choice |journal=Nature Immunology |volume=8 |issue=2 |pages=206–13 |year=2007 |pmid=17187071 |doi=10.1038/ni1419 |s2cid=23222989}}</ref> This convergently evolved characteristic allows them to have lymphocytes that work as the [[T cell]]s and [[B cell]]s present in higher vertebrates immune system.<ref name="Pancer et al.">{{Cite journal |last1=Pancer |first1=Z. |last2=Amemiya |first2=C. T. |last3=Ehrhardt |first3=G. T. R. A. |last4=Ceitlin |first4=J. |last5=Gartland |first5=G. |last6=Cooper |first6=M. D. |doi=10.1038/nature02740 |title=Somatic diversification of variable lymphocyte receptors in the agnathan sea lamprey |journal=Nature |volume=430 |issue=6996 |pages=174–180 |year=2004 |pmid=15241406 |url=https://deepblue.lib.umich.edu/bitstream/2027.42/62870/1/nature02740.pdf |bibcode=2004Natur.430..174P |hdl=2027.42/62870 |s2cid=876413 |hdl-access=free}}</ref>▼
▲Lampreys are the only extant vertebrate to have four eyes.<ref name="The Economist 2018">{{Cite news |title=Three-eyed lizards are not uncommon. Four-eyed ones are a novelty |newspaper=The Economist |date=2018-04-05 |url=https://www.economist.com/news/science-and-technology/21739951-they-had-languished-museum-drawer-1870s-three-eyed-lizards-are-not |access-date=2018-04-10}}</ref> Most lampreys have two additional [[parietal eye]]s: a pineal and parapineal one (the exception is members of ''[[Mordacia]]'').<ref name="Nieuwenhuys 1998">{{cite book |last=Nieuwenhuys |first=R |title=The central nervous system of vertebrates |publisher=Springer |location=Berlin New York |year=1998 |isbn=978-3-540-56013-5 |page=454}}</ref>
Different species of lamprey have many shared physical characteristics. The same anatomical structure can serve different functions in the lamprey depending on whether or not it is [[carnivorous]]. The mouth and suction capabilities of the lamprey not only allow it to cling to a fish as a [[Parasitism|parasite]],<ref name=":0">{{cite web |date=2017-07-06 |title=A Leap in Lampreys: Unlovely Fish Make Welcome Comback |url=https://eelriver.org/2017/07/06/a-leap-in-lampreys-unlovely-fish-make-welcome-comback/ |access-date=2021-03-27 |website=Friends of the Eel River |language=en-US}}</ref> but provide it with limited climbing ability so that it can travel upstream and up ramps or rocks to breed.<ref name=":1">{{Cite journal |last=Reinhardt |first=Ulrich |date=November 2008 |title=Lamprey climbing behavior |url=https://www.researchgate.net/publication/233600184 |journal=Canadian Journal of Zoology |volume=86 |doi=10.1139/Z08-112 |via=ResearchGate}}</ref><ref name=":0" /> 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.<ref name=":1" /> Some scientists are also hoping to design ramps<ref name=":1" /> that will optimize the lamprey's climbing ability, as lampreys are valued as food in the Northwest United States and need to travel upstream to reproduce.<ref name=":0" />
The [[Most recent common ancestor|last common ancestor]] of lampreys appears to have been specialized to feed on the blood and body fluids of other fish after metamorphosis.<ref>{{cite journal |doi=10.1016/S0380-1330(03)70480-8 |volume=29 |title=Adaptive Radiation of Lampreys |year=2003 |journal=Journal of Great Lakes Research |pages=95–112 |last1=Potter |first1=Ian C. |last2=Gill |first2=Howard S.|bibcode=2003JGLR...29...95P }}</ref> They attach their mouthparts to the target animal's body, then use three horny plates (laminae) on the tip of their piston-like tongue, one transversely and two longitudinally placed, to scrape through surface tissues until they reach body fluids.<ref>{{cite journal |doi=10.1023/A:1023961910547 |volume=66 |issue=3 |year=2003 |journal=Environmental Biology of Fishes |pages=271–278 |last1=Khidir |first1=K. Teresa |title=Oral fimbriae and papillae in parasitic lampreys (Petromyzontiformes) |s2cid=10254661}}</ref> The teeth on their oral disc are primarily used to help the animal attach itself to its prey.<ref>{{Cite book |url=https://books.google.com/books?id=ktTOf3l66QQC&pg=PT228 |title=Marine Parasitology |isbn=978-0-643-09927-2 |last1=Rohde |first1=Klaus |date=2005-09-13|publisher=Csiro }}</ref> Made of keratin and other proteins, lamprey teeth have a hollow core to give room for replacement teeth growing under the old ones.<ref>{{Cite book |url=https://books.google.com/books?id=QNSlBQAAQBAJ&q=lampreys+hollow+core+replacement+teeth&pg=PA7 |title=Biological Materials of Marine Origin: Vertebrates |isbn=978-94-007-5730-1 |last1=Ehrlich |first1=Hermann |date=2014-12-01|publisher=Springer }}</ref> Some of the original blood-feeding forms have evolved into species that feed on both blood and flesh, and some who have become specialized to eat flesh and may even invade the internal organs of the host. Tissue feeders can also involve the teeth on the oral disc in the excision of tissue.