Hagfish
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Eptatretus |
This article is about the hagfish. For the Punk Rock band, see Hagfish (band).
A hagfish is a marine chordate of the class Myxini, also known as Hyperotreti. Despite their name, there is some debate about whether they are strictly fish (as there is for lampreys), since they belong to a much more primitive lineage than any other group that is commonly defined fish (Chondrichthyes and Osteichthyes).
They are long, vermiform and can exude copious quantities of a sticky slime or mucus (from which the typical species Myxine glutinosa was named). When captured and held by the tail, they escape by secreting the fibrous slime, which turns into a thick and sticky gel when combined with water, and then cleaning off by tying themselves in an overhand knot which works its way from the head to the tail of the animal, scraping off the slime as it goes. Some authorities conjecture that this singular behavior may assist them in extricating themselves from the jaws of predatory fish. However, the "sliming" also seems to act as a distractant to predators, and free-swimming hagfish are seen to "slime" when agitated and will later clear the mucus off by way of the same traveling-knot behavior.
Instead of vertically articulating jaws like Gnathostomata (vertebrates with jaws), they have a pair of horizontally moving structures with toothlike projections for pulling off food. There are typically short tentacle-like protrusions around the mouth.
Hagfish enter both living and dead fish, feeding on the insides (polychaete marine worms are also prey). They tend to be quite common in their range, sometimes becoming a nuisance to fishermen by devouring the catch before it can be pulled to the surface. Not unlike leeches, they have a sluggish metabolism and can go months between feedings.
Hagfish average about half a metre (18 inches) in length; Eptatretus carlhubbsi is the largest known, with a specimen recorded at 116 cm, while Myxine kuoi and Myxine pequenoi seem to reach no more than 18 cm. An adult hagfish can secrete enough slime to turn a large bucket of water into gel in a matter of minutes.
There has been long discussion in scientific literature about the hagfish being non-vertebrate. Given their classification as Agnatha, Hagfish are seen as an elementary vertebrate inbetween Prevertebrate and Gnathostome. Thus, their classification is as an extremely primitive Vertebrate.
- They are part of the subphylum Vertebrata so, taxonomically speaking, they are vertebrates.
- They do not have vertebrae so, anatomically, they're not vertebrates.
Recent molecular biology analyses tend to classify hagfish as vertebrates (see references), their molecular evolutive distance from Vertebrata (sensu stricto) being short.
The circulatory system of the hagfish has both closed and open blood vessels, with a heart system that is the most primitive of all vertebrates, bearing some resemblance to that of some worms. This system comprises a "brachial heart", which functions as the main pump, and three types of accessory hearts: the "portal" heart(s) which carry blood from intestines to liver; the "cardinal" heart(s) which move blood from the head to the body, and the "caudal" heart(s) which pump blood from the trunk and kidneys to the body. None of these hearts are innervated, so their function is probably modulated, if at all, by hormones.
Individual hagfish are hermaphroditic, with both ovaries and testes, but the female gonads remain non-functional until the individual has reached a particular stage in the hagfish lifecycle. Hagfish do not have a larval stage, in contrast to lampreys, which have a long larval phase.
Hagfish are eaten in Japan and South Korea, and their skin is made into "eel leather" (used for so-called "eelskin" products ) in Korea.
In recent years hagfish have become of special interest for genetic analysis investigating the relationships among chordates. It has also recently been discovered that the mucus excreted by the hagfish is unique in that it includes strong, threadlike fibres similar to spider silk. Research continues into potential uses for this or a similar synthetic gel or of the included fibres. Some possibilities include new biodegradable polymers, space-filling gels, and as a means of stopping blood flow in accident victims and surgery patients.
