Editing Hagfish

{{About||the punk rock band|Hagfish (band)|the album|Hagfish (album)}}
{{Distinguish|Hogfish}}
{{Short description|Class of eel-shaped, slime-producing animal}}
{{Automatic taxobox
| fossil_range = [[Pennsylvanian (geology)|Pennsylvanian ]] - [[Holocene|Holocene ]], {{fossil range|311|0}} <ref>{{Cite journal |last=Bardack |first=David |date=November 1, 1991 |title=First Fossil Hagfish (Myxinoidea): A Record from the Pennsylvanian of Illinois |url=http://dx.doi.org/10.1126/science.254.5032.701 |journal=Science |volume=254 |issue=5032 |pages=701–703 |doi=10.1126/science.254.5032.701 |pmid=17774799 |issn=0036-8075}}</ref>
| image = Collage of hagfish genera.png
| image_caption = Hagfish and some examples of their diversity.
| display_parents = 4
| taxon = Myxiniformes
| authority = [[Constantine Samuel Rafinesque|Rafinesque]], 1815
| type_species = ''[[Myxine glutinosa]]''
| type_species_authority = [[Linnaeus]], [[10th edition of Systema Naturae|1758]]
| subdivision_ranks = Genera
| subdivision_ref = <ref name="Nelson">{{cite book|last1=Nelson|first1=Joseph S.|last2=Grande|first2=Terry C.|last3=Wilson|first3=Mark V. H.|year=2016|title=''Fishes of the World''|edition=5th|publisher=[[John Wiley & Sons]]|isbn=9781118342336}}</ref>
| subdivision = *†''[[Myxinikela]]'' <small>Bardack, 1991</small>
* '''Myxinidae'''
** '''[[Rubicundinae]]''' <small>Fernholm ''et al.'', 2013</small>
*** ''[[Rubicundus]]'' <small>Fernholm ''et al.'', 2013</small>
***†''[[Tethymyxine]]'' <small>Miyashita ''et al.'', 2019</small>
** '''[[Eptatretinae]]''' <small>Bonaparte, 1850</small>
*** ''[[Eptatretus]]'' <small>Cloquet, 1819</small>
** '''[[Myxininae]]''' <small>Nelson, 1976</small>
*** ''[[Myxine]]'' <small>[[Carl Linnaeus|Linnaeus]], [[10th edition of Systema Naturae|1758]]</small>
*** ''[[Nemamyxine]]'' <small>Richardson, 1958</small>
*** ''[[Neomyxine]]'' <small>Richardson, 1953</small>
*** ''[[Notomyxine]]'' <small>Nani & Gneri, 1951</small>
| synonyms = * Bdellostomatidae <small>Gill, 1872</small>
* Homeidae <small>Garman, 1899</small>
* Paramyxinidae <small>Berg, 1940</small>
* Diporobranchia <small>Latreille, 1825</small><ref name="van der Laan et al.">{{cite journal|last1=van der Laan|first1=Richard|last2=Eschmeyer|first2=William N.|last3=Fricke |first3=Ronald|year=2014|title=''Family-group names of Recent fishes''|volume=3882|issue=2|journal=[[Zootaxa]]|pages=001–230|issn=1175-5326|doi=10.11646/zootaxa.3882.1.1|pmid=25543675|s2cid=31014657|doi-access=free}}</ref>
}}

'''Hagfish''', of the [[Class (biology)|class]] '''Myxini''' {{IPAc-en|m|I|k|'|s|ai|n|ai}} (also known as '''Hyperotreti''') and [[Order (biology)|order]] '''Myxiniformes''' {{IPAc-en|m|I|k|'|s|I|n|ᵻ|f|ɔr|m|iː|z}}, are [[eel]]-shaped [[Agnatha|jawless fish]] (occasionally called '''slime eels'''). Hagfish are the only known living [[Animal|animals]] that have a [[skull]] but no [[vertebral column]], although they do have rudimentary [[vertebra]]e.<ref name=":0">{{Cite book|title=Campbell Biology|last=Reece|first=Jane|publisher=Pearson|year=2014|isbn=978-0321775658|location=Boston|pages=717}}</ref> Hagfish are [[marine animal|marine]] [[predator]]s and [[scavenger]]s<ref name=":7">{{Cite book |last=Freeborn |first=Michelle |url=https://www.wikidata.org/wiki/Q58012425 |title=The fishes of New Zealand |date=2015-01-01 |publisher=Te Papa Press |isbn=978-0-9941041-6-8 |editor-last=Roberts |editor-first=Clive Douglas |volume=2 |pages=24 |editor-last2=Stewart |editor-first2=Andrew L. |editor-last3=Struthers |editor-first3=Carl D. |access-date=2024-05-29 |archive-date=2024-08-29 |archive-url=https://web.archive.org/web/20240829210750/https://www.wikidata.org/wiki/Q58012425 |url-status=live }}</ref> that can defend themselves against other larger predators by releasing copious amounts of [[slime coat|slime]] from [[mucous gland]]s in their [[skin]].<ref name=":5">{{Cite journal |last1=Zeng |first1=Yu |last2=Plachetzki |first2=David C |last3=Nieders |first3=Kristen |last4=Campbell |first4=Hannah |last5=Cartee |first5=Marissa |last6=Pankey |first6=M Sabrina |last7=Guillen |first7=Kennedy |last8=Fudge |first8=Douglas |date=2023-03-10 |title=Epidermal threads reveal the origin of hagfish slime |journal=eLife |language=en |volume=12 |doi=10.7554/eLife.81405 |issn=2050-084X |pmc=10005773 |pmid=36897815 |doi-access=free }}</ref>

Although their exact relationship to the only other [[extant taxon|living]] group of [[Agnatha|jawless fish]], the [[lamprey]]s, was long the subject of controversy, genetic evidence suggests that hagfish and lampreys are more closely related to each other than to [[jawed vertebrates]], thus forming the superclass [[Cyclostomi]].<ref name=":1"/> The oldest-known [[stem group]] hagfish are known from the [[Pennsylvanian (geology)|Late Carboniferous]], around [[Moscovian (Carboniferous)|310]] million years ago,<ref name=":3"/> with modern representatives first being recorded in the mid-[[Cretaceous]] around [[Cenomanian|100]] million years ago.<ref name=":1"/>

==Physical characteristics==
[[File:NIE 1905 Hagfish - Myxine glutinosa.jpg|thumb|left|Two views of the hagfish (''Myxini glutinosa'') with analytical overlays and dissection, published 1905]]

===Body features===
Hagfish are typically about {{cvt|50|cm|1}} in length. The largest-known species is ''[[Eptatretus goliath]]'', with a specimen recorded at {{cvt|127|cm|ftin}}, while ''[[Myxine kuoi]]'' and ''[[Myxine pequenoi]]'' seem to reach no more than {{cvt|18|cm}}. Some have been seen as small as {{cvt|4|cm}}.{{Citation needed|date=July 2022}}

Hagfish have elongated, eel-like bodies, and paddle-like tails. The skin is naked and covers the body like a loosely fitting sock. They are generally a dull pink color and look quite worm-like. They have [[cartilage|cartilaginous]] skulls (although the part surrounding the brain is composed primarily of a fibrous sheath) and tooth-like structures composed of [[keratin]]. Colors depend on the species, ranging from pink to blue-grey, and black or white spots may be present. Eyes are simple eyespots, not lensed eyes that can resolve images. Hagfish have no true fins and have six or eight [[barbel (anatomy)|barbel]]s around the mouth and a single nostril. Instead of vertically articulating jaws like [[Gnathostomata]] ([[vertebrate]]s with jaws), they have a pair of horizontally moving structures with tooth-like projections for pulling off food. The mouth of the hagfish has two pairs of horny, comb-shaped teeth on a cartilaginous plate that protracts and retracts. These teeth are used to grasp food and draw it toward the pharynx.<ref name=r1>[http://tolweb.org/Hyperotreti Hyperotreti] {{Webarchive|url=https://web.archive.org/web/20130206033343/http://tolweb.org/Hyperotreti |date=2013-02-06 }}. Tree of Life</ref>
[[Image:Eptatretus stoutii.jpg|thumb|[[Pacific hagfish]] at 150 m depth, [[California]], [[Cordell Bank National Marine Sanctuary]]]]

