Hagfish

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Biography

Hagfish , the clade Myxini (also known as Hyperotreti ), are eel-shaped slime-producing marine animals (occasionally called slime eels ). They are the only living animals that have a skull but not a vertebral column . Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all (other) vertebrates. Hagfish have been summarised as Lovecraftian horror|Lovecraftian . - University of California Museum of Paleontology Hagfish have often been described by marine biologists as the most disgusting animal that lives in the oceans.Eyers, Jonathan (2012). How to Snog a Hagfish!: Disgusting Things in the Sea . Bloomsbury Publishing, London, UK. ISBN 978-1-4081-4042-0.

The classification of hagfish has been controversial. The issue is whether the hagfish is itself a degenerate type of vertebrate-fish (most closely related to lampreys), or else may represent a stage which precedes the evolution of the vertebral column (as do lancelets ). The original scheme groups hagfish and lampreys together as cyclostomata|cyclostomes (or historically, Agnatha ), as the oldest surviving clade of vertebrates alongside gnathostomes (the now-ubiquitous jawed-vertebrates). An 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 introduces the category craniata to group vertebrates near hagfish. Recent DNA evidence has supported the original scheme. Philippe Janvier|Janvier, P. 2010. "MicroRNAs revive old views about jawless vertebrate divergence and evolution." Proceedings of the National Academy of Sciences (USA) 107:19137-19138. " 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, hag?shes) are strongly degenerate and have lost many characters over time. "

Physical characteristics

Body features

Hagfish average about half a metre (18& nbsp;in); The largest known species is Eptatretus goliath with a specimen recorded at 127& nbsp;cm, while Myxine kuoi and Myxine pequenoi seem to reach no more than 18& nbsp;cm (some have been seen as small as 4& nbsp;cm).

Hagfish have elongated, eel-like bodies, and paddle -like tail s. They have cartilage|cartilaginous skull s (although the part surrounding the brain is composed primarily of a fibrous sheath) and tooth -like structures composed of keratin . Color s depend on the species , ranging from pink to blue - grey , and black or white spots may be present. Eye s are simple eyespots, not compound eyes that can resolve images. Hagfish have no true fin s and have six or eight barbel (anatomy)|barbel s around the mouth and a single nostril . Instead of vertically articulating jaw s like Gnathostomata ( vertebrate s with jaws), they have a pair of horizontal plane|horizontally moving structures with tooth-like projections for pulling off food. The mouth of the hagfish has two pairs of horny, comb-shaped teeth on a cartilaginous plate that protracts and retracts. These teeth are used to grasp food and draw it toward the pharynx. http://tolweb.org/Hyperotreti

Slime

Hagfish are long and wikt:vermiform|vermiform , and can exude copious quantities of a slime or mucus (from which the typical species Myxine glutinosa was named) of unusual composition. When captured and held, e.g., by the tail, they secrete the microfibrous slime, which expands into a gelatinous and sticky goo when combined with water; if they remain captured, they can tie 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 and freeing them from their captor, as well as the slime. It has been conjectured that this singular behavior assists them in extricating themselves from the jaws of predatory fish or from the interior of their own "prey", and that the "sliming" might act as a distraction to predators.

Recently it has been reported that the slime entrains water in its microfilaments, creating a slow-to-dissipate viscoelastic substance, rather than a simple gel, and it has been proposed that the primary protective effect of the slime is related to impairment of the function of a predator fish's gills. It has been observed that most of the known predators of hagfish are varieties of birds or mammals; it has been proposed that the lack of marine predators can be explained by a "gill-clogging hypothesis", wherein one purpose of the slime is to impair the gill function of marine animals that attempt to prey on the hagfish. If true, it could be regarded as a highly successful evolutionary strategy against predatory fish. ibid.

Free-swimming hagfish also "slime" when agitated and will later clear the mucus off by way of the same travelling-knot behavior. The reported gill-clogging effect suggests that the travelling-knot behavior is useful or even necessary to restore the hagfish's own gill function after "sliming".

An adult hagfish can secrete enough slime to turn a 20 litre (5 gallon) bucket of water into slime in a matter of minutes.

Research is ongoing regarding the properties and possible applications of the components of hagfish slime filament protein.

