Hagfish
Myxini is the only class in the clade Craniata that does not also belong to the subphylum Vertebrata. That is, they are the only animals which have a skull but not a vertebral column.
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). Their unusual feeding habits and slime-producing capabilities have led members of the scientific and popular media to dub the hagfish as the most "disgusting" of all sea creatures. Although hagfish are sometimes called "slime eels," they are not eels at all.
Physical characteristics
Body features
Hagfish average about half a metre (18 in) long; The largest known species is Eptatretus goliath with a specimen recorded at 127 cm, while Myxine kuoi and Myxine pequenoi seem to reach no more than 18 cm.
Hagfish have elongated, eel-like bodies, and paddle-like tails. Colours depend on the species, ranging from pink to blue-grey, and may have black or white spots.
Hagfish have no true fins and have six barbels around the mouth and a single nostril. Instead of vertically articulating jaws like Gnathostomata (vertebrates with jaws), they have a pair of horizontally moving structures with tooth-like projections for pulling off food.
Circulatory system
The circulatory systems of the hagfish have both closed and open blood vessels, with a heart system that is more primitive than that of 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.
A group of pacific hagfish.
Slime
Hagfish are long and 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.
The "sliming" also seems to act as a distraction 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.
An adult hagfish can secrete enough slime to turn a large bucket of water into gel in a matter of minutes.
Eye
In December 2003, an article was published by the University of Queensland claiming the hagfish's eye as being significant to the evolution of more complex eyes.
Reproduction
Very little is known about hagfish reproduction. In some species, sex ratio can be as high as 100:1 (but if population is dying out then they can switch between male and female) in favour of females.
In other species, individual hagfish which 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, are not uncommon. Females typically lay 20 to 30 yolky eggs that tend to aggregate due to having Velcro-like tufts at either end.
Hagfish do not have a larval stage, in contrast to lampreys, which have a long larval phase.
Feeding
Hagfish enter both living and dead fish, feeding on the insides (polychaete marine worms are also prey).
While having no ability to enter through skin, they often enter through natural openings such as the mouth, gills or anus and consume their prey from the inside out. They can be a great nuisance to fishermen, as they are known to infiltrate and devour a catch before it can be pulled to the surface.
Like leeches, they have a sluggish metabolism and can survive months between feedings.
Classification
Drawing of a New Zealand hagfish.
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 in between Prevertebrate and Gnathostome. Thus their classification is as an invertebrate within subphylum Craniata.
Recent molecular biology analyses tend to classify hagfish as invertebrates (see references) within subphylum Craniata, because of their short molecular evolutive distance from Vertebrata (sensu stricto).
A single fossil of hagfish shows that there has been little evolutionary change in the last 300 million years.
Genetic analysis
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.
No other slime secretion known is reinforced with fibres in the way hagfish slime is. The fibres are about as fine as spider silk (averaging 2 micrometres), but can be 12 cm long.
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, or a means of stopping blood flow in accident victims and surgery patients .
Species
Drawing of eptatretus minor
About 66 species are known, in 7 genera.
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
Eptatretus goliath Mincarone & Stewart, 2006
Sixgill hagfish, Eptatretus hexatrema (Müller, 1836)
Eptatretus lopheliae Fernholm & Quattrini, 2008
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)
Eptatretus strickrotti Møller & Jones, 2007
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
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