Difference between revisions of "Hagfish" - New World Encyclopedia

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{{Taxobox
 
{{Taxobox
 
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'''Hagfish''' is the common name for the marine [[craniate]]s (animals with skulls) of the class (or subphylum) '''Myxini''', characterized by a scaleless, [[eel]]-like body that lacks both paired fins and [[vertebrae]], but does have a cranium. Hagfish are the only animals that have a [[skull]] but not a [[vertebral column]]. Despite their lack of vertebrae, the hagfish have traditionally been classified among the vertebrates. This traditional classification is currently under dispute.
  
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'''Hagfish''' is the common name for the marine [[craniate]]s 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 to not consider hagfish to be a [[fish]]. Hagfish are the only animals that have a [[skull]] but not a [[vertebral column]].
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Although hagfish have an ancient history, possibly tracing back 300 million years ago, to the [[Carboniferous]], there remain today living species of 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 eat primarily the insides of dying or dead fish and [[invertebrate]]s. They are unique in being the only "vertebrate" in which the body fluids are isosmotic (having the same [[osmosis|osmotic pressure]]) with seawater (Nelson 1994). Although hagfish are sometimes called "slime eels," they are not [[eel]]s at all, which are part of the bony fish.
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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 [[invertebrate]]s (Nelson 1994). The extant Myxinidae are unique in being the only vertebrate in which the body fluids are isomotic with seawater (Nelson 1994).
 
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Although hagfish are sometimes called "slime eels," they are not [[eel]]s at all, which are part of the bony fish.
 
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While a staple food in [[Korea]], the unusual feeding habits and slime-producing capabilities of hagfish have led members of the scientific and popular media to dub the hagfish as the most "disgusting" of all sea creatures (URI 2002; Ressem 2003; Frank 2004). Nonetheless, they provide important values ***
 
  
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The unusual feeding habits and slime-producing capabilities of hagfish have led members of the scientific and popular media to dub the hagfish as the most "disgusting" of all sea creatures (URI 2002; Ressem 2003; Frank 2004). Nonetheless, they play an important ecological role and for humans are of value for both commerce and research. In [[food chain]]s, hagfish are consumed by seabirds, [[pinniped]]s (seals and walruses), and [[crustacean]]s such as lobsters and crabs. In some areas of the world, they are consumed by people, being important commercially in Korea for this reason. They are used also in the study of tumors (Lee 2002) and for investigating [[chordate]] relationships through genetic analyses. Scientists are exploring possible practical applications for the hagfish slime (Vowles 2007). Finally, their mysterious and unusual behaviors and forms, and their connection to an ancient past, add greatly to the wonder of nature.
  
 
==Overview==
 
==Overview==
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[[Image:Eptatretus stoutii heads.jpg|thumb|right|A group of [[Pacific hagfish]].]]
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The hagfish family, Myxinidae, is the only family in the order Myxiniformes (also known as Hyperotreti), which itself is the only order in the class Myxini. Thus, hagfish is variously used for any of the three taxonomic levels (ITIS 2003; Nelson 1994).
  
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Hagfish are jawless and generally classified with the [[lamprey]]s into the superclass [[Agnatha]] (jawless vertebrates) within the subphylum [[Vertebrata]]. However, hagfish actually lack [[vertebrae]]. For this reason, they sometimes are separated from the [[vertebrate]]s. Janvier (1981) and a number of others 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 and hagfish (Myxini) as within the vertebrates (Nelson 1994). The other living member of Agnatha, the lamprey, has primitive vertebrae made of [[cartilage]].
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Hagfish are jawless and generally classified with the [[lamprey]]s into the superclass [[Agnatha]] (jawless vertebrates) within the subphylum [[Vertebrata]]. However, hagfish actually lack [[vertebrae]]. For this reason, they sometimes are separated from the [[vertebrate]]s and not even considered to be [[fish]]. Janvier (1981) and a number of others put hagfish in a separate ''subphylum'' Myxini, which along with the subphylum Vertebrata comprises the taxon [[Craniata]], recognizing the common possession of a cranium (Janvier 1981). Others, however, place Vertebrata and Craniata as synonyms at the same level of classification, and thereby retain hagfish (Myxini) as members of the superclass Agnatha within the vertebrates (Nelson 1994). The other living member of Agnatha, the lamprey, has primitive vertebrae made of [[cartilage]].
  
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While some place Myxini as a subphylum equal to the subphylum Vertebrata (Janvier 1981), others place Myxini as a ''class'', whether within the subphylum Vertebrata (ITIS 2003) or outside as the only class in the [[clade]] Craniata that does not also belong to the [[phylum|subphylum]] [[Vertebrata]] (Campbell and Reece 2005).
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Yet other classifications place Myxini as a ''class'' that in one instance lies within the subphylum Vertebrata (ITIS 2003) and, in another instance, lies within—and is the only class in—the [[clade]] Craniata, which is considered to be separate from the [[phylum|subphylum]] [[Vertebrata]] (Campbell and Reece 2005).
  
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As members of Agnatha ([[Greek Language|Greek]], "no jaws"), hagfish are characterized by the absence of jaws derived from gill arches, although they do 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 [[fin]]s, absence of pelvic fins, the presence of a [[notochord]] both in larvae and adults, and seven or more paired [[gill]] pouches. In addition, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (Nelson 1994).  
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As members of Agnatha ([[Greek Language|Greek]], "no jaws"), hagfish are characterized by the absence of jaws derived from [[gill]] arches (bone or cartilage supporting the gills), although they do 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 [[fin]]s, absence of pelvic fins, the presence of a [[notochord]] both in larvae and adults, and seven or more paired gill pouches. In addition, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (the portion of the skull the protects the brain) (Nelson 1994).  
  