Its skin is attached to the body only along the center ridge of the back and at the slime glands, and is filled with close to a third of the body's blood volume, giving the impression of a blood-filled sack. It is assumed this is an adaptation to survive predator attacks.<ref>{{Cite web |last=Chodosh |first=Sara |date=14 December 2017 |title=The world's fastest shark is no match for a sack of flaccid hagfish skin |url=https://www.popsci.com/shark-hagfish-flaccid-skin-sack/ |access-date=29 August 2024 |website=[[Popular Science]] |language=en-US |archive-date=13 April 2024 |archive-url=https://web.archive.org/web/20240413072251/https://www.popsci.com/shark-hagfish-flaccid-skin-sack/ |url-status=live }}</ref> The Atlantic hagfish, representative of the subfamily Myxininae, and the Pacific hagfish, representative of the subfamily Eptatretinae, differ in that the latter has muscle fibers embedded in the skin. The resting position of the Pacific hagfish also tends to be coiled, while that of the Atlantic hagfish is stretched.<ref>{{Cite web |last=Pennisi |first=Elizabeth |date=6 January 2017 |title=How the slimy hagfish ties itself up in knots—and survives shark attacks |url=https://www.science.org/content/article/how-slimy-hagfish-ties-itself-knots-and-survives-shark-attacks |access-date=30 August 2024 |website=[[Science (journal)|Science]] |archive-date=15 June 2022 |archive-url=https://web.archive.org/web/20220615080958/https://www.science.org/content/article/how-slimy-hagfish-ties-itself-knots-and-survives-shark-attacks |url-status=live }}</ref><ref>{{Cite web |url=http://www.sicb.org/meetings/2017/schedule/abstractdetails.php?id=391 |title=Comparative Biomechanics of Hagfish Skins SICB - 2017 meeting - Abstract Details |access-date=2018-05-17 |archive-date=2018-05-21 |archive-url=https://web.archive.org/web/20180521021841/http://www.sicb.org/meetings/2017/schedule/abstractdetails.php?id=391 |url-status=dead }}</ref>

===Slime===
[[File:Hagfish Slime Predator Deterrence.jpg|thumb|An [[Myxine glutinosa|Atlantic hagfish]] (''Myxine glutinosa'') using its slime to get away from a [[kitefin shark]] (''Dalatias licha'') and an [[Atlantic wreckfish]] (''Polyprion americanus'')]]
[[File:Eptatretus stoutii 1.jpg|thumb|right|[[Pacific hagfish]] trying to hide under a rock]]
Hagfish can exude copious quantities of a milky and fibrous slime or [[mucus]], from specialized slime glands.<ref name=":5"/> When released in seawater, the slime expands to 10,000 times its original size in 0.4 seconds.<ref>{{Cite web |last=Chodosh |first=Sara |date=25 August 2021 |title=Here's how hagfish slime gets 10,000 times bigger in 0.4 seconds |url=https://www.popsci.com/hagfish-slime-expands/ |access-date=29 August 2024 |website=[[Popular Science]] |language=en-US}}</ref> This slime that hagfish excrete has very thin fibers that make it more durable and retentive than the slime excreted by other animals.<ref>{{cite journal |last1=Fudge |first1=Douglas |last2=Levy |first2=Nimrod |last3=Chiu |first3=Scott |last4=Gosline |first4=John |title=Composition, morphology and mechanics of hagfish slime |journal=Journal of Experimental Biology |date=2005 |volume=208 |issue=24 |pages=4613–4625 |doi=10.1242/jeb.01963|pmid=16326943 |s2cid=16606815 |doi-access= }}</ref> The fibers are made of proteins and also make the slime flexible. If they are caught by a predator, they can quickly release a large amount of slime to escape.<ref>{{cite journal |last1=Böni |first1=Lukas |last2=Fischer |first2=Peter |last3=Böcker |first3=Lukas |last4=Kuster |first4=Simon |last5=Rühs |first5=Patrick |title=Hagfish slime and mucin flow properties and their implications for defense |journal=Scientific Reports |date=2016 |volume=6 |page=30371 |doi=10.1038/srep30371|pmid=27460842 |pmc=4961968 |bibcode=2016NatSR...630371B }}</ref> If they remain captured, they can tie themselves in an [[overhand knot]], and work their way from the head to the tail of the animal, scraping off the slime and freeing themselves from their captor. [[Rheology|Rheological]] investigations showed that hagfish slime [[viscosity]] increases in elongational flow which favors gill clogging of [[Aquatic feeding mechanisms|suction feeding fish]], while its viscosity decreases in [[Shear stress|shear]] which facilitates scraping off the slime by the travelling-knot.<ref>{{cite journal |last1=Böni |first1=Lukas |last2=Fischer |first2=Peter |last3=Böcker |first3=Lukas |last4=Kuster |first4=Simon |last5=Rühs |first5=Patrick A. |title=Hagfish slime and mucin flow properties and their implications for defense |journal=Scientific Reports |date=September 2016 |volume=6 |issue=1 |pages=30371 |doi=10.1038/srep30371 |pmid=27460842 |pmc=4961968 |bibcode=2016NatSR...630371B |doi-access=free }}</ref>

Recently, the slime was reported to entrain water in its [[keratin]]-like [[intermediate filament]]s excreted by [[Gland thread cell|gland thread cells]], creating a slow-to-dissipate, viscoelastic substance, rather than a simple gel. It has been shown to impair the function of a predator fish's [[gill]]s. In this case, the hagfish's mucus would clog the predator's gills, disabling their ability to respire. The predator would release the hagfish to avoid suffocation. Because of the mucus, few marine predators target the hagfish. Other predators of hagfish are varieties of birds or mammals.<ref>{{cite journal |doi=10.1242/jeb.02067 |last1=Lim |first1=J |last2=Fudge |first2=DS |last3=Levy |first3=N |last4=Gosline |first4=JM |title=Hagfish slime ecomechanics: testing the gill-clogging hypothesis |date=January 31, 2006 |journal=Journal of Experimental Biology |volume=209|issue=Pt 4 |pages=702–710 |pmid=16449564 |doi-access=free }}</ref>

Free-swimming hagfish also slime when agitated, and later clear the mucus using the same travelling-knot behavior.<ref>{{Cite book |last=Martini |first=F. H. |year=1998 |chapter=The ecology of hagfishes |title=The Biology of Hagfishes |editor-first=J. M. |editor-last=Jørgensen |editor2-first=J. P. |editor2-last=Lomholt |editor3-first=R. E. |editor3-last=Weber |editor4-first=H. |editor4-last=Malte |pages=57–77 |place=London |publisher=Chapman and Hall |isbn=978-0-412-78530-6 |chapter-url=https://books.google.com/books?id=vu1uzL0p7xsC&pg=PA57 }}</ref><ref>{{Cite journal |author=Strahan, R. |year=1963 |title=The behavior of myxinoids |journal=Acta Zoologica |volume=44 |issue=1–2 |pages=73–102 |doi=10.1111/j.1463-6395.1963.tb00402.x}}</ref> The reported gill-clogging effect suggests that the travelling-knot behavior is useful or even necessary to restore the hagfish's own gill function after sliming.

Hagfish thread keratin ([[Pacific hagfish|EsTKα]] and EsTKγ; {{UniProt|Q90501}} and {{UniProt|Q90502}}), the protein that make up its slime filaments, is under investigation as an alternative to [[spider silk]] for use in applications such as body armor.<ref>{{Cite news|url=https://nypost.com/2017/10/25/slime-from-this-300-million-year-old-creature-could-create-bulletproof-body-armor/|title=Slime from this 300 million-year-old creature could create bulletproof body armor|date=2017-10-25|work=New York Post|access-date=2017-10-26|language=en-US|archive-date=2017-10-25|archive-url=https://web.archive.org/web/20171025192741/http://nypost.com/2017/10/25/slime-from-this-300-million-year-old-creature-could-create-bulletproof-body-armor/|url-status=live}}</ref> These [[alpha-keratin]] proteins in hagfish slime transform from an [[alpha helix|α-helical]] structure to a stiffer [[β sheet]] structure when stretched.<ref>{{cite journal |last1=Fu |first1=Jing |last2=Guerette |first2=Paul A. |last3=Miserez |first3=Ali |title=Self-Assembly of Recombinant Hagfish Thread Keratins Amenable to a Strain-Induced α-Helix to β-Sheet Transition |journal=Biomacromolecules |date=8 July 2015 |volume=16 |issue=8 |pages=2327–2339 |doi=10.1021/acs.biomac.5b00552|pmid=26102237 }}</ref> With combined draw-processing (stretching) and chemical crosslinking, [[recombinant DNA|recombinant]] slime keratin turns into a very strong fiber with an [[Young's modulus|elastic modulus]] reaching 20&nbsp;GPa.<ref>{{cite journal |last1=Fu |first1=Jing |last2=Guerette |first2=Paul A. |last3=Pavesi |first3=Andrea |last4=Horbelt |first4=Nils |last5=Lim |first5=Chwee Teck |last6=Harrington |first6=Matthew J. |last7=Miserez |first7=Ali |title=Artificial hagfish protein fibers with ultra-high and tunable stiffness |journal=Nanoscale |date=2017 |volume=9 |issue=35 |pages=12908–12915 |doi=10.1039/c7nr02527k |pmid=28832693}}
*{{cite magazine |author=Harriet Brewerton |date=15 September 2017 |title=Hagfish slime turned into ultra-stiff fibre |magazine=Chemistry World |url=https://www.chemistryworld.com/news/hagfish-slime-turned-into-ultra-stiff-fibre/3007995.article |access-date=1 June 2019 |archive-date=1 June 2019 |archive-url=https://web.archive.org/web/20190601194038/https://www.chemistryworld.com/news/hagfish-slime-turned-into-ultra-stiff-fibre/3007995.article |url-status=live }}</ref>