Respiration

Hagfish generally respire through taking in water through their pharynx , past the velar chamber and bringing the water through 6 internal gill pouches. The gill pouches lead to a common aperture on the ventral side of the hagfish. The esophagus is also connected to the common aperture on the ventral side through a pharyngocutaneous duct (esophageocutaneous duct), which has no respiratory tissue. It is likely that this pharyngocutaneous duct is used to cough up indigestible materials. Hagfish also have some cutaneous respiration via the blood Sinus (anatomy)|sinuses under their skin. This can be essential for hagfish to respire while feeding, since they do not have Operculum (fish)|operculi to beat to produce current across the gills (as in the case of teleost fish).

Eye

In December 2003, an article was published by the University of Queensland claiming the hagfish's eye, which lacks both lens and extrinsic musculature, as being significant to the Evolution of the eye|evolution of more complex eyes . Hagfish eyespots, when present, can detect light, but as far as is known none can resolve detailed images. In Myxine and Neomyxine, the eyes are partly covered by the trunk musculature.

Reproduction

Very little is known about hagfish reproduction. In some species, sex ratio has been reported to be as high as 100:1 in favor of females. Some hagfish species are thought to be hermaphroditic , having both an ovary and a testicle (there is only one gamete production organ in both females and males). In some cases it is thought that the ovary remains non-functional until the individual has reached a particular age or encounters a particular environmental stress. These two factors in combination suggest that the survival rate of hagfish is quite high.

Depending on species, females lay from 1 or 2, to 20-30, tough, yolky eggs. These tend to aggregate due to having Velcro -like tufts at either end. Hagfish are sometimes seen curled around small clutches of eggs. It is not certain if this constitutes actual brooding behavior.

Hagfish do not have a larva l stage, in contrast to lamprey s, which have a long larval phase.

Hagfish have a mesonephros|mesonephric kidney and are often Neoteny|neotenic of their pronephros|pronephric kidney . The kidney(s) are drained via mesonephric/ archinephric duct . Unlike many other vertebrates, this duct is separate from the reproductive tract. Unlike all other vertebrates, the proximal tubule of the nephron is also connected with the coelom , provided lubrication.
The single testicle or ovary has no transportation duct. Instead, the gametes are released into the coelom until they find their way to the posterior end of the caudal region, whereby they find an opening in the digestive system.

Feeding

in a curled position at the left of the photo.
While polychaete marine worm s on or near the sea floor are a major source of nutrition, hagfish can feed upon and often even enter and eviscerate the bodies of dead and dying/injured sea creatures much larger than themselves. They are known to devour their victims from the inside. Hagfish have the ability to absorb dissolved organic matter across the skin and gill, which may be an adaptation to a scavenging lifestyle, allowing hagfish to maximize sporadic opportunities for feeding. From an evolutionary perspective, hagfish represent a transitory state between the generalized nutrient absorption pathways of aquatic invertebrates and the more specialized digestive systems of aquatic vertebrates.

Like leech es, they have a sluggish metabolism and can survive months between feedings; their feeding behavior, however, appears quite vigorous.

In captivity, hagfish are observed to use the overhand-knot behavior "in reverse" (tail-to-head) to assist them in gaining mechanical advantage to pull out hunks of flesh from carrion fish or cetaceans, eventually making an opening to permit entry to the interior of the body cavity of larger carcasses. It is to be expected that a healthy larger sea creature would be able to outfight or outswim this sort of assault.

However, this energetic opportunism on the part of the hagfish can be a great nuisance to fishermen, as they can devour or spoil entire deep-drag netted catches before they can be pulled to the surface. Since hagfish are typically found in large clusters on and near the bottom, a single trawler's catch could contain several dozen or even hundreds of hagfish as bycatch, and all the other struggling, captive sealife make easy prey for them.

The digestive tract of the hagfish is unique among the vertebrates because the food in the gut is enclosed in a permeable membrane, analogous to the peritrophic matrix of insects. Ross Piper|Piper, Ross (2007), Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals , Greenwood Press (publisher)|Greenwood Press .