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Despite their name, there is some debate about whether hagfish are strictly [[fish]] , since they  belong to a much more primitive lineage than any other group that is commonly defined [[fish]] ([[Chondrichthyes]] and [[Osteichthyes]]), and because of the lack of a vertebrae commonly associated with the definition of fish. However, many authors do place hagfish as a primitive fish.
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Despite their name, there is some debate about whether hagfish are strictly [[fish]], since they  belong to a much more primitive lineage than any other group that is commonly defined as fish ([[Chondrichthyes]] and [[Osteichthyes]]), and because of the lack of a vertebrae commonly associated with the definition of fish. However, many authors do place hagfish as a primitive fish.
  
 
==Description==
 
==Description==
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[[Image:Eptatretus cirrhatus (New Zealand hagfish).gif|thumb|right|240px|Drawing of a [[New Zealand hagfish]].]]
 
Extant hagfish are placed in the family Myxinidae within the order Myxiniformes (Hyperotreti) and subphylum or class Myxini.  
 
Extant hagfish are placed in the family Myxinidae within the order Myxiniformes (Hyperotreti) and subphylum or class Myxini.  
  
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Members of the order Myxiniformes are characterized by one semicircular canal, absence of eye musculature, a single olfactory capsule with few folds in sensory epithelium, no bone, and 1 to 16 pairs of external gill openings (Nelson 1994).  
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Members of the order Myxiniformes are characterized by one semicircular canal, absence of eye musculature, a single olfactory capsule with few folds in the sensory epithelium, no bone, and one to 16 pairs of external gill openings (Nelson 1994).  
  
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Members of the family Myxinidae are characterized by the lack of a dorsal fin, barbels present around the mouth, degenerate eyes, teeth only on the tongue, no metamorphosis, and ovaries and testes in the same individual but only one functional gonad (Nelson 1994). Note that many of these features differ from the other [[agnatha]]n, lampreys, which have one or two dorsal fins, well-developed eyes, absence of barbels, separate sexes, a larval stage that undergoes radical metamorphosis, and teeth bothon the oral disc and tongue (Nelson 1994).
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Members of the family Myxinidae are characterized by the lack of a dorsal fin, barbels present around the mouth, degenerate eyes, teeth only on the tongue, no metamorphosis, and ovaries and testes in the same individual but only one functional gonad (Nelson 1994). Note that many of these features differ from the other [[agnatha]]n, lampreys, which have one or two dorsal fins, well-developed eyes, absence of barbels, separate sexes, a larval stage that undergoes radical metamorphosis, and teeth both on the oral disc and tongue (Nelson 1994).
  
 
Hagfish have a scaleless, elongated, eel-like body without paired fins. The extant hagfish average about half a meter (18 inches) long. The largest known species is ''[[Eptatretus goliath]]'' with a specimen recorded at 127 centimeters, while ''[[Myxine kuoi]]'' and ''[[Myxine pequenoi]]'' seem to reach no more than 18 centimeters.
 
Hagfish have a scaleless, elongated, eel-like body without paired fins. The extant hagfish average about half a meter (18 inches) long. The largest known species is ''[[Eptatretus goliath]]'' with a specimen recorded at 127 centimeters, while ''[[Myxine kuoi]]'' and ''[[Myxine pequenoi]]'' seem to reach no more than 18 centimeters.
  
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Extant hagfish have [[paddle]]-like [[tail]]s, [[cartilage|cartilaginous]] [[skull]]s, and [[tooth]]-like structures composed of [[keratin]]. [[Color]]s depend on the [[species]], ranging from [[pink]] to [[blue]]-[[gray]], and may have [[black]] or [[white]] spots. [[Eye]]s may be vestigial or absent. Hagfish have no true [[fin]]s and have six [[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]]ly moving structures with tooth-like projections for pulling off food.
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Extant hagfish have [[paddle]]-like [[tail]]s, [[cartilage|cartilaginous]] [[skull]]s, and [[tooth]]-like structures composed of [[keratin]]. [[Color]]s range from [[pink]] to [[blue]]-[[gray]] depending on the [[species]], and may include [[black]] or [[white]] spots. [[Eye]]s may be vestigial or absent. Hagfish have no true [[fin]]s and have six [[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]]ly moving structures with tooth-like projections for pulling off food.
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The [[circulatory system]]s of the extant hagfish have both closed and open [[blood vessel]]s, with a [[heart]] system that is more primitive than that of [[vertebra]]tes, bearing some resemblance to that of some [[worm]]s. 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 [[intestine]]s 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 [[kidney]]s to the body. None of these hearts are [[innervate]]d, so their function is probably modulated, if at all, by [[hormones]].
 
  
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[[Image:Eptatretus stoutii heads.jpg|thumb|right|A group of [[pacific hagfish]].]]
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The [[circulatory system]]s of the extant hagfish have both closed and open [[blood vessel]]s, with a [[heart]] system that is more primitive than that of [[vertebra]]tes, bearing some resemblance to that of some [[worm]]s. 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 [[intestine]]s 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 [[kidney]]s to the body. None of these hearts are [[innervate]]d, so their function is probably modulated, if at all, by [[hormones]].
  