When in 2017 a road accident on [[U.S. Route 101 in Oregon|U.S. Highway 101]] resulted in {{convert|7500|lb}} of hagfish being spilled, they emitted sufficient slime to cover the road and a nearby car.<ref>{{cite web|url=https://edition.cnn.com/2017/07/14/us/slime-eels-highway-accident-oregon-trnd/index.html|title=Slime eels cause multiple car pileup on Oregon highway|website=CNN.com|last=LeBlanc|first=Paul|date=14 July 2017|access-date=21 April 2022|archive-date=20 June 2022|archive-url=https://web.archive.org/web/20220620103114/https://amp.cnn.com/cnn/2017/07/14/us/slime-eels-highway-accident-oregon-trnd/index.html|url-status=live}}</ref>

===Respiration===
A hagfish generally respires by taking in water through its [[pharynx]], past the velar chamber, and bringing the water through the internal [[gill]] pouches, which can vary in number from five to 16 pairs, depending on species.<ref>{{cite book|author1=Springer, Joseph|author2=Holley, Dennis|title=An Introduction to Zoology|url=https://books.google.com/books?id=BzgNZca_L5AC&pg=PA376|date=2012|publisher=Jones & Bartlett Publishers|isbn=978-1-4496-9544-6|pages=376–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210757/https://books.google.com/books?id=BzgNZca_L5AC&pg=PA376|url-status=live}}</ref> The gill pouches open individually, but in'' Myxine'', the openings have coalesced, with canals running backwards from each opening under the skin, uniting to form a common aperture on the [[ventral]] side known as the branchial opening. The [[esophagus]] is also connected to the left branchial opening, which is therefore larger than the right one, through a pharyngocutaneous duct (esophageocutaneous duct), which has no respiratory tissue. This pharyngocutaneous duct is used to clear large particles from the pharynx, a function also partly taking place through the nasopharyngeal canal. In other species, the coalescence of the gill openings is less complete, and in ''Bdellostoma'', each pouch opens separately to the outside, as in lampreys.<ref>{{cite book|last=Hughes |first=George Morgan |title=Comparative Physiology of Vertebrate Respiration|url=https://archive.org/details/ost-biology-comparativephysi00hugh|date=1963|publisher=Harvard University Press|isbn=978-0-674-15250-2|pages=[https://archive.org/details/ost-biology-comparativephysi00hugh/page/n26 9]–}}</ref><ref>{{cite book|author=Wake, Marvalee H.|title=Hyman's Comparative Vertebrate Anatomy|url=https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA81|date=1992|publisher=University of Chicago Press|isbn=978-0-226-87013-7|pages=81–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210815/https://books.google.com/books?id=VKlWjdOkiMwC&pg=PA81#v=onepage&q&f=false|url-status=live}}</ref> The unidirectional water flow passing the gills is produced by rolling and unrolling velar folds located inside a chamber developed from the nasohypophyseal tract, and is operated by a complex set of muscles inserting into cartilages of the neurocranium, assisted by peristaltic contractions of the gill pouches and their ducts.<ref>{{cite book|author1=Bone, Quentin|author2=Moore, Richard|title=Biology of Fishes|url=https://books.google.com/books?id=e2N4AgAAQBAJ&pg=PA128|date=2008|publisher=Taylor & Francis|isbn=978-1-134-18631-0|pages=128–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210751/https://books.google.com/books?id=e2N4AgAAQBAJ&pg=PA128#v=onepage&q&f=false|url-status=live}}</ref> Hagfish also have a well-developed dermal capillary network that supplies the skin with oxygen when the animal is buried in anoxic mud, as well as a high tolerance for both hypoxia and anoxia, with a well-developed anaerobic metabolism.<ref name="Jørgensen">{{cite book|author=Jørgensen, Jørgen Mørup |title=The Biology of Hagfishes|url=https://books.google.com/books?id=vu1uzL0p7xsC&pg=PA231|year=1998|publisher=Springer Science & Business Media|isbn=978-0-412-78530-6|pages=231–}}</ref> Members of the group have spent 36 hours in water completely devoid of dissolved oxygen, and made a complete recovery.<ref>[https://www.sciencedirect.com/science/article/abs/pii/S1095643321001501 Hypoxia modifies calcium handling in the Pacific hagfish, Eptatretus stoutii]</ref> The skin has also been suggested to be capable of [[cutaneous respiration]].<ref>{{cite book|author1=Helfman, Gene|author2=Collette, Bruce B.|author3=Facey, Douglas E.|author4=Bowen, Brian W.|title=The Diversity of Fishes: Biology, Evolution, and Ecology|url=https://books.google.com/books?id=FyehAR6hsUUC&pg=PA235|date=2009|publisher=John Wiley & Sons|isbn=978-1-4443-1190-7|pages=235–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210753/https://books.google.com/books?id=FyehAR6hsUUC&pg=PA235#v=onepage&q&f=false|url-status=live}}</ref>

===Nervous system===

[[File:Dorsal, left lateral views of dissected hagfish brain.jpg|thumb|left|Dorsal / left lateral views of dissected hagfish brain, scale bar added for size]]

The origins of the vertebrate nervous system are of considerable interest to evolutionary biologists, and cyclostomes (hagfish and lampreys) are an important group for answering this question. The complexity of the hagfish brain has been an issue of debate since the late 19th century, with some morphologists suggesting that they do not possess a [[cerebellum]], while others suggest that it is continuous with the [[midbrain]].<ref>{{Citation| last1 = Larsell | first1 = O | title = The cerebellum of myxinoids and petromyzonts including developmental stages in the lampreys. | journal = Journal of Experimental Biology | volume = 210 | issue = 22 | year = 1947 | pages = 3897–3909 | doi = 10.1002/cne.900860303 | pmid = 20239748 | s2cid = 36764239 }}</ref> It is now considered that the hagfish neuroanatomy is similar to that of lampreys.<ref>{{Citation| last1 = Wicht | first1 = H | title = The brains of lampreys and hagfishes: Characteristics, characters, and comparisons. | journal = Brain, Behavior and Evolution | volume = 48 | issue = 5 | year = 1996 | pages = 248–261 | doi = 10.1159/000113204 | pmid = 8932866 }}</ref> A common feature of both cyclostomes is the absence of [[myelin]] in neurons.<ref>{{Cite journal| last1 = Bullock | first1 = T.H. | last2 = Moore | first2 = J.K. | last3 = Fields | first3 = R.D. | title = Evolution of myelin sheaths: both lamprey and hagfish lack myelin. | journal = Neuroscience Letters | volume = 48 | issue = 2 | year = 1984 | pages = 145–148 | doi = 10.1016/0304-3940(84)90010-7| pmid = 6483278 | s2cid = 46488707 }}</ref> The brain of a hagfish has specific parts similar to the brains of other vertebrates.<ref name=":4">{{cite journal |last1=Ota |first1=Kinya |last2=Kuratani |first2=Shigeru |title=Developmental Biology of Hagfishes, with a Report on Newly Obtained Embryos of the Japanese Inshore Hagfish, Eptatretus burgeri |journal=Zoological Science |date=2008 |volume=25 |issue=10 |pages=999–1011 |doi=10.2108/zsj.25.999 |pmid=19267636 |s2cid=25855686 }}</ref> The dorsal and ventral muscles located towards the side of the hagfish body are connected to [[spinal nerves]]. The spinal nerves that connect to the muscles of the pharyngeal wall grow individually to reach them.<ref>{{cite journal |last1=Oisi |first1=Yasuhiro |last2=Fujimoto |first2=Satoko |last3=Ota |first3=Kinya |last4=Kuratani |first4=Shigeru |title=On the peculiar morphology and development of the hypoglossal, glossopharyngeal and vagus nerves and hypobranchial muscles in the hagfish |journal=Zoological Letters |date=2015 |volume=1 |issue=6 |page=6 |doi=10.1186/s40851-014-0005-9|pmid=26605051 |pmc=4604111 |doi-access=free }}</ref>