Gastronomy

Hagfish are usually not eaten owing to their repugnant looks, as well as their viscosity and unpleasant habits. However, a particular species, the inshore hagfish , found in the Northwest Pacific, is valued as food in the Korean Peninsula . The hagfish is kept alive and irritated by rattling its container with a stick, prompting it to produce slime in large quantities. This slime is used in a similar manner as egg whites in various forms of cookery in the region.

The inshore hagfish, known as kkomjangeo (???) or meokjango (???) in Korean language|Korean and Nuta-unagi (?????) in Japanese language|Japanese , is the only member of the hagfish family that has a seasonal reproductive cycle.

Classification

In recent years hagfish have become of special interest for genetic analysis investigating the relationships among chordate s. Their classification as agnatha ns places hagfish as elementary vertebrate s in between invertebrate s and gnathostome s. However there has been long discussion in scientific literature about whether the hagfish were even Invertebrate|non-vertebrate . This position is supported by recent molecular biology analyses which tend to classify hagfish as invertebrates (see references) within subphylum Craniata , because of their molecular evolutionary distance from Vertebrata (sensu stricto) . A single fossil of hagfish shows that there has been little evolutionary change in the last 300 million years.

Genera

As of February 2011, about 77 species are known, in 5 genera (two are being retained in the no-longer recognized genus Paramyxine and are awaiting taxonomic issues to be resolved before being transferred to Eptatretus (see below)). A number of the species have only been recently discovered, living at depths of several hundred metres.

Eptatretus

Myxine

Nemamyxine

Neomyxine

Notomyxine

Paramyxine (Note that this genus is considered Synonym (taxonomy)|synonymous with Eptatretus and most species have been moved into that genus with the exception of the following two which are Homonym (biology)|homonyms of current Eptatretus species. Since the current Eptatretus species are junior (i.e., described later) to the two following species, they must be renamed and then the two remaining "paramyxines" will be transferred to Eptatretus .)

Notes

References

Ewoldt, R. H., Winegard, T. M. and Fudge D. S. (2010). Non-linear viscoelasticity of hagfish slime. Int. J. Lin. Mech. 46: 627-636.

Fudge, D. (2001). Nature Australia, Spring 2001 ed., Australian Museum Trust, Sydney. pp.& nbsp;61–69.

Fudge, D. S., Gardner, K. H., Forsyth, V. T., Riekel, C. and Gosline, J. M. (2003) Biophys. J. 85 : 2015–2027.

Fudge, D. S., Hillis, S., Levy, N. and Gosline, J. M. (2010) Bioinsp. Biomim. 5 : 1–8.

Fudge, D. S., Levy, N., Chiu, S. and Gosline, J. M. (2005) J. Exp. Biol. 208 : 4613–4625.

Lim, J., Fudge, D. S., Levy, N. and Gosline, J. M. (2006) J. Exp. Biol. 209 : 702–710.

Winegard, T. M. and Fudge, D. S. (2010) J. Exp. Biol. 213 : 1235–1240.

External links

3 News , 28 Oct 2011. Video.

Category:Edible fish
Category:Myxinidae|
Category:Vertebrates without jaws
Category:Fish of the Pacific Ocean
Category:Scavengers

az:Miksinl?r
zh-min-nan:Bông-moâ
be:???????
be-x-old:???????
ca:Mixinoïdeu
cs:Sliznatky
da:Slimål
de:Schleimaale
nv:Lóó' bíígháántsiighaa' ádaadinígíí
et:Pihklased
es:Myxini
eo:Mukofisoj
eu:Mixinea
fr:Myxinidae
ko:???
hr:Sljepulje
id:Remang
it:Myxiniformes
ka:?????????
kk:??????????
la:Myxiniformes
lt:Miksiniai apskritažiomeniai
lij:Myxini
hu:Nyálkahalak
nl:Slijmprikken
ja:?????
no:Slimåler
nn:Slimålar
oc:Myxinidae
pl:Sluzice
pt:Peixe-bruxa
ru:???????
simple:Hagfish
sk:Sliznatky
sr:???????
sh:Sljepulje
fi:Viiksiympyräsuiset
sv:Pirålar
te:???? ???
th:??????
tr:Myxini
uk:???????
zh:??

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