 
=== Slime ===  
 
=== Slime ===  
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Extant hagfish are long, [[vermiform]], and can exude copious quantities of a sticky [[slime]] or [[mucus]] (from which the typical species ''Myxine glutinosa'' was named). There are from 70 to 200 slime glands found in each of two ventrolateral lines from head to tail (Nelson 1994). The slime glands contain both mucous cells and thread cells, with the thread from the thread cells probably adding tensile strength to the slime (Nelson 1994). Indeed, the [[mucus]] excreted by hagfish is unique in that it includes strong, threadlike fibers similar to [[spider silk]], causing the slime to be fiber-reinforced. No other slime secretion known is reinforced with fibers in the same manner as hagfish slime. The fibers are about as fine as spider silk (averaging 2 [[micrometer]]s), but can be 12 centimeters long. When the [[coil]]ed fibers leave the gland, they unravel quickly to their full length without tangling.  
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Extant hagfish are long, [[vermiform]] (worm-like), and can exude copious quantities of a sticky [[slime]] or [[mucus]] (from which the typical species ''Myxine glutinosa'' was named). There are from 70 to 200 slime glands found in each of two ventrolateral lines from head to tail (Nelson 1994). The slime glands contain both mucous cells and thread cells, with the thread from the thread cells probably adding tensile strength to the slime (Nelson 1994). Indeed, the mucus excreted by hagfish is unique in that it includes strong, threadlike fibers similar to [[spider silk]], causing the slime to be fiber-reinforced. No other slime secretion known is reinforced with fibers in the same manner as hagfish slime. The fibers are about as fine as spider silk (averaging two [[micrometer]]s), but can be 12 centimeters long. When the [[coil]]ed fibers leave the gland, they unravel quickly to their full length without tangling.  
  
 
When captured and held by the tail, hagfish escape by secreting the fibrous slime, which turns into a thick and sticky gel when combined with water. They clean off the slime 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 knotting 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.
 
When captured and held by the tail, hagfish escape by secreting the fibrous slime, which turns into a thick and sticky gel when combined with water. They clean off the slime 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 knotting 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.
  
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Slime appears to be particularly effective at clogging gills of fish and thus it is speculated that sliming may be an effective defense mechanism against fish, which are not among the main hagfish predators (Lim et al. 2006).
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The slime appears to be particularly effective at clogging gills of fish and thus it is speculated that sliming may be an effective defense mechanism against [[fish]], which are not among the main hagfish predators (Lim et al. 2006).
  
 
An adult hagfish can secrete enough slime to turn a large bucket of water into gel in a matter of minutes.
 
An adult hagfish can secrete enough slime to turn a large bucket of water into gel in a matter of minutes.
  
 
==Behavior and reproduction==
 
==Behavior and reproduction==
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Hagfish tend to burrow under rocks or into mud, in highly saline [[water]]s, and away from bright light (Lee 2002). They are mostly found either near the mouths of [[river]]s or at depths of 25 or more meters, with ''Myxiine circifrons'' found at more than 1000 meters below the [[ocean]] surface (Lee 2002).
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Hagfish tend to burrow under rocks or in mud, in highly saline waters, and away from bright light (Lee 2002). They are mostly found either near the mouths of rivers or at depths of 25 or more meters, with ''Myxiine circifrons'' found at more than 1000 meters below the ocean surface (Lee 2002).
 
  
 
Hagfish typically are scavengers, entering decaying and dead [[fish]] and invertebrates (including ([[polychaete]] [[marine worm]]s and shrimp), feeding on the insides. Living organisms also are consumed. While having no ability to enter through [[skin]], they often enter through natural openings such as the [[mouth]], [[gill]]s, 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.
 
Hagfish typically are scavengers, entering decaying and dead [[fish]] and invertebrates (including ([[polychaete]] [[marine worm]]s and shrimp), feeding on the insides. Living organisms also are consumed. While having no ability to enter through [[skin]], they often enter through natural openings such as the [[mouth]], [[gill]]s, 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.
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Like [[leech]]es, they have a sluggish metabolism and can survive months between feedings.
 
Like [[leech]]es, they have a sluggish metabolism and can survive months between feedings.
  
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Very little is known about hagfish reproduction. In some species, sex ratio can be as high as 100 to 1 in favor of females. In other species, individual hagfish that are [[hermaphroditic]] are not uncommon. These individuals have both [[ovaries]] and [[testes]], but the female [[gonad]]s remain non-functional until the individual has reached a particular stage in the hagfish lifecycle. Females typically lay 20 to 30 yolky eggs that tend to aggregate due to having [[Velcro]]-like tufts at either end.
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Very little is known about hagfish reproduction. In some species, the sex ratio can be as high as 100 to 1 in favor of females. In other species, individual hagfish that are [[hermaphroditic]] are not uncommon. These individuals have both [[ovaries]] and [[testes]], but the female [[gonad]]s remain non-functional until the individual has reached a particular stage in the hagfish lifecycle. 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 [[larva]]l stage, in contrast to [[lamprey]]s, which have a long larval phase.
 
Hagfish do not have a [[larva]]l stage, in contrast to [[lamprey]]s, which have a long larval phase.
  
 
== Classification and evolution==
 
== Classification and evolution==
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[[Image:Eptatretus cirrhatus (New Zealand hagfish).gif|thumb|right|Drawing of a [[New Zealand hagfish]].]]
 
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Hagfish appear to have branched off from the chordates before the vertebral column appeared (Lee 2002). A single fossil of hagfish shows that there has been little evolutionary change in the last 300 million years (Marshall 2001). There have been claims that the hagfish eye is significant to the evolution of more complex eyes (UQ 2003).
 
Hagfish appear to have branched off from the chordates before the vertebral column appeared (Lee 2002). A single fossil of hagfish shows that there has been little evolutionary change in the last 300 million years (Marshall 2001). There have been claims that the hagfish eye is significant to the evolution of more complex eyes (UQ 2003).
  
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There has been long discussion in scientific literature about the hagfish being classified as [[vertebrate]]s versus [[invertebrate]]s. Given their classification as [[Agnatha]], Hagfish are seen as an elementary vertebrate in between [[Prevertebrate]] and [[Gnathostome]]. They tend to be classified either under the subphylum Vertebrata or as an invertebrate within subphylum [[Craniata]].  
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There has been long discussion in scientific literature about the hagfish being classified as [[vertebrate]]s versus [[invertebrate]]s. Given their classification as [[Agnatha]], Hagfish are seen as an elementary vertebrate in between [[Prevertebrate]] and [[Gnathostome]]. They tend to be classified either under the subphylum Vertebrata or as an invertebrate within subphylum [[Craniata]].  
  