===Eye===
The hagfish eye lacks a lens, [[extraocular muscles]], and the three motor cranial nerves (III, IV, and VI) found in more complex vertebrates, which is significant to the study of the [[Evolution of the eye|evolution of more complex eyes]]. A [[parietal eye]] is also absent in extant hagfish.<ref>{{cite book|author=Ostrander, Gary Kent|title=The Laboratory Fish|url=https://books.google.com/books?id=Hp4YSFiSD0IC&pg=PT129|year=2000|publisher=Elsevier|isbn=978-0-12-529650-2|pages=129–|access-date=2016-03-13|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829210925/https://books.google.ps/books?id=Hp4YSFiSD0IC&pg=PT129&redir_esc=y#v=onepage&q&f=false|url-status=live}}</ref><ref>{{cite web|url=http://www.physorg.com/news115919015.html|title=Keeping an eye on evolution|date=2007-12-03|access-date=2007-12-04|publisher=PhysOrg.com|archive-date=2012-03-15|archive-url=https://web.archive.org/web/20120315125907/http://www.physorg.com/news115919015.html|url-status=live}}</ref> Hagfish eyespots, when present, can detect light, but as far as it is known, none can resolve detailed images. In ''Myxine'' and ''Neomyxine'', the eyes are partly covered by the trunk musculature.<ref name=r1/> [[Paleontological]] evidence suggests, however, that the hagfish eye is not [[Cladistics#Terminology_for_character_states|plesiomorphic]] but rather degenerative, as fossils from the [[Carboniferous]] have revealed hagfish-like vertebrates with complex eyes. This would suggest that ancestrally Myxini possessed complex eyes.<ref>{{Citation| last1 = Gabbott | first1 = S.E | last2 = Donoghu | first2 = P.C | last3 = Sansom | first3 = R.S | last4 = Vinther | first4 = J | display-authors = 2 | title = Pigmented anatomy in Carboniferous cyclostomes and the evolution of the vertebrate eye. | journal = Proc. R. Soc. B | volume = 283 | issue = 1836 | pages = 20161151 | year = 2016 | doi = 10.1098/rspb.2016.1151 | pmid = 27488650 | pmc = 5013770 }}</ref><ref>{{Citation| last1 = Bardack | first1 = D | title = First fossil hagfish (Myxinoidea): a record from the Pennsylvanian of Illinois | journal = Science | volume = 254 | issue = 5032 | pages = 701–3 | year = 1991 | doi = 10.1126/science.254.5032.701 | pmid = 17774799 | bibcode = 1991Sci...254..701B | s2cid = 43062184 }}</ref>

===Cardiac function, circulation, and fluid balance===
Hagfish are known to have one of the lowest blood pressures among the vertebrates.<ref name="Forster 1985–1992">{{Cite journal|last1=Forster|first1=Malcolm E.|last2=Axelsson|first2=Michael|last3=Farrell|first3=Anthony P.|last4=Nilsson|first4=Stefan|date=1991-07-01|title=Cardiac function and circulation in hagfishes|journal=Canadian Journal of Zoology|volume=69|issue=7|pages=1985–1992|doi=10.1139/z91-277|issn=0008-4301}}</ref> One of the most primitive types of fluid balance found in animals is among these creatures; whenever a rise in extracellular fluid occurs, the blood pressure rises and this, in turn, is sensed by the kidney, which excretes excess fluid.<ref name ="Jørgensen"/> They also have the highest blood volume to body mass of any chordate, with 17 ml of blood per 100 g of mass.<ref>{{Cite web |url=http://cronodon.com/BioTech/hagfish.html |title=Hagfish - Cronodon |access-date=2018-05-04 |archive-date=2018-05-04 |archive-url=https://web.archive.org/web/20180504091106/http://cronodon.com/BioTech/hagfish.html |url-status=live }}</ref>

The hagfish circulatory system has been of considerable interest to evolutionary biologists and present day readers of physiology. Some observers first believed that the hagfish heart was not innervated (as the hearts of jawed vertebrates are),<ref>{{Citation| last1 = Jensen | first1 = D | title = The aneural heart of the hagfish. | journal = Annals of the New York Academy of Sciences | volume = 127 | issue = 1 | pages = 443–58 | year = 1965 | bibcode = 1965NYASA.127..443J | doi = 10.1111/j.1749-6632.1965.tb49418.x | pmid = 5217274 | s2cid = 5646370 }}</ref> but further investigation revealed that the hagfish does have a true innervated heart. The hagfish circulatory system also includes multiple accessory pumps throughout the body, which are considered auxiliary "hearts".<ref name="Forster 1985–1992"/>

Hagfish are the only known vertebrates with osmoregulation isosmotic to their external environment. Their renal function remains poorly described. There is a hypothesis that they excrete ions in bile salts.<ref>{{Citation| last1 = Robertson | first1 = J.D | title = Chemical composition of the body fluids and muscle of the hagfish Myxine glutinosa and the rabbit-fish Chimaera monstros. | journal = Journal of Zoology | volume = 178 | issue = 2 | year = 1976 | pages = 261–277 | doi = 10.1111/j.1469-7998.1976.tb06012.x }}</ref>

===Musculoskeletal system===
Hagfish musculature differs from jawed vertebrates in that they have neither a horizontal septum nor a vertical septum, which in jawed vertebrates are junctions of connective tissue that separate the [[Epaxial and hypaxial muscles|hypaxial musculature and epaxial musculature]]. They do, however, have true [[myomere]]s and myosepta like all vertebrates. The mechanics of their craniofacial muscles in feeding have been investigated, revealing advantages and disadvantages of their dental plate. In particular, hagfish muscles have increased force and gape size compared to similar-sized jawed vertebrates, but lack the speed amplification given by jawed vertebrates' muscles, suggesting that jaws are faster acting than hagfish dental plates.<ref>{{Cite journal| last1 = Clark | first1 = A.J. | last2 = Summers | first2 = A.P. | title = Morphology and kinematics of feeding in hagfish: possible functional advantages of jaws. | journal = Journal of Experimental Biology | volume = 210 | issue = 22 | year = 2007 | pages = 3897–3909 | doi = 10.1242/jeb.006940 | pmid = 17981857 | doi-access =  }}</ref>

[[File:Hagfish trunk.jpg|thumb|left|Vertical section of hagfish midline trunk: The notochord is the only skeletal element, and the musculature has no septum, neither horizontal nor vertical.]]

[[File:Hagfish skull Fig 74 in Kingsley 1912.png|thumb|Hagfish skull Fig 74 in Kingsley 1912]]
The hagfish skeleton comprises the skull, the [[notochord]], and the caudal fin rays. The first diagram of the hagfish endoskeleton was made by Frederick Cole in 1905.<ref>{{Citation| last= Cole | first= F.J. | title = A Monograph on the general Morphology of the Myxinoid Fishes, based on a study of Myxine. Part I. The Anatomy of the Skeleton. | journal = Earth and Environmental Science Transactions of the Royal Society of Edinburgh | volume = 41 | issue = 3 | year = 1906 }}</ref> In Cole's monograph, he described sections of the skeleton that he termed "pseudo-cartilage", referring to its distinct properties compared to jawed chordates. The lingual apparatus of hagfish is composed of a cartilage base bearing two teeth-covered plates (dental plates) articulated with a series of large cartilage shafts<!-- citation -->. The nasal capsule is considerably expanded in hagfish, comprising a fibrous sheath lined with cartilage rings. In contrast to lampreys, the braincase is noncartilaginous. The role of their branchial arches is still highly speculative, as hagfish embryos undergo a caudal shift of the posterior pharyngeal pouches; thus, the branchial arches do not support gills.<ref>{{Cite journal| last1 = Oisi | first1 = Y. | last2 = Fujimoto | first2 = S. | last3 = Ota | first3 = K.G. | last4 = Kuratani | first4 = S. | title = On the peculiar morphology and development of the hypoglossal, glossopharyngeal and vagus nerves and hypobranchial muscles in the hagfish. | journal = Zoological Letters | volume = 1 | issue = 1 | pages = 6 | year = 2015 | doi = 10.1186/s40851-014-0005-9 |doi-access=free | pmid = 26605051 | pmc = 4604111 }}</ref> While parts of the hagfish skull are thought to be homologous with lampreys, they are thought to have very few elements homologous with jawed vertebrates.<ref>{{Cite journal| last1 = Oisi | first1 = Y. | last3 = Fujimoto | first3 = S. | last2 = Ota | first2 = K.G. | last4 = Kuratani | first4 = S. | title = Development of the chondrocranium in hagfishes, with special reference to the early evolution of vertebrates | journal = Zoological Science | volume = 30 | issue = 11 | year = 2013 | pages = 944–961 | doi = 10.2108/zsj.30.944| pmid = 24199860 | s2cid = 6704672 | doi-access = free }}</ref>