<ref>{{Cite book |url=https://books.google.com/books?id=iUrcAwAAQBAJ&pg=PA121 |title=Freshwater Fishes of North America: Volume 1: Petromyzontidae to Catostomidae |isbn=978-1-4214-1201-6 |last1=Warren |first1=Melvin L. Jr. |last2=Burr |first2=Brooks M. |date=2014-07-10|publisher=JHU Press }}</ref> 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.<ref>{{Cite journal |url=https://www.researchgate.net/publication/227745051 |doi=10.1111/j.1469-7998.2009.00571.x |title=Relationships between the diets and characteristics of the dentition, buccal glands and velar tentacles of the adults of the parasitic species of lamprey |journal=Journal of Zoology |volume=278 |issue=3 |pages=231–242 |year=2009 |last1=Renaud |first1=C. B. |last2=Gill |first2=H. S. |last3=Potter |first3=I. C.}}</ref> A study of the stomach content of some lampreys has shown the remains of intestines, fins and vertebrae from their prey.<ref>{{cite news |url=https://www.adn.com/alaska-news/science/2016/07/07/what-we-know-about-lampreys-the-arctic-bloodsuckers-that-swarm-alaska-rivers-by-the-millions/ |title=What we know about lampreys – the arctic bloodsuckers that swarm Alaska rivers by the millions}}</ref> Although attacks on humans do occur,<ref>{{cite magazine |url=http://www.time.com/time/magazine/article/0,9171,861450,00.html |archive-url=https://web.archive.org/web/20081215055314/http://www.time.com/time/magazine/article/0,9171,861450,00.html |url-status=dead |archive-date=15 December 2008 |title=CANADA: A Surfeit of Lampreys |magazine=[[Time (magazine)|Time]] |date=9 May 1955 |access-date=7 June 2008}}</ref> they will generally not attack humans unless starved.<ref>{{cite book |last=Liem |first=Karel F. |author2=William E. Bemis |author3=Warren F. Walker Jr. |author4=Lance Grande |title=Functional Anatomy of the Vertebrates |publisher=Thomson: Brooks/Cole |year=2001 |location=The United States of America |page=50 |isbn=978-0-03-022369-3}}</ref><ref name="Mikko">{{cite web |last=Haaramo |first=Mikko |title=Mikko's Phylogeny Archive |date=11 March 2008 |url=http://www.helsinki.fi/~mhaaramo/metazoa/deuterostoma/chordata/vertebrata.html#Chondrichthyes |access-date=26 January 2009}}</ref>
[[File:スナヤツメ（南方種）.jpg|alt=Lamprey camouflage|thumb|The lamprey's light-colored underside and darker back allow it to blend in when viewed from above or below, an example of [[countershading]]]]
Lamprey coloration can also vary according to the region and specific environment in which the species is found. Some species can be distinguished by their unique markings – for example, [[Pouched lamprey|''Geotria australis'']] individuals display two bluish stripes running the length of its body as an adult.<ref name=":2">{{Cite journal |last1=Todd |first1=P. R. |last2=Wilson |first2=R. D. |date=1983-03-01 |title=Epidermal pigmentation and liver coloration in the southern hemisphere lamprey, Geotria austral is Gray |journal=New Zealand Journal of Marine and Freshwater Research |volume=17 |issue=1 |pages=21–26 |doi=10.1080/00288330.1983.9515983 |issn=0028-8330 |doi-access=free}}</ref> These markings can also sometimes be used to determine what stage of the life cycle the lamprey is in; [[Pouched lamprey|''G. australis'']] individuals lose these stripes when they approach the reproductive phase and begin to travel upstream.<ref name=":2" /> Another example is ''[[Petromyzon marinus]]'', which shifts to more of an orange color as it reaches the reproductive stage in its life cycle.
=== Genetics and immunology ===
▲Northern lampreys (Petromyzontidae) have the highest number of [[chromosome]]s (164–174) among vertebrates.<ref>{{FishBase family|family=Petromyzontidae|month=February|year=2017}}</ref> Due to certain peculiarities in their [[adaptive immune system]], the study of lampreys provides valuable insight into the evolution of vertebrate adaptive immunity. Generated from a somatic recombination of [[leucine-rich repeat]] gene segments, lamprey leukocytes express surface [[variable lymphocyte receptors]] (VLRs).<ref name="Nagawa et al.">{{cite journal |last1=Nagawa |first1=Fumikiyo |
=== Lifecycle ===
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Anadromous lampreys spend up to four years in the sea before migrating back to freshwater, where they spawn. Adults create nests (called [[Spawn (biology)#Brood hiders|redds]]) by moving rocks, and females release thousands of eggs, sometimes up to 100,000.<ref name="alaska.gov" /> The male, intertwined with the female, fertilizes the eggs simultaneously. Being [[Semelparity and iteroparity|semelparous]], both adults die after the eggs are fertilized.<ref>{{cite journal |last1=Beamish |first1=F W H |last2=Medland |first2=T E |title=Age Determination for Lampreys |journal=Transactions of the American Fisheries Society |date=1988 |volume=117 |issue=1 |pages=63–71 |doi=10.1577/1548-8659(1988)117<0063:ADFL>2.3.CO;2}}</ref>
▲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. After having attracted a female with pheromones, the heat detected by the female through body contact will encourage spawning.<ref>{{Cite news |
== Classification ==
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