 
=== Species ===  
 
=== Species ===  
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[[Image:Eptatretus minor.JPG|thumb|right|Drawing of ''[[eptatretus minor]]'']]
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[[Image:Eptatretus minor.JPG|thumb|right|Drawing of ''[[Eptatretus minor]]'']]
  
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About 66 species are known, in 7 genera. A number of the species have only been recently discovered, living at depths of several hundred meters. Some of the species are listed here:
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About 66 species are known, in seven genera. A number of the species have only been recently discovered, living at depths of several hundred meters. Some of the species are listed here:
  
 
* Genus ''[[Eptatretus]]''
 
* Genus ''[[Eptatretus]]''
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** [[New Zealand hagfish]], ''[[Eptatretus cirrhatus]]'' <small>(Forster, 1801)</small>
 
** [[New Zealand hagfish]], ''[[Eptatretus cirrhatus]]'' <small>(Forster, 1801)</small>
 
** [[Black hagfish]], ''[[Eptatretus deani]]'' <small>(Evermann & Goldsborough, 1907)</small>
 
** [[Black hagfish]], ''[[Eptatretus deani]]'' <small>(Evermann & Goldsborough, 1907)</small>
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** [[Guadalupe hagfish]], ''[[Eptatretus fritzi]]'' <small>Wisner & McMillan, 1990</small>
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** [[Guadalupe hagfish]], ''[[Eptatretus fritzi]]'' <small>(Wisner & McMillan, 1990)</small>
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** ''[[Eptatretus goliath]]'' <small>Mincarone & Stewart, 2006</small>
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** ''[[Eptatretus goliath]]'' <small>(Mincarone & Stewart, 2006)</small>
 
** [[Sixgill hagfish]], ''[[Eptatretus hexatrema]]'' <small>(Müller, 1836)</small>
 
** [[Sixgill hagfish]], ''[[Eptatretus hexatrema]]'' <small>(Müller, 1836)</small>
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** ''[[Eptatretus lopheliae]]'' <small>Fernholm & Quattrini, 2008</small><!--Copeia 2008, No. 1, 126–132—>
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** ''[[Eptatretus lopheliae]]'' <small>(Fernholm & Quattrini, 2008)</small><!--Copeia 2008, No. 1, 126–132—>
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** [[Shorthead hagfish]], ''[[Eptatretus mcconnaugheyi]]'' <small>Wisner & McMillan, 1990</small>
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** [[Shorthead hagfish]], ''[[Eptatretus mcconnaugheyi]]'' <small>(Wisner & McMillan, 1990)</small>
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** ''[[Eptatretus mendozai]]'' <small>Hensley, 1985</small>
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** ''[[Eptatretus mendozai]]'' <small>(Hensley, 1985)</small>
 
** [[Eightgill hagfish]], ''[[Eptatretus octatrema]]'' <small>(Barnard, 1923)</small>
 
** [[Eightgill hagfish]], ''[[Eptatretus octatrema]]'' <small>(Barnard, 1923)</small>
 
** [[Fourteen-gill hagfish]], ''[[Eptatretus polytrema]]'' <small>(Girard, 1855)</small>
 
** [[Fourteen-gill hagfish]], ''[[Eptatretus polytrema]]'' <small>(Girard, 1855)</small>
 
** [[Fivegill hagfish]], ''[[Eptatretus profundus]]'' <small>(Barnard, 1923)</small>
 
** [[Fivegill hagfish]], ''[[Eptatretus profundus]]'' <small>(Barnard, 1923)</small>
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** [[Cortez hagfish]], ''[[Eptatretus sinus]]'' <small>Wisner & McMillan, 1990</small>
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** [[Cortez hagfish]], ''[[Eptatretus sinus]]'' <small>(Wisner & McMillan, 1990)</small>
 
** [[Gulf hagfish]], ''[[Eptatretus springeri]]'' <small>(Bigelow & Schroeder, 1952)</small>
 
** [[Gulf hagfish]], ''[[Eptatretus springeri]]'' <small>(Bigelow & Schroeder, 1952)</small>
 
** [[Pacific hagfish]], ''[[Eptatretus stoutii]]'' <small>(Lockington, 1878)</small>
 
** [[Pacific hagfish]], ''[[Eptatretus stoutii]]'' <small>(Lockington, 1878)</small>
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** ''[[Eptatretus strickrotti]]'' <small>Møller & Jones, 2007</small>
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** ''[[Eptatretus strickrotti]]'' <small>(Møller & Jones, 2007)</small>
 
* Genus ''[[Myxine]]''
 
* Genus ''[[Myxine]]''
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** [[Patagonian hagfish]] ''[[Myxine affinis]]'' <small>Günther, 1870</small>
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** [[Patagonian hagfish]] ''[[Myxine affinis]]'' <small>(Günther, 1870)</small>
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** ''[[Myxine australis]]'' <small>Jenyns, 1842</small>
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** ''[[Myxine australis]]'' <small>(Jenyns, 1842)</small>
 