==Reproduction==
[[File:Black_hagfish,_Eptatretus_deani_gonads_01.jpg|thumb|Egg development in a female black hagfish, ''Eptatretus deani'']]
[[Image:Eptatretus polytrema.jpg|thumb|right|Drawing of ''[[Eptatretus polytrema]]'']]
Very little is known about hagfish reproduction. Obtaining embryos and observing reproductive behavior are difficult due to the deep-sea habitat of many hagfish species.<ref name=":2">{{Cite journal|last1=Ota|first1=Kinya G.|last2=Kuratani|first2=Shigeru|title=The History of Scientific Endeavors Towards Understanding Hagfish Embryology|journal=Zoological Science|year=2006|volume=23|issue=5|pages=403–418|doi=10.2108/zsj.23.403|pmid=16766859|s2cid=20666604|issn=0289-0003|doi-access=free}}</ref> In the wild, females outnumber males, with the exact sex-ratio differing depending on the species. ''E. burgeri'', for example, has nearly a 1:1 ratio, while ''M. glutinosa'' females are significantly more common than males.<ref name=":2"/> Some species of hagfish are sexually undifferentiated before maturation, and possess gonadal tissue for both ovaries and testis.<ref>{{Cite journal|last1=Martini|first1=Frederic H.|last2=Beulig|first2=Alfred|date=2013-11-08|title=Morphometics and Gonadal Development of the Hagfish Eptatretus cirrhatus in New Zealand|journal=PLOS ONE|language=en|volume=8|issue=11|pages=e78740|doi=10.1371/journal.pone.0078740|issn=1932-6203|pmc=3826707|pmid=24250811|bibcode=2013PLoSO...878740M|doi-access=free}}</ref> It has been suggested that females develop earlier than males, and that this may be the reason for unequal sex ratios. Hagfish testis are relatively small.<ref name=":2"/>

Depending on species, females lay from one to 30 tough, yolky eggs. These tend to aggregate due to having [[Hook and loop fastener|Velcro]]-like tufts at either end.<ref name=":2"/> It is unclear how hagfish go about laying eggs, although researchers have proposed three hypotheses based on observations of the low percentage of males and small testis. The hypotheses are that female hagfish lay eggs in small crevices in rock formations, the eggs are laid in burrow beneath the sand, and the slime produced by the hagfish is used to hold the eggs in a small area.<ref name=":2"/> It is worth noting that no direct evidence has been found to support any of these hypotheses. Hagfish do not have a [[larva]]l stage, in contrast to [[lamprey]]s.<ref name=":2"/>

Hagfish have a [[mesonephros|mesonephric kidney]] and are often [[Neoteny|neotenic]] of their [[pronephros|pronephric kidney]]. The kidney(s) are drained via mesonephric/[[archinephric duct]]. Unlike many other vertebrates, this duct is separate from the reproductive tract, and the proximal tubule of the [[nephron]] is also connected with the [[coelom]], providing lubrication.<ref>{{Cite book|last=Kardong|first=Kenneth V. |title=Vertebrates: comparative anatomy, function, evolution|date=2019|isbn=978-1-259-70091-0|edition=Eighth |location=New York |oclc=1053847969}}</ref> The single testicle or ovary has no transportation duct. Instead, the gametes are released into the coelom until they find their way to the posterior end of the [[Caudal (anatomical term)|caudal]] region, whereby they find an opening in the digestive system.

The hagfish embryo can develop for as long as 11 months before hatching, which is shorter in comparison to other jawless vertebrates.<ref>{{cite journal| year=1997|last1=Gorbman|first1=A|title=Hagfish development|journal = Zoological Science | volume = 14|issue=3| pages=375–390 |doi=10.2108/zsj.14.375|s2cid=198158310|doi-access=free}}</ref> Not much was known about hagfish embryology until recently, when husbandry advances enabled considerable insight into the group's evolutionary development. New insights into the evolution of [[neural crest cells]], support the consensus that all vertebrates share these cells, which might be regulated by a common subset of genes.<ref>{{cite journal| year=2007 |last1=Ota|first1=K.G| last2=Kuraku | first2 = S. | last3 = Kuratani | first3 = S. | title = Hagfish embryology with reference to the evolution of the neural crest|journal =Nature | volume = 446|issue=7136|pages=672–5|doi=10.1038/nature05633|pmid=17377535|bibcode=2007Natur.446..672O|s2cid=4414164}}</ref> Their genome has a large number of microchromosomes which are lost during the animal's development, leaving only the reproductive organs with a complete genome.<ref>{{Cite web |url=https://www.bristol.ac.uk/news/2024/january/hagfish.html |title=First genome of slime eels uncovers the deep evolutionary history of our genomes and bodies |access-date=2024-01-18 |archive-date=2024-08-29 |archive-url=https://web.archive.org/web/20240829211316/https://www.bristol.ac.uk/news/2024/january/hagfish.html |url-status=live }}</ref> Hagfish possess [[gonadotropin]]s which secrete from pituitary glands to the gonads to stimulate development.<ref>{{cite journal|last=Nozaki|first=Masumi|date=2013|title=Hypothalamic-Pituitary-Gonadal Endocrine System in the Hagfish|journal=Frontiers in Endocrinology|language=English|volume=4|page=200|doi=10.3389/fendo.2013.00200|pmid=24416029|pmc=3874551|issn=1664-2392|doi-access=free}}</ref> This suggests that hagfish have an early version of the [[hypothalamic–pituitary–gonadal axis]], a system which once thought to be exclusive to the [[Gnathostomata|Gnathostomes]].[[Image:Eptatretus cirrhatus (New Zealand hagfish).gif|thumb|right|Drawing of a [[New Zealand hagfish]]]]Some species of hagfish reproduce seasonally, stimulated by hormones from their pituitary gland. ''E. burgeri'' is known to reproduce and migrate annually.<ref name=":6">{{Cite journal|last1=Powell|first1=Mickie L.|last2=Kavanaugh|first2=Scott I.|last3=Sower|first3=Stacia A.|date=2005-01-01|title=Current Knowledge of Hagfish Reproduction: Implications for Fisheries Management|url=https://academic.oup.com/icb/article/45/1/158/604629|journal=Integrative and Comparative Biology|language=en|volume=45|issue=1|pages=158–165|doi=10.1093/icb/45.1.158|pmid=21676757|issn=1540-7063|doi-access=free|citeseerx=10.1.1.491.7210|access-date=2021-05-11|archive-date=2021-05-12|archive-url=https://web.archive.org/web/20210512090157/https://academic.oup.com/icb/article/45/1/158/604629|url-status=live}}</ref>

==Feeding==
[[Image:Pacific hagfish feeding 01.jpg|thumb|left|Two Pacific hagfish feeding on a dead sharpchin rockfish, ''[[Sebastes zacentrus]]'', while one remains in a curled position at the left of the photo]]
While [[polychaete]] [[marine worm]]s on or near the sea floor are a major food source, hagfish can feed upon and often even enter and eviscerate the bodies of dead and dying/injured sea creatures much larger than themselves. They are known to devour their prey from the inside.<ref>Wilson, Hugh (November 2009) [https://web.archive.org/web/20091116002431/http://green.ca.msn.com/green-living/gallery.aspx?cp-documentid=22580760&page=6 Hagfish – World's weirdest animals]. green.ca.msn.com</ref> Hagfish have the ability to absorb dissolved organic matter across the skin and gill, which may be an adaptation to a scavenging lifestyle, allowing them to maximize sporadic opportunities for feeding. From an evolutionary perspective, hagfish represent a transitory state between the generalized nutrient absorption pathways of aquatic invertebrates and the more specialized digestive systems of aquatic vertebrates.<ref>{{cite journal|last1=Glover|first1=CN|last2=Bucking |first2=C |last3=Wood |first3=CM |title=Adaptations to in situ feeding: novel nutrient acquisition pathways in an ancient vertebrate|journal=Proceedings of the Royal Society B: Biological Sciences |date=2011-03-02|pmid=21367787|doi=10.1098/rspb.2010.2784|volume=278|issue=1721|pages=3096–101|pmc=3158932}}</ref>

Like [[leech]]es, they have a sluggish metabolism and can survive months between feedings;<ref>{{cite web|title=Introduction to the Myxini|url=http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html|access-date=2009-01-25|work=Berkeley.edu website|archive-url=https://web.archive.org/web/20171215173214/http://www.ucmp.berkeley.edu/vertebrates/basalfish/myxini.html|archive-date=2017-12-15|url-status=dead}}</ref><ref>{{cite journal|doi=10.1016/S0022-0981(96)02665-2|last1=Lesser|first1=M|title=Ecology of the hagfish, Myxine glutinosa L. in the Gulf of Maine I. Metabolic rates and energetics |journal=Journal of Experimental Marine Biology and Ecology | volume=208 | date=3 January 1997|issue=1–2 | pages=215–225|last2=Martini|first2=Frederic H.|last3=Heiser|first3=John B. |doi-access=|bibcode=1997JEMBE.208..215L }}</ref> their feeding behavior, however, appears quite vigorous. Analysis of the stomach content of several species has revealed a large variety of prey, including [[polychaete]]s, shrimp, [[hermit crab]]s, [[cephalopod]]s, [[brittle star]]s, [[bony fish]]es, sharks, birds, and whale flesh.<ref>{{Cite journal | doi = 10.3354/meps10341|url=https://www.int-res.com/articles/meps_oa/m485p223.pdf | title = Hagfish feeding habits along a depth gradient inferred from stable isotopes| journal = Marine Ecology Progress Series| volume = 485| pages = 223–234| year = 2013| last1 = Zintzen | first1 = V. | last2 = Rogers | first2 = K. M. | last3 = Roberts | first3 = C. D. | last4 = Stewart | first4 = A. L. | last5 = Anderson | first5 = M. J. | bibcode = 2013MEPS..485..223Z| doi-access = free }}</ref>

In captivity, hagfish are observed to use the overhand-knot behavior in reverse (tail-to-head) to assist them in gaining mechanical advantage to pull out chunks of flesh from carrion fish or cetaceans, eventually making an opening to permit entry to the interior of the body cavity of larger carcasses. A healthy larger sea creature likely would be able to outfight or outswim this sort of assault.

