Temporal range:
Eptatretus hexatrema 15727711.jpg
Sixgill hagfish, Eptatretus hexatrema
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Infraphylum: Agnatha
Superclass: Cyclostomi
Class: Myxini
Order: Myxiniformes
Family: Myxinidae
Rafinesque, 1815
  • Bdellostomatidae Gill, 1872
  • Homeidae Garman, 1899
  • Paramyxinidae Berg, 1940
  • Diporobranchia Latreille, 1825[2]

Hagfish, of the class Myxini /mɪkˈsn/ (also known as Hyperotreti) and order Myxiniformes /mɪkˈsɪnɪfɔːrmz/, are eel-shaped, slime-producing marine fish (occasionally called slime eels). They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae.[3] Along with lampreys, hagfish are jawless; the two form the sister group to jawed vertebrates, and living hagfish remain similar to hagfish from around 300 million years ago.[4]

The classification of hagfish was once the subject of debate: was the hagfish a type of vertebrate that through evolution had lost its vertebrae, most closely related to lampreys (the earlier view), or did the hagfish represent a stage preceding the evolution of the vertebral column, as is the case with lancelets (the alternative view)? Recent DNA evidence has supported the earlier view.[5]

The original scheme groups hagfish and lampreys together as cyclostomes (or historically, Agnatha), as the oldest surviving class of vertebrates alongside gnathostomes (the now-ubiquitous jawed vertebrates). The alternative scheme proposed that jawed vertebrates are more closely related to lampreys than to hagfish (i.e., that vertebrates include lampreys but exclude hagfish), and introduced the category craniata to group vertebrates near hagfish.

  1. ^ Nelson, Joseph S.; Grande, Terry C.; Wilson, Mark V. H. (2016). Fishes of the World (5th ed.). John Wiley & Sons. ISBN 9781118342336.
  2. ^ van der Laan, Richard; Eschmeyer, William N.; Fricke, Ronald (2014). "Family-group names of Recent fishes". Zootaxa. 3882 (2): 001–230. doi:10.11646/zootaxa.3882.1.1. ISSN 1175-5326. PMID 25543675. S2CID 31014657.
  3. ^ Reece, Jane (2014). Campbell Biology. Boston: Pearson. p. 717. ISBN 978-0321775658.
  4. ^ Myxini Archived 2017-12-15 at the Wayback Machine – University of California Museum of Paleontology
  5. ^ Janvier, P. (2010). "MicroRNAs revive old views about jawless vertebrate divergence and evolution". Proceedings of the National Academy of Sciences. 107 (45): 19137–19138. Bibcode:2010PNAS..10719137J. doi:10.1073/pnas.1014583107. PMC 2984170. PMID 21041649. 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

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