** [[Cape hagfish]], ''[[Myxine capensis]]''
 
** [[Cape hagfish]], ''[[Myxine capensis]]''
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** [[Whiteface hagfish]], ''[[Myxine circifrons]]'' <small>Garman, 1899</small>
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** [[Whiteface hagfish]], ''[[Myxine circifrons]]'' <small>(Garman, 1899)</small>
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** ''[[Myxine debueni]]'' <small>Wisner & McMillan, 1995</small>
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** ''[[Myxine debueni]]'' <small>(Wisner & McMillan, 1995)</small>
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** ''[[Myxine dorsum]]'' <small>Wisner & McMillan, 1995</small>
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** ''[[Myxine dorsum]]'' <small>(Wisner & McMillan, 1995)</small>
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** ''[[Myxine fernholmi]]'' <small>Wisner & McMillan, 1995</small>
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** ''[[Myxine fernholmi]]'' <small>(Wisner & McMillan, 1995)</small>
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** ''[[Myxine formosana]]'' <small>Mok & Kuo, 2001</small>
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** ''[[Myxine formosana]]'' <small>(Mok & Kuo, 2001)</small>
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** ''[[Myxine garmani]]'' <small>Jordan & Snyder, 1901</small>
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** ''[[Myxine garmani]]'' <small>(Jordan & Snyder, 1901)</small>
 
** Hagfish (or [[Atlantic hagfish]]), ''[[Myxine glutinosa]]''
 
** Hagfish (or [[Atlantic hagfish]]), ''[[Myxine glutinosa]]''
−
** ''[[Myxine hubbsi]]'' <small>Wisner & McMillan, 1995</small>
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** ''[[Myxine hubbsi]]'' <small>(Wisner & McMillan, 1995)</small>
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** ''[[Myxine hubbsoides]]'' <small>Wisner & McMillan, 1995</small>
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** ''[[Myxine hubbsoides]]'' <small>(Wisner & McMillan, 1995)</small>
 
** [[White-headed hagfish]], ''[[Myxine ios]]''
 
** [[White-headed hagfish]], ''[[Myxine ios]]''
−
** ''[[Myxine jespersenae]]'' <small>Møller, Feld, Poulsen, Thomsen & Thormar, 2005</small>
+
** ''[[Myxine jespersenae]]'' <small>(Møller, Feld, Poulsen, Thomsen & Thormar, 2005)</small>
−
** ''[[Myxine knappi]]'' <small>Wisner & McMillan, 1995</small>
+
** ''[[Myxine knappi]]'' <small>(Wisner & McMillan, 1995)</small>
−
** ''[[Myxine kuoi]]'' <small>Mok, 2002</small>
+
** ''[[Myxine kuoi]]'' <small>(Mok, 2002)</small>
−
** ''[[Myxine limosa]]'' <small>Girard, 1859</small>
+
** ''[[Myxine limosa]]'' <small>(Girard, 1859)</small>
−
** ''[[Myxine mccoskeri]]'' <small>Wisner & McMillan, 1995</small>
+
** ''[[Myxine mccoskeri]]'' <small>(Wisner & McMillan, 1995)</small>
−
** ''[[Myxine mcmillanae]]'' <small>Hensley, 1991</small>
+
** ''[[Myxine mcmillanae]]'' <small>(Hensley, 1991)</small>
−
** ''[[Myxine paucidens]]'' <small>Regan, 1913</small>
+
** ''[[Myxine paucidens]]'' <small>(Regan, 1913)</small>
−
** ''[[Myxine pequenoi]]'' <small>Wisner & McMillan, 1995</small>
+
** ''[[Myxine pequenoi]]'' <small>(Wisner & McMillan, 1995)</small>
−
** ''[[Myxine robinsorum]]'' <small>Wisner & McMillan, 1995</small>
+
** ''[[Myxine robinsorum]]'' <small>(Wisner & McMillan, 1995)</small>
−
** ''[[Myxine sotoi]]'' <small>Mincarone, 2001</small>
+
** ''[[Myxine sotoi]]'' <small>(Mincarone, 2001)</small>
 
* Genus ''[[Nemamyxine]]''
 
* Genus ''[[Nemamyxine]]''
−
** ''[[Nemamyxine elongata]]'' <small>Richardson, 1958</small>
+
** ''[[Nemamyxine elongata]]'' <small>(Richardson, 1958)</small>
−
** ''[[Nemamyxine kreffti]]'' <small>McMillan and Wisner, 1982</small>
+
** ''[[Nemamyxine kreffti]]'' <small>(McMillan and Wisner, 1982)</small>
 
* Genus ''[[Neomyxine]]''
 
* Genus ''[[Neomyxine]]''
 
** ''[[Neomyxine biniplicata]]'' <small>(Richardson and Jowett, 1951)</small>
 
** ''[[Neomyxine biniplicata]]'' <small>(Richardson and Jowett, 1951)</small>
Line 136: Line 131:
 
** ''[[Notomyxine tridentiger]]'' <small>(Garman, 1899)</small>
 
** ''[[Notomyxine tridentiger]]'' <small>(Garman, 1899)</small>
 
* Genus ''[[Paramyxine]]''
 
* Genus ''[[Paramyxine]]''
−
** ''[[Paramyxine atami]]'' <small>Dean, 1904</small>
+
** ''[[Paramyxine atami]]'' <small>(Dean, 1904)</small>
−
** ''[[Paramyxine cheni]]'' <small>Shen and Tao, 1975</small>
+
** ''[[Paramyxine cheni]]'' <small>(Shen and Tao, 1975)</small>
−
** ''[[Paramyxine fernholmi]]'' <small>Kuo, Huang and Mok, 1994</small>
+
** ''[[Paramyxine fernholmi]]'' <small>(Kuo, Huang and Mok, 1994)</small>
−
** ''[[Paramyxine sheni]]'' <small>Kuo, Huang and Mok, 1994</small>
+
** ''[[Paramyxine sheni]]'' <small>(Kuo, Huang and Mok, 1994)</small>
−
** ''[[Paramyxine wisneri]]'' <small>Kuo, Huang and Mok, 1994</small>
+
** ''[[Paramyxine wisneri]]'' <small>(Kuo, Huang and Mok, 1994)</small>
 
* Genus ''[[Quadratus (genus)|Quadratus]]''
 