This energetic opportunism on the part of the hagfish can be a great nuisance to fishermen, as they can devour or spoil entire deep drag-netted catches before they can be pulled to the surface. Since hagfish are typically found in large clusters on and near the bottom, a single trawler's catch could contain several dozen or even hundreds of hagfish as bycatch, and all the other struggling, captive sea life make easy prey for them.

The digestive tract of the hagfish is unique among chordates because the food in the gut is enclosed in a permeable membrane, analogous to the [[peritrophic matrix]] of insects.<ref>[[Ross Piper|Piper, Ross]] (2007), ''Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals'', [[Greenwood Press (publisher)|Greenwood Press]].</ref> They are also able to absorb nutrients directly through their skin.<ref>{{Cite web |last=Reardon |first=Sara |date=2 March 2011 |title=Hagfish Just Got More Disgusting |url=https://www.science.org/content/article/hagfish-just-got-more-disgusting |access-date=30 August 2024 |website=[[Science (journal)|Science]] |archive-date=29 August 2024 |archive-url=https://web.archive.org/web/20240829211417/https://www.science.org/content/article/hagfish-just-got-more-disgusting |url-status=live }}</ref>

Hagfish have also been observed actively [[predation|hunting]] the red bandfish, ''[[Cepola haastii]]'', in its burrow, possibly using their slime to suffocate the fish before grasping it with their dental plates and dragging it from the burrow.<ref>{{Cite journal | last1 = Zintzen | first1 = V. | last2 = Roberts | first2 = C. D. | last3 = Anderson | first3 = M. J. | last4 = Stewart | first4 = A. L. | last5 = Struthers | first5 = C. D. | last6 = Harvey | first6 = E. S. | title = Hagfish predatory behaviour and slime defence mechanism | doi = 10.1038/srep00131 | journal = Scientific Reports | volume = 1 | pages = 131 | year = 2011 | pmid =  22355648| pmc = 3216612| bibcode = 2011NatSR...1E.131Z }}</ref>

==Classification==
[[File:Pacific_hagfish_Myxine.jpg|thumb|[[Pacific hagfish]] resting on the ocean bottom, at 280&nbsp;m depth off the [[Oregon]] coast]]
Originally, ''Myxine'' was included by [[Linnaeus]] ([[10th edition of Systema Naturae|1758]]) in [[Vermes]]. The fossil hagfish ''[[Myxinikela|Myxinikela siroka]]'', from the Late Carboniferous of the United States, is the oldest-known member of the group. It is in some respects more similar to lampreys, but shows key [[autapomorphies]] of hagfish.<ref name=":3">{{Cite journal|last=Miyashita|first=Tetsuto|date=23 November 2020|title=A Paleozoic stem hagfish Myxinikela siroka — revised anatomy and implications for evolution of the living jawless vertebrate lineages|url=https://www.researchgate.net/publication/346740531|journal=Canadian Journal of Zoology|language=en|volume=98|issue=12|pages=850–865|doi=10.1139/cjz-2020-0046|s2cid=229489559|issn=0008-4301}}</ref> In recent years, hagfish have become of special interest for genetic analysis investigating the relationships among [[chordate]]s. Their classification as [[agnatha]]ns places hagfish as elementary [[vertebrate]]s in between [[invertebrate]]s and [[gnathostome]]s. However, discussion has long occurred in scientific literature about whether the hagfish were even [[invertebrate]]. Using fossil data, paleontologists posited that lampreys are more closely related to gnathostomes than hagfish. The term "Craniata" was used to refer to animals that had a developed skull, but were not considered true vertebrates.<ref>{{cite journal| year=1993 |last1=Forey|first1=P.| last2=Janvier | first2 = P. | title = Agnathans and the origin of jawed vertebrates|journal =Nature | volume = 361|issue=6408 |pages=129–134| doi = 10.1038/361129a0|bibcode=1993Natur.361..129F|s2cid=43389789}}</ref> Molecular evidence in the early 1990s first began suggesting that lampreys and hagfish were more closely related to each other than to gnathostomes.<ref>{{cite journal| year=1992 |last1=Stock|first1=D.W.| last2=Whitt| first2 = G.S. | title = Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group. |journal =Science | volume = 257|issue=5071 |pages=787–9| doi = 10.1126/science.1496398 |pmid=1496398|bibcode=1992Sci...257..787S}}</ref> The validity of the taxon "Craniata" was further examined by Delarbre et al. (2002) using [[mtDNA]] [[DNA sequence|sequence]] data, concluding the Myxini are more closely related to the [[Hyperoartia]] than to the Gnathostomata—i.e., that modern jawless fishes form a clade called the [[Cyclostomata]]. The argument is that if the Cyclostomata are indeed monophyletic, Vertebrata would return to its old content ([[Gnathostomata]] + Cyclostomata) and the name Craniata, being superfluous, would become a junior synonym.<ref name="r3">{{cite journal |author=Janvier, P. |author-link=Philippe Janvier |year=2010 |title=MicroRNAs revive old views about jawless vertebrate divergence and evolution |journal=Proceedings of the National Academy of Sciences |volume=107 |issue=45 |pages=19137–19138 |bibcode=2010PNAS..10719137J |doi=10.1073/pnas.1014583107 |pmc=2984170 |pmid=21041649 |quote=Although I was among the early supporters of vertebrate paraphyly, I am impressed by the evidence provided by Heimberg et al. and prepared to admit that cyclostomes are, in fact, monophyletic. The consequence is that they may tell us little, if anything, about the dawn of vertebrate evolution, except that the intuitions of 19th century zoologists were correct in assuming that these odd vertebrates (notably, hagfishes) are strongly degenerate and have lost many characters over time |doi-access=free}}</ref> Nowadays, molecular data are almost unanimously in consensus of cyclostome monophyly, with more recent work being directed at shared microRNAs between cyclostomes and gnathostomes.<ref>{{cite journal| year=2010 |last1=Heimberg|first1=A.M| last2=Cowper-Sal| first2 = R | last3 = Semon | first3 = M | last4 = Donoghue | first4 = P.C | last5 = Peterson | first5 = K.J | display-authors = 1 | 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| volume = 107|issue=45 |pages=19379–83|doi=10.1073/pnas.1010350107|pmid=20959416|pmc=2984222|doi-access=free}}</ref> The current classification supported by molecular analyses (which show that lampreys and hagfishes are sister taxa), as well as the fact that hagfishes do, in fact, have rudimentary vertebrae, which places hagfishes in Cyclostomata.<ref name=":0"/>

==Phylogeny==
Hagfish are in the group [[Cyclostomata]] which includes jawless fish. The group [[Cyclostomata]] is characterized by two significant characteristics; [[keratin]]ous tooth plates and movement of postotic [[myomere]]s to the orbitals.<ref name=":1">{{Cite journal|last1=Miyashita|first1=Tetsuto|last2=Coates|first2=Michael I.|last3=Farrar|first3=Robert|last4=Larson|first4=Peter|last5=Manning|first5=Phillip L.|last6=Wogelius|first6=Roy A.|last7=Edwards|first7=Nicholas P.|last8=Anné|first8=Jennifer|last9=Bergmann|first9=Uwe|last10=Palmer|first10=A. Richard|last11=Currie|first11=Philip J.|date=2019-02-05|title=Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny|journal=Proceedings of the National Academy of Sciences|language=en|volume=116|issue=6|pages=2146–2151|doi=10.1073/pnas.1814794116|issn=0027-8424|pmc=6369785|pmid=30670644|bibcode=2019PNAS..116.2146M|doi-access=free}}</ref> According to fossil record, hagfish and [[lamprey]]s have been estimated to have diverged from one another during the [[Paleozoic]] period.<ref name=":1"/> An experiment used an estimation of synonymous and nonsynonymous substitutions for [[nucleotide]]s and supplemented that data with pre-existing data into a clock that would calculate divergence times for the [[taxon]]s ''[[Myxine]]'' and ''[[Eptatretus]]''.<ref name="Kuraku">{{cite journal |last1=Kuraku |first1=S. |last2=Kuratani |first2=S. |year=2006 |title=Time scale for cyclostome evolution inferred with a phylogenetic diagnosis of hagfish and lamprey cDNA sequences |journal=Zoological Science |volume=23 |issue=12 |pages=1053–1064 |doi=10.2108/zsj.23.1053 |pmid=17261918 |s2cid=7354005|doi-access=free }}</ref> This data found that the lineage diverged around 93–28 Mya; however, later studies have found even earlier divergence times within the group, with ''Myxine'' and ''Eptatretus'' diverging during the Triassic and the ancestor of  ''[[Rubicundus]]'' diverging during other extant hagfishes during the [[Permian]].<ref name="Kuraku"/><ref>{{Cite journal |last1=Brownstein |first1=Chase Doran |last2=Near |first2=Thomas |date=2024-06-13 |title=Colonization of the ocean floor by jawless vertebrates across three mass extinctions |journal=BMC Ecology and Evolution |volume=24 |issue=1 |pages=79 |doi=10.1186/s12862-024-02253-y |doi-access=free |issn=2730-7182 |pmc=11170801 |pmid=38867201|bibcode=2024BMCEE..24...79B }}</ref> Hagfish are excluded from the subphylum [[Gnathostomata]] because of [[Morphology (biology)|morphological]] characteristics including the hagfish arched tongue.<ref name=":4"/> Hagfish embryos have characteristics of gnathostomes and may be plesiomorphic;<ref name=":4"/> however, these characteristics drastically change [[Morphology (biology)|morphologically]] as the hagfish matures.<ref name=":4"/> The following hagfish and lamprey phylogeny is an adaptation based on the 2019 work of Miyashita et al.<ref>{{Cite journal |last1=Miyashita |first1=Tetsuto |last2=Coates |first2=Michael I. |last3=Farrar |first3=Robert |last4=Larson |first4=Peter |last5=Manning |first5=Phillip L. |last6=Wogelius |first6=Roy A. |last7=Edwards |first7=Nicholas P. |last8=Anné |first8=Jennifer |last9=Bergmann |first9=Uwe |last10=Palmer |first10=A. Richard |last11=Currie |first11=Philip J. |date=2019-02-05 |title=Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny |journal=Proceedings of the National Academy of Sciences |language=en |volume=116 |issue=6 |pages=2146–2151 |doi=10.1073/pnas.1814794116 |issn=0027-8424 |pmc=6369785 |pmid=30670644|bibcode=2019PNAS..116.2146M |doi-access=free }}</ref>