[edit] Species
About 64 species are known, in 5 genera. A number of the species have only been recently discovered, living at depths of several hundred metres. Some of the species are listed here:
- Genus Eptatretus:
- Inshore hagfish, Eptatretus burgeri (Girard, 1855)
- New Zealand hagfish, Eptatretus cirrhatus (Forster, 1801)
- Black hagfish, Eptatretus deani (Evermann & Goldsborough, 1907)
- Guadalupe hagfish, Eptatretus fritzi Wisner & McMillan, 1990
- Sixgill hagfish, Eptatretus hexatrema (Müller, 1836)
- Shorthead hagfish, Eptatretus mcconnaugheyi Wisner & McMillan, 1990
- Eptatretus mendozai Hensley, 1985
- Eightgill hagfish, Eptatretus octatrema (Barnard, 1923)
- Fourteen-gill hagfish, Eptatretus polytrema (Girard, 1855)
- Fivegill hagfish, Eptatretus profundus (Barnard, 1923)
- Cortez hagfish, Eptatretus sinus Wisner & McMillan, 1990
- Gulf hagfish, Eptatretus springeri (Bigelow & Schroeder, 1952)
- Pacific hagfish, Eptatretus stoutii (Lockington, 1878)
- Genus Myxine:
- Patagonian hagfish Myxine affinis Günther, 1870
- Myxine australis Jenyns, 1842
- Cape hagfish, Myxine capensis
- Whiteface hagfish, Myxine circifrons Garman, 1899
- Myxine debueni Wisner & McMillan, 1995
- Myxine dorsum Wisner & McMillan, 1995
- Myxine fernholmi Wisner & McMillan, 1995
- Myxine formosana Mok & Kuo, 2001
- Myxine garmani Jordan & Snyder, 1901
- Hagfish (or Atlantic hagfish), Myxine glutinosa
- Myxine hubbsi Wisner & McMillan, 1995
- Myxine hubbsoides Wisner & McMillan, 1995
- White-headed hagfish, Myxine ios
- Myxine jespersenae Møller, Feld, Poulsen, Thomsen & Thormar, 2005
- Myxine knappi Wisner & McMillan, 1995
- Myxine kuoi Mok, 2002
- Myxine limosa Girard, 1859
- Myxine mccoskeri Wisner & McMillan, 1995
- Myxine mcmillanae Hensley, 1991
- Myxine paucidens Regan, 1913
- Myxine pequenoi Wisner & McMillan, 1995
- Myxine robinsorum Wisner & McMillan, 1995
- Myxine sotoi Mincarone, 2001
- Genus Nemamyxine:
- Nemamyxine elongata Richardson, 1958
- Nemamyxine kreffti McMillan and Wisner, 1982
- Genus Neomyxine:
- Neomyxine biniplicata (Richardson and Jowett, 1951)
- Genus Notomyxine:
- Notomyxine tridentiger (Garman, 1899)
- Genus Paramyxine:
- Paramyxine atami Dean, 1904
- Paramyxine cheni Shen and Tao, 1975
- Paramyxine fernholmi Kuo, Huang and Mok, 1994
- Paramyxine sheni Kuo, Huang and Mok, 1994
- Paramyxine wisneri Kuo, Huang and Mok, 1994
- Genus Quadratus:
- Quadratus ancon Mok, Saavedra-Diaz and Acero P., 2001
- Quadratus nelsoni (Kuo, Huang and Mok, 1994)
- Quadratus taiwanae (Shen and Tao, 1975)
- Quadratus yangi
[edit] References
- J.M. Jørgensen, J.P. Lomholt, R.E. Weber and H. Malte (eds.) (1997). The biology of hagfishes. London: Chapman & Hall.
- Delarbre et al (2002). "Complete Mitochondrial DNA of the Hagfish, Eptatretus burgeri: The Comparative Analysis of Mitochondrial DNA Sequences Strongly Supports the Cyclostome Monophyly". Molecular Phylogenetics and Evolution 22 (2): 184-192.
- Bondareva and Schmidt (2003). "Early Vertebrate Evolution of the TATA-Binding Protein, TBP". Molecular Biology and Evolution 20 (11): 1932-1939.
[edit] External links
- FishBase entry for Myxinidae
- OceanLink description of hagfish
- Quicktime movie of hagfish in repose and "sliming"