* Genus ''[[Quadratus (genus)|Quadratus]]''
−
** ''[[Quadratus ancon]]'' <small>Mok, Saavedra-Diaz and Acero P., 2001</small>
+
** ''[[Quadratus ancon]]'' <small>(Mok, Saavedra-Diaz and Acero P., 2001)</small>
 
** ''[[Quadratus nelsoni]]'' <small>(Kuo, Huang and Mok, 1994)</small>
 
** ''[[Quadratus nelsoni]]'' <small>(Kuo, Huang and Mok, 1994)</small>
 
** ''[[Quadratus taiwanae]]'' <small>(Shen and Tao, 1975)</small>
 
** ''[[Quadratus taiwanae]]'' <small>(Shen and Tao, 1975)</small>
Line 148: Line 143:
  
 
==Importance==
 
==Importance==
−
 
 
Hagfish are important in food chains, serving as scavengers, while themselves being consumed by seabirds, [[pinniped]]s, and [[crustacean]]s (Lim et al. 2006). Fish, however, are not among their main predators (Lim et al. 2006).
 
Hagfish are important in food chains, serving as scavengers, while themselves being consumed by seabirds, [[pinniped]]s, and [[crustacean]]s (Lim et al. 2006). Fish, however, are not among their main predators (Lim et al. 2006).
  
Line 156: Line 150:
  
 
== References ==
 
== References ==
−
 
+
* Bondareva, A. A., and E. E. Schmidt. 2003. [http://mbe.oxfordjournals.org/cgi/content/abstract/20/11/1932 Early vertebrate evolution of the TATA-binding protein, TBP.] ''Molecular Biology and Evolution'' 20(11): 1932–1939. Retrieved June 1, 2008.  
−
* Bondareva, A. A., and E. E. Schmidt. 2003. [http://mbe.oxfordjournals.org/cgi/content/abstract/20/11/1932 Early vertebrate evolution of the TATA-binding protein, TBP]. ''Molecular Biology and Evolution'' 20(11): 1932–1939. Retrieved June 1, 2008.  
+
* Campbell, N. A., and J. B. Reece. 2005. ''Biology,'' 7th edition. San Francisco: Benjamin Cummings. ISBN 0536964173.
−
 
+
*  Delarbre, C., C. Gallutb, V. Barriel, et al. 2002. [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WNH-4575RHY-3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a0d701a108518c19d83eaac48d3a903b 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.
−
* Campbell, N. A., and J. B. Reece. 2005. ''Biology'', 7th edition. San Francisco, CA: Benjamin Cummings. ISBN 0536964173.
+
* Frank, T. 2004. [http://oceanexplorer.noaa.gov/explorations/04deepscope/logs/aug9/aug9.html Disgusting hagfish and magnificent sharks.] ''NOAA Ocean Explorer''. Retrieved May 31, 2008.
−
 
 
−
*  Delarbre, C., C. Gallutb, V. Barriel, et al. 2002. [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WNH-4575RHY-3&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=a0d701a108518c19d83eaac48d3a903b 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.
 
−
 
 
−
* Frank, T. 2004. [http://oceanexplorer.noaa.gov/explorations/04deepscope/logs/aug9/aug9.html Disgusting hagfish and magnificent sharks]. ''NOAA Ocean Explorer''. Retrieved May 31, 2008.
 
−
 
 
 
* Fudge, D. 2001. Hagfishes: Champions of slime. ''Nature Australia (Spring 2001): 61–69.
 
* Fudge, D. 2001. Hagfishes: Champions of slime. ''Nature Australia (Spring 2001): 61–69.
−
 
+
* Integrated Taxonomic Information System (ITIS). 2003. [http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=159693 Agnatha.] ''ITIS Taxonomic Serial No.: 159693''. Retrieved May 31, 2008.
−
* Integrated Taxonomic Information System (ITIS). 2003. [http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=159693 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.
 
* Janvier, P. 1981. The phylogeny of the Craniata, with particular reference to the significance of fossil "agnathans." ''J. Vertebr. Paleont.'' 1(2):121-159.
−
 
 
* Jørgensen, J. M., J.P. Lomholt, R. E. Weber, and H. Malte (eds.). 1998. ''The Biology of Hagfishes''. London: Chapman & Hall. ISBN 0412785307.
 
* Jørgensen, J. M., J.P. Lomholt, R. E. Weber, and H. Malte (eds.). 1998. ''The Biology of Hagfishes''. London: Chapman & Hall. ISBN 0412785307.
−
 
+
* Lee, J. 2002. [http://www.jyi.org/volumes/volume5/issue7/features/lee.html Hagfish aren't so horrible after all.] ''Journal of Young Investigators'' 5(7). Retrieved June 1, 2008.
−
* Lee, J. 2002. [http://www.jyi.org/volumes/volume5/issue7/features/lee.html Hagfish aren't so horrible after all]. ''Journal of Young Investigators'' 5(7). (Undergraduate, but peer-reviewed science journal). Retrieved June 1, 2008.
+
* Lim, J., D. S. Fudge, N. Levy, and J. M. Gosline. 2006. [http://jeb.biologists.org/cgi/content/full/209/4/702 Hagfish slime ecomechanics: Testing the gill-clogging hypothesis.] ''Journal of Experimental Biology'' 209: 702-710.  
−
 
+
* Marshall, P. M. 2001. [http://www.networksplus.net/maxmush/myxinidae.html Myxinidae information.] ''Mudminnow Information Services''. Retrieved June 1, 2008.
−
* Lim, J., D. S. Fudge, N. Levy, and J. M. Gosline. 2006. [http://jeb.biologists.org/cgi/content/full/209/4/702 Hagfish slime ecomechanics: Testing the gill-clogging hypothesis]. ''Journal of Experimental Biology'' 209: 702-710.  
+
* Mincarone, M. M., and A. L. Stewart. 2006. [http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1643%2F0045-8511%282006%296%5B225%3AANSOGS%5D2.0.CO%3B2 A new species of giant seven-gilled hagfish (Myxinidae: Eptatretus) from New Zealand.] ''Copeia'' 2006(2): 225-229. Retrieved June 1, 2008.
−
 