{{Clade|{{Clade
   |1=†''[[Haikouella]]''
   |2={{Clade
      |1={{Clade
         |1=†''[[Haikouichthys]]''
         |2=†''[[Myllokunmingia]]''
         }}
      |2={{Clade
         |1=†''[[Metaspriggina]]''
         |label2=[[Vertebrata]]|sublabel2=(crown group)
         |2={{Clade
            |1=[[Gnathostomata]] (jawed fish)
            |2={{Clade
               |label1=†[[Anaspida]]
               |1={{Clade
                  |1=†''[[Cornovichthys]]''
                  |2={{Clade
                     |1={{Clade
                        |1=†''[[Achanarella]]''
                        |2=†''[[Ciderius]]''
                        }}
                     |2={{Clade
                        |1=†[[Birkeniida]]
                        |2={{Clade
                           |1=†''[[Lasanius]]''
                           |2={{Clade
                              |1=†''[[Euphanerops]]''
                              |2=†''[[Jamoytius]]''
                  }} }} }} }} }}
               |2={{Clade
                  |1=†''[[Pipiscius]]''
                  |2={{Clade
                     |1=†[[conodont|Conodonta]] (conodonts)
                     |label2=[[Cyclostomi]]|sublabel2=(crown group)
                     |2={{Clade
                        |1={{Clade
                           |1=†''[[Myxinikela]]''
                           |label2=[[Myxinoidea]]|sublabel2=(crown group)
                           |2={{Clade
                              |1={{Clade
                                 |1=†''[[Tethymyxine tapirostrum]]''
                                 |2={{Clade
                                    |1=''[[Rubicundus eos]]''
                                    |2=''[[Rubicundus lopheliae]]''
                                 }} }}
                              |2={{Clade
                                 |1={{Clade
                                    |1=''[[Myxine glutinosa]]''
                                    |2=''[[Neomyxine biniplicata]]''
                                    }}
                                 |2={{Clade
                                    |1=''[[Eptatretus stoutii]]''
                                    |2={{Clade
                                       |1=''[[Eptatretus burgeri]]''
                                       |2="''[[Paramyxine]]''" spp.
                           }} }} }} }} }}
                        |2={{Clade
                           |1=†''[[Gilpichthys]]''
                           |2={{Clade
                              |1={{Clade
                                 |1=†''[[Hardistiella]]''
                                 |2=†''[[Mayomyzon]]''
                                 }}
                              |2={{Clade
                                 |1=†''[[Myxineidus]]''
                                 |2={{Clade
                                    |1=†''[[Priscomyzon]]''
                                    |2={{Clade
                                       |1=†''[[Mesomyzon]]''
                                       |label2=[[Petromyzontiformes]]|sublabel2=(crown group)
                                       |2={{Clade
                                          |1={{Clade
                                             |1=''[[Geotria australis]]''
                                             |2=''[[Mordacia mordax]]''
                                             }}
                                          |2={{Clade
                                             |1=''[[Petromyzon marinus]]''
                                             |2={{Clade
                                                |1=''[[Lampetra fluviatilis]]''
                                                |2=''[[Lethenteron camtschaticum]]''
}} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }}|style=font-size:85%; line-height:85%}}

==Commercial use==
[[File:Korean cuisine-Kkomjangeo bokkeum-01.jpg|thumb|right|''Kkomjangeo bokkeum'' (꼼장어 볶음), a Korean stir-fried fish dish made with the hagfish ''[[Eptatretus burgeri]]'']]

===As food===
In most of the world, hagfish are not often eaten. But in [[Korea]], the hagfish is a valued food, where it is generally skinned, coated in spicy sauce, and grilled over charcoal or stir-fried. It is especially popular in the southern port cities of the peninsula, such as [[Busan]] and coastal cities in [[South Gyeongsang Province]].{{Citation needed|date=July 2022}} 

Due to their value in [[Korean cuisine]], most hagfish caught for food elsewhere in the world is fished with intent of being exported to [[South Korea]].<ref name=":7" /> The [[inshore hagfish]], found in the northwest Pacific, is eaten in [[Japan]]<ref name="froese">{{Cite web|url=https://www.fishbase.se/summary/8712|title=''Epatretus burgeri'' Inshore hagfish|last=Froese|first=Rainer|website=Fishbase|access-date=18 April 2019}}</ref> and South Korea. As hagfish slime binds vast amounts of liquid even at low temperatures, it was proposed as an energy-saving alternative for the production of [[tofu]] that does not require heating.<ref>{{cite journal |last1=Böni |first1=Lukas |last2=Rühs |first2=Patrick A. |last3=Windhab |first3=Erich J. |last4=Fischer |first4=Peter |last5=Kuster |first5=Simon |title=Gelation of Soy Milk with Hagfish Exudate Creates a Flocculated and Fibrous Emulsion- and Particle Gel |journal=PLOS ONE |date=25 January 2016 |volume=11 |issue=1 |pages=e0147022 |doi=10.1371/journal.pone.0147022 |pmid=26808048 |pmc=4726539 |bibcode=2016PLoSO..1147022B |doi-access=free }}</ref>

===In textiles===
The hagfish slime threads can be used as ultra-strong fiber for clothing. Douglas Fudge, of [[Chapman University]], has conducted research in this area.<ref>{{Cite web |last=Crosbie |first=Jack |date=6 July 2016 |title=Say Hello to Fish Slime Bulletproof Vests |url=https://www.inverse.com/article/17893-fish-slime-bulletproof-vest |access-date=29 August 2024 |website=[[Inverse (website)|Inverse]] |language=en |archive-date=17 April 2024 |archive-url=https://web.archive.org/web/20240417063149/https://www.inverse.com/article/17893-fish-slime-bulletproof-vest |url-status=live }}</ref><ref>{{Cite web |date=2014-04-04 |title=Guelph Researchers Solve Part of Hagfish Slime Mystery |url=https://www.uoguelph.ca/news/2014/04/guelph_research.html |access-date=2023-12-07 |website=[[University of Guelph]] |archive-date=2023-12-07 |archive-url=https://web.archive.org/web/20231207180839/https://www.uoguelph.ca/news/2014/04/guelph_research.html |url-status=live }}</ref>

===Skins===
Hagfish skin, used in a variety of clothing accessories,<ref name=":7" /> is usually referred to as "eel skin". It produces a particularly durable leather, especially suitable for wallets and belts.<ref>{{Cite news|last=Dillman|first=Terry|date=1 February 2013|title=Slimed: Ugly Hagfish Yields Somewhat Pretty Income|work=Fishermen's News|url=http://www.fishermensnews.com/story/2013/02/01/features/slimed-ugly-hagfish-yields-somewhat-pretty-income/149.html|url-status=dead|archive-url=https://web.archive.org/web/20141026194850/http://www.fishermensnews.com/story/2013/02/01/features/slimed-ugly-hagfish-yields-somewhat-pretty-income/149.html|archive-date=26 October 2014|access-date=22 June 2014}}</ref>
{{Clear}}

