AskDefine | Define hagfish

Dictionary Definition

hagfish n : eellike cyclostome having a tongue with horny teeth in a round mouth surrounded by eight tentacles; feeds on dead or trapped fishes by boring into their bodies [syn: hag, slime eels] [also: hagfishes (pl)]

User Contributed Dictionary

English

Noun

hagfish
  1. any of several primitive eellike creatures, of the family Myxindae, having a sucking mouth with rasping teeth; considered edible in Japan, their skin is used to make a form of leather

Extensive Definition

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.

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. 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.

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.

Slime

Hagfish 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. 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

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. 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 .

Species

About 66 species are known, in 7 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:

References

  • Fudge, D. (2001). Hagfishes: Champions of Slime Nature Australia, Spring 2001 ed., Australian Museum Trust, Sydney. pp. 61–69.
hagfish in Catalan: Mixinoïdeu
hagfish in Czech: Sliznatky
hagfish in Danish: Slimål
hagfish in German: Schleimaale
hagfish in Estonian: Pihklased
hagfish in Spanish: Myxinoidea
hagfish in Esperanto: Mukofiŝoj
hagfish in Basque: Mixinea
hagfish in French: Myxinidae
hagfish in Croatian: Sljepulje
hagfish in Italian: Myxiniformes
hagfish in Lithuanian: Miksiniai apskritažiomeniai
hagfish in Hungarian: Nyálkahalfélék
hagfish in Dutch: Slijmprikken
hagfish in Norwegian: Slimåler
hagfish in Norwegian Nynorsk: Slimålar
hagfish in Occitan (post 1500): Myxinidae
hagfish in Polish: Śluzice
hagfish in Portuguese: Peixe-bruxa
hagfish in Slovak: Sliznatky
hagfish in Serbian: Слепуље
hagfish in Finnish: Limaaja
hagfish in Swedish: Pirål
hagfish in Chinese: 盲鰻
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