+
* Nelson, J. S. 1994. ''Fishes of the World,'' 3rd edition. New York: John Wiley & Sons. ISBN 0471547131.
−
* Marshall, P. M. 2001. [http://www.networksplus.net/maxmush/myxinidae.html Myxinidae information]. ''Mudminnow Information Services''. Retrieved June 1, 2008.
+
* Ressem, S. 2003. [http://www.ntnu.no/gemini/2003-06e/26-27.htm Slimy, disgusting and useful.] ''Norwegian University of Science and Technology''. Retrieved May 31, 2008.
−
 
+
* University of Queensland (UQ). 2003. [http://www.physorg.com/news115919015.html Keeping an eye on evolution.] ''PhysOrg.com''. Retrieved June 1, 2008.
−
* Mincarone, M. M., and A. L. Stewart. 2006. [http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1643%2F0045-8511%282006%296%5B225%3AANSOGS%5D2.0.CO%3B2 A new species of giant seven-gilled hagfish (Myxinidae: Eptatretus) from New Zealand]. ''Copeia'' 2006(2): 225-229. Retrieved June 1, 2008.
+
* University of Rhode Island (URI), Department of Communications. 2002. [http://www.uri.edu/news/releases/html/02-0325-01.html Friends of Oceanography Public Lecture Series explores the strange, wondrous, and disgusting hagfish.] ''University of Rhode Island'' March 25, 2002. Retrieved May 31, 2008.
−
 
+
* Vowles, A. 2007. [http://www.uoguelph.ca/atguelph/05-11-09/featuresslime.shtml From slime to "bio-steel."] ''University of Guelph''. Retrieved June 1, 2008.
−
* Nelson, J. S. 1994. ''Fishes of the World'', 3rd ed. New York: John Wiley & Sons. ISBN 0471547131.
 
−
 
 
−
* Ressem, S. 2003. [http://www.ntnu.no/gemini/2003-06e/26-27.htm Slimy, disgusting and useful]. ''Norwegian University of Science and Technology''. Retrieved May 31, 2008.
 
−
 
 
−
* University of Queensland (UQ). 2003. [http://www.physorg.com/news115919015.html Keeping an eye on evolution]. ''PhysOrg.com''. Retrieved June 1, 2008.
 
−
 
 
−
* University of Rhode Island (URI), Department of Communications. 2002. [http://www.uri.edu/news/releases/html/02-0325-01.html Friends of Oceanography Public Lecture Series
 
−
explores the strange, wondrous, and disgusting hagfish]. ''University of Rhode Island'' marcy 25, 2002. Retrieved May 31, 2008.
 
−
 
 
−
* Vowles, A. 2007. [http://www.uoguelph.ca/atguelph/05-11-09/featuresslime.shtml From slime to "bio-steel"]. ''University of Guelph''. Retrieved June 1, 2008.
 
  
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
 
[[Category:Animals]]
 
[[Category:Animals]]
−
[[Category:Fishes]]
 
  
 
{{Credit|Hagfish|214726110}}
 
{{Credit|Hagfish|214726110}}

Latest revision as of 19:07, 24 September 2008

Hagfish
Pacific hagfish resting on bottom 280 m depth off Oregon coast
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 (animals with skulls) of the class (or subphylum) Myxini, characterized by a scaleless, eel-like body that lacks both paired fins and vertebrae, but does have a cranium. Hagfish are the only animals that have a skull but not a vertebral column. Despite their lack of vertebrae, the hagfish have traditionally been classified among the vertebrates. This traditional classification is currently under dispute.

Although hagfish have an ancient history, possibly tracing back 300 million years ago, to the Carboniferous, there remain today living species of 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 eat primarily the insides of dying or dead fish and invertebrates. They are unique in being the only "vertebrate" in which the body fluids are isosmotic (having the same osmotic pressure) with seawater (Nelson 1994). Although hagfish are sometimes called "slime eels," they are not eels at all, which are part of the bony fish.

The unusual feeding habits and slime-producing capabilities of hagfish have led members of the scientific and popular media to dub the hagfish as the most "disgusting" of all sea creatures (URI 2002; Ressem 2003; Frank 2004). Nonetheless, they play an important ecological role and for humans are of value for both commerce and research. In food chains, hagfish are consumed by seabirds, pinnipeds (seals and walruses), and crustaceans such as lobsters and crabs. In some areas of the world, they are consumed by people, being important commercially in Korea for this reason. They are used also in the study of tumors (Lee 2002) and for investigating chordate relationships through genetic analyses. Scientists are exploring possible practical applications for the hagfish slime (Vowles 2007). Finally, their mysterious and unusual behaviors and forms, and their connection to an ancient past, add greatly to the wonder of nature.

Overview

A group of Pacific hagfish.

The hagfish family, Myxinidae, is the only family in the order Myxiniformes (also known as Hyperotreti), which itself is the only order in the class Myxini. Thus, hagfish is variously used for any of the three taxonomic levels (ITIS 2003; Nelson 1994).

Hagfish are jawless and generally classified with the lampreys into the superclass Agnatha (jawless vertebrates) within the subphylum Vertebrata. However, hagfish actually lack vertebrae. For this reason, they sometimes are separated from the vertebrates and not even considered to be fish. Janvier (1981) and a number of others put hagfish in a separate subphylum Myxini, which along with the subphylum Vertebrata comprises the taxon Craniata, recognizing the common possession of a cranium (Janvier 1981). Others, however, place Vertebrata and Craniata as synonyms at the same level of classification, and thereby retain hagfish (Myxini) as members of the superclass Agnatha within the vertebrates (Nelson 1994). The other living member of Agnatha, the lamprey, has primitive vertebrae made of cartilage.

