Hagfish

Hagfish
Pacific hagfish resting on bottom 280 m depth off Oregon coast
Scientific classification
Kingdom: Animalia Phylum: Chordata Subphylum: Craniata Class: Myxini Order: Myxiniformes Family: Myxinidae
Genera
Eptatretus Myxine Nemamyxine Neomyxine Notomyxine Paramyxine Quadratus

Hagfish is the common name for the marine craniates comprising the family Myxinidae of the order Myxiniformes (Hyperotreti) of the class (or subphylum) Myxini, characterized by a scaleless, eel-like body, without paired fins and without a vertebrae, but having a cranium. The Myxinidae is the only family in Myxiniformes, which is the only order in the class Myxini, and thus hagfish is variously used for any of the three taxonomic levels (ITIS 2003; Nelson 1994). Hagfish has traditionally been considered part of the superclass Agnatha within the sub-phylum Vertebrata, but the lack of a vertebrae has led some to place it outside of the Vertebrata, and thus also not considered a fish. Hagfish are the only animals that have a skull but not a vertebral column.

Although hagfish have an ancient history, possibly tracing back 300 million years ago to the Carboniferous, there remain extant hagfish. These animals, which are characterized by degenerate eyes, barbels present around the mouth, and teeth only on the tongue, are found in marine environments and are scavengers that mostly eat the insides of dying or dead fish and invertebrates (Nelson 1994). The extant Myxinidae are unique in being the only vertebrate in which the body fluids are isomotic with seawater (Nelson 1994).

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.

Overview

hagfish, like lampreys, are jawless, and thus generally classified into the group Agnatha (jawless vertebrates).

Hagfish, while generally classified in Agnatha and in the subphylum Vertebrata, actually lack vertebrae. For this reason, they sometimes are separated from the vertebrates. Janvier (1981) and a number of others, for example, put hagfish in the subphylum Myxini, which along with the subphylum Vertebrata comprises the taxon Craniata, recognizing the common possession of a cranium (Janvier 1981). Others, however, use the terms Vertebrata and Craniata as synonyms, rather than different levels of classification, and retain the use of Agnatha (Nelson 1994).

The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage.

Members of Agnatha are characterized by the absence of jaws derived from gill arches, although hagfish and some fossil forms have a biting apparatus that is not considered to have been derived from gill arches (Nelson 1994). Other common characteristics of Agnatha include the absence of paired fins, absence of pelvic fins, the presence of a notochord both in larvae and adults, and seven or more paired gill pouches. There is a lack of a vertebral centra, presence of one or two vertical semicircular canals, the gills are directed internally and covered with endoderm, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (Nelson 1994). The bronchial arches supporting the gill pouches lie close to the body surface. There is a light sensitive pineal eye (homologous to the pineal gland in mammals).

All living and most extinct agnathans do not have an identifiable stomach or any paired appendages, although the hagfish and lampreys do have a tail and a caudal fin. Both hagfish and lamprey have slimy skin without scales or plates. Some extinct agnathans reveal thick body plates. The internal skeleton of the Agnatha is not bony but rather cartilaginous (made up of dense connective tissue).

Myxini is the only class in the clade Craniata that does not also belong to the subphylum Vertebrata.

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).

Description

Extant hagfish are placed in the family Myxinidae within the order Myxiniformes (Hyperotreti) or subphylum Myxini. They have a scaleless, eel-like body without paired fins. They are characterized by the lack of a dorsal fin, barbels present around the mouth, degenerate eyes, teeth only on the tongue, no metamorphosis, one semicircular canal, a single olfactory capsule with folds in sensory epithelium, no bone, absence of eye musculature, and 1 to 16 pairs of external gill openings (Nelson 1994). Sometimes known as "slime eels," they are a staple food in Korea.

Members of the family characterized by

They are characterized by the lack of a dorsal fin, barbels present around the mouth, degenerate eyes, teeth only on the tongue,

members of the order characterized by

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.

A group of pacific hagfish.

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.^{[citation needed]}

An adult hagfish can secrete enough slime to turn a large bucket of water into gel in a matter of minutes.^{[citation needed]}

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.^{[citation needed]}

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. 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

Drawing of eptatretus minor

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:

• 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

References

ISBN links support NWE through referral fees

• New species Eptatretus goliath. BIOONE Online Journals. Retrieved 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 (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.
• Fudge, D. (2001). Hagfishes: Champions of Slime Nature Australia, Spring 2001 ed., Australian Museum Trust, Sydney. pp. 61–69.

• Integrated Taxonomic Information System (ITIS). 2003. Agnatha ITIS Taxonomic Serial No.: 159693. Retrieved May 31, 2008.

• Janvier, P. 1981. The phylogeny of the Craniata, with particular reference to the significance of fossil "agnathans." J. Vertebr. Paleont. 1(2):121-159.

• Nelson, J. S. 1994. Fishes of the World, 3rd ed. New York: John Wiley & Sons. ISBN 0471547131.

External links

• FishBase entry for Myxinidae
• OceanLink description of hagfish
• Tree of life illustration showing hagfish's relation to other organisms
• Quicktime movie of hagfish in repose and "sliming"

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