==References==
{{Reflist|28em}}

==Further reading==
{{Refbegin|28em}}
*{{FishBase family|family=Myxinidae|year=2011|month=February}}
*{{cite journal | last1=Bardack | first1=D | year=1991 | title=First fossil hagfish (Myxinoidea): a record from the Pennsylvanian of Illinois | journal=Science | volume=254 | issue=5032 | pages=701–703 | doi=10.1126/science.254.5032.701| pmid=17774799 | bibcode=1991Sci...254..701B | s2cid=43062184 }}
*{{cite journal | last1=Bardack | first1=D. | last2=Richardson | first2=E. S. Jr | year=1977 | title=New agnathous fishes from the Pennsylvanian of Illinois | journal=Fieldiana |series=Geology | volume=33 | pages=489–510 | doi=10.5962/bhl.title.5167| doi-access=free }}
*Brodal, A. and Fänge, R. (ed.) (1963). The Biology of Myxine, Universitetsforlaget, Oslo.
*{{cite journal | last1=Fernholm | first1=B. | last2=Holmberg | first2=K. | year=1975 | title=The eyes in three genera of hagfish (''Eptatretus'', ''Paramyxine'' and ''Myxine'') – A case of degenerative evolution | journal=Vision Research | volume=15 | issue=2 | pages=253–259 | doi=10.1016/0042-6989(75)90215-1| pmid=1129982 | s2cid=29476956 }}
*Hardisty, M. W. (1982). Lampreys and hagfishes: Analysis of cyclostome relationships. In The Biology of Lampreys, (ed. M. W. Hardisty and I. C. Potter), Vol.4B, pp.&nbsp;165–259. Academic Press, London.
*Janvier, P. (1996). Early vertebrates. Oxford Monographs in Geology and Geophysics, 33, Oxford University Press, Oxford.
*{{cite book|last=Marinelli|first=Wilhelm|title=Vergleichende Anatomie und Morphologie der Wirbeltiere: 2. Lieferung. Myxine glutinosa (L.)|url=https://books.google.com/books?id=FC2UjgEACAAJ|year=1956|publisher=Franz Deuticke|access-date=2017-10-26|archive-date=2024-08-29|archive-url=https://web.archive.org/web/20240829211259/https://books.google.com/books?id=FC2UjgEACAAJ|url-status=live}}
*{{cite journal | last1=Yalden | first1=D.W. | year=1985 | title=Feeding mechanisms as evidence for cyclostome monophyly | journal=Zoological Journal of the Linnean Society | volume=84 | issue=3 | pages=291–300 | doi=10.1111/j.1096-3642.1985.tb01802.x}}
*{{cite journal | last1=Stock | first1=D. W. | last2=Whitt | first2=G. S. | year=1992 | title=Evidence from 18S ribosomal RNA that lampreys and hagfishes form a natural group | journal=Science | volume=257 | issue=5071 | pages=787–789 | doi=10.1126/science.1496398 | pmid=1496398| bibcode=1992Sci...257..787S }}
*{{cite journal |doi=10.1643/0045-8511(2006)6[225:ANSOGS]2.0.CO;2 |last1=Mincarone |first1=Michael M. |last2=Stewart |first2=Andrew L. |title=A new species of giant seven-gilled hagfish (Myxinidae: ''Eptatretus'') from New Zealand |journal=Copeia |volume=2006 |issue=2 |pages=225–229 |year=2006|s2cid=85854373 }}
*{{cite book|editor=J.M. Jørgensen|editor2=J.P. Lomholt|editor3=R.E. Weber|editor4=H. Malte|title=The biology of hagfishes|location=London |publisher=[[Chapman & Hall]]|year=1997}}
*{{cite journal|last1=Delarbre |first1=C |year=2002 |title=Complete Mitochondrial DNA of the Hagfish, ''Eptatretus burgeri'': The Comparative Analysis of Mitochondrial DNA Sequences Strongly Supports the Cyclostome Monophyly |journal=Molecular Phylogenetics and Evolution |volume=22 |issue=2 |pages=184–192 |doi=10.1006/mpev.2001.1045 |pmid=11820840|last2=Gallut|first2=C |last3=Barriel |first3=V|last4=Janvier |first4=P |last5=Gachelin |first5=G |bibcode=2002MolPE..22..184D |display-authors=1}}
*{{cite journal |author=Bondareva |last2=Schmidt |first2=EE |name-list-style=amp |date=November 2003 |title=Early Vertebrate Evolution of the TATA-Binding Protein, TBP |journal=Molecular Biology and Evolution |volume=20 |issue=11 |pages=1932–1939 |doi=10.1093/molbev/msg205 |pmid=12885957|pmc=2577151}}
*Ewoldt, R. H., Winegard, T. M. and Fudge D. S. (2010). Non-linear viscoelasticity of hagfish slime. Int. J. Lin. Mech. 46: 627–636.
*Fudge, D. (2001). [https://museum-publications.australian.museum/media/dd/documents/AMS389_27_02_2001_LowRes.e7f21b3.pdf ''Hagfishes: Champions of Slime''] Nature Australia, Spring 2001 ed., Australian Museum Trust, Sydney. pp.&nbsp;61–69.
*{{cite journal | last1=Fudge | first1=D. S. | last2=Gardner | first2=K. H. | last3=Forsyth | first3=V. T. | last4=Riekel | first4=C. | last5=Gosline | first5=J. M. | year=2003 | title=The mechanical properties of hydrated intermediate filaments: Insights from hagfish gland thread cells | journal=Biophysical Journal | volume=85 | issue=3| pages=2015–2027 | pmc=1303373 | pmid=12944314 | doi=10.1016/S0006-3495(03)74629-3| bibcode=2003BpJ....85.2015F }}
*{{cite journal | last1=Fudge | first1=D. S. | last2=Hillis | first2=S. | last3=Levy | first3=N. | last4=Gosline | first4=J. M. | year=2010 | title=Hagfish slime threads as a biomimetic model for high performance protein fibres | url=http://www.comparativephys.ca/system/files/Fudge_etal_Bioinspir%2526Biomim.pdf | journal=Bioinspiration & Biomimetics | volume=5 | issue=3 | pages=1–8 | doi=10.1088/1748-3182/5/3/035002 | pmid=20729569 | bibcode=2010BiBi....5c5002F | s2cid=19817946 }}{{dead link|date=October 2017 |bot=InternetArchiveBot |fix-attempted=yes }}
*{{cite journal | last1=Fudge | first1=D. S. | last2=Levy | first2=N. | last3=Chiu | first3=S. | last4=Gosline | first4=J. M. | year=2005 | title=Composition, morphology and mechanics of hagfish slime | journal=Journal of Experimental Biology | volume=208 | issue=24| pages=4613–4625 | doi=10.1242/jeb.01963| pmid=16326943 | doi-access= }}
*{{cite journal | last1=Winegard | first1=T. M. | last2=Fudge | first2=D. S. | year=2010 | title=Deployment of hagfish slime thread skeins requires the transmission of mixing forces via mucin strands | journal=Journal of Experimental Biology | volume=213 | issue=8| pages=1235–1240 | doi=10.1242/jeb.038075| pmid=20348334 | doi-access= }}
{{Refend}}

==External links==
{{Commons category|Myxinidae}}
{{NIE Poster|year=1905|Hagfish}}
*[http://www.fishbase.org/Summary/FamilySummary.cfm?ID=2 FishBase entry for Myxinidae]
*[https://www.youtube.com/watch?v=NYRr_MrjebA YouTube 5+ minute video of Scripps scientist/diver on hagfish]
*[http://www.metacafe.com/watch/1597296/hagfish_and_the_disgusting_slime/ Metacafe video of a University of Alberta grad student showing slime production of hagfish while in Bamfield, British Columbia] {{Webarchive|url=https://web.archive.org/web/20110825100652/http://www.metacafe.com/watch/1597296/hagfish_and_the_disgusting_slime/ |date=2011-08-25 }}
* [https://web.archive.org/web/20131203002245/http://www.3news.co.nz/Beware-the-hagfish---repeller-of-sharks/tabid/1216/articleID/231051/Default.aspx Beware the hagfish – repeller of sharks] ''3 News'', 28 Oct 2011. Video.
* [http://blog.tepapa.govt.nz/2011/10/28/hagfish-versus-sharks-1-0/ Hagfish versus sharks : 1-0] ''Te Papa Blog'', 28 October 2011.
* [https://www.youtube.com/watch?v=nzMB8jqioV0 Teen Spots Hagfish-Slurping Elephant Seal – YouTube] (2:11)
* [https://www.bbc.co.uk/news/magazine-21966514 What happens when a shark attacks a hagfish – BBC] (0:39)
* [https://www.youtube.com/watch?v=pmaal7Hf0WA Vancouver Aquarium Hagfish Slime]

{{Animalia}}
{{Chordata}}
{{Taxonbar|from=Q75713}}
{{Authority control}}

[[Category:Myxinidae| ]]
[[Category:Scavengers]]
[[Category:Taxa named by Constantine Samuel Rafinesque]]