Yet other classifications place Myxini as a class that in one instance lies within the subphylum Vertebrata (ITIS 2003) and, in another instance, lies within—and is the only class in—the clade Craniata, which is considered to be separate from the subphylum Vertebrata (Campbell and Reece 2005).

As members of Agnatha (Greek, "no jaws"), hagfish are characterized by the absence of jaws derived from gill arches (bone or cartilage supporting the gills), although they do 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. In addition, the gills open to the surface through pores rather than slits, and the gill arch skeleton is fused with neurocranium (the portion of the skull the protects the brain) (Nelson 1994).

Despite their name, there is some debate about whether hagfish are strictly fish, since they belong to a much more primitive lineage than any other group that is commonly defined as fish (Chondrichthyes and Osteichthyes), and because of the lack of a vertebrae commonly associated with the definition of fish. However, many authors do place hagfish as a primitive fish.

Description

Drawing of a New Zealand hagfish.

Extant hagfish are placed in the family Myxinidae within the order Myxiniformes (Hyperotreti) and subphylum or class Myxini.

Members of the order Myxiniformes are characterized by one semicircular canal, absence of eye musculature, a single olfactory capsule with few folds in the sensory epithelium, no bone, and one to 16 pairs of external gill openings (Nelson 1994).

Members of the family Myxinidae are characterized by the lack of a dorsal fin, barbels present around the mouth, degenerate eyes, teeth only on the tongue, no metamorphosis, and ovaries and testes in the same individual but only one functional gonad (Nelson 1994). Note that many of these features differ from the other agnathan, lampreys, which have one or two dorsal fins, well-developed eyes, absence of barbels, separate sexes, a larval stage that undergoes radical metamorphosis, and teeth both on the oral disc and tongue (Nelson 1994).

Hagfish have a scaleless, elongated, eel-like body without paired fins. The extant hagfish average about half a meter (18 inches) long. The largest known species is Eptatretus goliath with a specimen recorded at 127 centimeters, while Myxine kuoi and Myxine pequenoi seem to reach no more than 18 centimeters.

Extant hagfish have paddle-like tails, cartilaginous skulls, and tooth-like structures composed of keratin. Colors range from pink to blue-gray depending on the species, and may include 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 extant 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

Extant hagfish are long, vermiform (worm-like), and can exude copious quantities of a sticky slime or mucus (from which the typical species Myxine glutinosa was named). There are from 70 to 200 slime glands found in each of two ventrolateral lines from head to tail (Nelson 1994). The slime glands contain both mucous cells and thread cells, with the thread from the thread cells probably adding tensile strength to the slime (Nelson 1994). Indeed, the mucus excreted by hagfish is unique in that it includes strong, threadlike fibers similar to spider silk, causing the slime to be fiber-reinforced. No other slime secretion known is reinforced with fibers in the same manner as hagfish slime. The fibers are about as fine as spider silk (averaging two micrometers), but can be 12 centimeters long. When the coiled fibers leave the gland, they unravel quickly to their full length without tangling.

When captured and held by the tail, hagfish escape by secreting the fibrous slime, which turns into a thick and sticky gel when combined with water. They clean off the slime 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 knotting 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.

The slime appears to be particularly effective at clogging gills of fish and thus it is speculated that sliming may be an effective defense mechanism against fish, which are not among the main hagfish predators (Lim et al. 2006).

An adult hagfish can secrete enough slime to turn a large bucket of water into gel in a matter of minutes.

Behavior and reproduction

Hagfish tend to burrow under rocks or into mud, in highly saline waters, and away from bright light (Lee 2002). They are mostly found either near the mouths of rivers or at depths of 25 or more meters, with Myxiine circifrons found at more than 1000 meters below the ocean surface (Lee 2002).

Hagfish typically are scavengers, entering decaying and dead fish and invertebrates (including (polychaete marine worms and shrimp), feeding on the insides. Living organisms also are consumed. 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.

Very little is known about hagfish reproduction. In some species, the sex ratio can be as high as 100 to 1 in favor of females. In other species, individual hagfish that are hermaphroditic are not uncommon. These individuals have both ovaries and testes, but the female gonads remain non-functional until the individual has reached a particular stage in the hagfish lifecycle. 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.

Classification and evolution

Hagfish appear to have branched off from the chordates before the vertebral column appeared (Lee 2002). A single fossil of hagfish shows that there has been little evolutionary change in the last 300 million years (Marshall 2001). There have been claims that the hagfish eye is significant to the evolution of more complex eyes (UQ 2003).

There has been long discussion in scientific literature about the hagfish being classified as vertebrates versus invertebrates. Given their classification as Agnatha, Hagfish are seen as an elementary vertebrate in between Prevertebrate and Gnathostome. They tend to be classified either under the subphylum Vertebrata or as an invertebrate within subphylum Craniata.

Species

Drawing of Eptatretus minor

About 66 species are known, in seven genera. A number of the species have only been recently discovered, living at depths of several hundred meters. 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

Importance

Hagfish are important in food chains, serving as scavengers, while themselves being consumed by seabirds, pinnipeds, and crustaceans (Lim et al. 2006). Fish, however, are not among their main predators (Lim et al. 2006).

Hagfish are consumed in some regions of the world, being particularly important commercially in Korea (Lee 2002). They also are made into leather items (Lee 2002).

Hagfish also are important in scientific research. They are useful in the study of tumors (Lee 2002), and genetic analysis investigating the relationships among chordates. Research is being done regarding potential uses for their slime or a similar synthetic gel including the fibers. Some possibilities include new biodegradable polymers, space-filling gels, or a means of stopping blood flow in accident victims and surgery patients (Vowles 2007).

References
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