Hagfish are marine craniates of the class Myxini, also known as Hyperotreti. 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.
Hagfish average about half a metre
) 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. They have cartilaginous skulls and tooth-like structures composed of keratin. Colours depend on the species, ranging from pink to blue-grey, and may have black or white spots. Eyes may be vestigial or absent. 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.
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
, 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
Hagfish are long and vermiform
, and can exude copious quantities of a sticky slime
(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.
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
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
, but with female gonads
which 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.
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
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.
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.
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
. What is interesting about hagfish slime is that it is fibre-reinforced. 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. When the coiled
fibres leave the hagfishes' 'slime' gland, they unravel quickly to their full length without tangling. 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.
About 60 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
- 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
- Genus Neomyxine
- Genus Notomyxine
- Genus Paramyxine
- Genus Quadratus
- New species Eptatretus goliath. BIOONE Online Journals. Retrieved on 2008-02-19..
- J.M. Jørgensen, J.P. Lomholt, R.E. Weber and H. Malte (eds.) (1997). The biology of hagfishes. London: Chapman & Hall.
- Delarbre et al "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 "Early Vertebrate Evolution of the TATA-Binding Protein, TBP". Molecular Biology and Evolution 20 (11): 1932–1939.
- Fudge, D. (2001). Hagfishes: Champions of Slime Nature Australia, Spring 2001 ed., Australian Museum Trust, Sydney. pp. 61–69.