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89-601: Hagfish , of the class Myxini / m ɪ k ˈ s aɪ n aɪ / (also known as Hyperotreti ) and order Myxiniformes / m ɪ k ˈ s ɪ n ɪ f ɔːr m iː z / , are eel -shaped jawless fish (occasionally called slime eels ). Hagfish are the only known living animals that have a skull but no vertebral column , although they do have rudimentary vertebrae . Hagfish are marine predators and scavengers who can defend themselves against other larger predators by releasing copious amounts of slime from mucous glands in their skin . Although their exact relationship to

178-425: A durophagous diet have also evolved skull morphologies to crush the hard-shelled prey that is a part of their diet. Durophagous species skulls consistently have more fused skulls and shorter jaw lengths. This morphology leads to the skulls being less kinetic than their piscivorous counterparts. Having shorter jaw lengths, with a more akinetic skull allows for an individual to have a higher bite force, compromising

267-681: A cheap dish in the United Kingdom . During the 1990s, their numbers collapsed across Europe. They became a delicacy, and the UK's most expensive species. Eels, particularly the moray eel , are popular among marine aquarists . Eel blood is toxic to humans and other mammals, but both cooking and the digestive process destroy the toxic protein. High consumption of eels is seen in European countries leading to those eel species being considered endangered. In 2010, Greenpeace International added

356-437: A few milliseconds ( filter feeding ). The fish all open their mouths and opercula wide at the same time (the red gills are visible in the photo below—click to enlarge). The fish swim in a grid where the distance between them is the same as the jump length of the copepods. Rorquals feed on plankton by a technique called lunge feeding . Lunge feeding could be regarded as a kind of inverted suction feeding, during which

445-583: A large number of microchromosomes which are lost during the animal's development, leaving only the reproductive organs with a complete genome. Hagfish possess gonadotropins which secrete from pituitary glands to the gonads to stimulate development. This suggests that hagfish have an early version of the hypothalamic–pituitary–gonadal axis , a system which once thought to be exclusive to the Gnathostomes . Some species of hagfish reproduce seasonally, stimulated by hormones from their pituitary gland. E. burgeri

534-506: A mobile premaxilla can only be seen in fishes within the teleostei clade. However, a common misconception of these fishes is that suction feeding is the only or primary method employed. In Micropterus salmoides , ram feeding is the primary method for prey capture; however, they can modulate between the two methods or use both as with many teleosts. Also, it is commonly thought that fishes with more primitive characteristics also exhibit suction feeding. Although suction may be created upon

623-416: A result of more complex skull linkages that allow greater expansion of the buccal cavity and thereby create a greater negative pressure. Most commonly, this is achieved by increasing the lateral expansion of the skull. In addition, the derived trait of anterior protrusion via the premaxillary bone in the upper jaw is acknowledged to increase the force exerted on the prey to be engulfed. Protrusible jaws via

712-502: A road accident on U.S. Highway 101 resulted in 7,500 pounds (3,400 kg) of hagfish being spilled, they emitted sufficient slime to cover the road and a nearby car. A hagfish generally respires by taking in water through its pharynx , past the velar chamber, and bringing the water through the internal gill pouches, which can vary in number from five to 16 pairs, depending on species. The gill pouches open individually, but in Myxine ,

801-461: A scavenging lifestyle, allowing them 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 leeches , they have a sluggish metabolism and can survive months between feedings; their feeding behavior, however, appears quite vigorous. Analysis of

890-401: A smaller gape with increased accuracy but the size of prey is limited. The three main tradeoffs within the fish skull have occurred because of the high kinesis in the skull and the elusiveness of some prey types. However, having kinesis in the skull can enable a predator to evolve new techniques on increasing the performance of prey capture. Ram feeding is a method of feeding underwater in which

979-411: A specialized adaptation to this feeding mode. Jaw protrusion is the outward movement of the premaxilla or mouthparts towards the prey, which is achieved via more mobile mechanical linkages in the articulations of the mouth. Vertebrate jaw protrusion is known only among modern bony fishes , which possess many forms of coupled linkages in their head. Remarkable examples are the slingjaw wrasse and

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1068-487: A swept-back mouth, as in balaenid whales, or by allowing water to flow out through the gills, as in sharks and herring . A number of species have evolved narrow snouts, as in gar fish and water snakes . Herrings often hunt copepods . If they encounter copepods schooling in high concentrations, the herrings switch to ram feeding. They swim with their mouth wide open and their opercula fully expanded. Every several feet, they close and clean their gill rakers for

1157-443: A whale takes a huge gulp of water, which is then filtered through the baleen . Biomechanically this is a unique and extreme feeding method, for which the animal at first must accelerate to gain enough momentum to fold its elastic throat ( buccal cavity ) around the volume of water to be swallowed. Subsequently, the water flows back through the baleen, keeping back the food particles. The highly elastic and muscular buccal rills are

1246-567: Is a monophyletic group that originated among the deep-sea eels. The earliest fossil eels are known from the Late Cretaceous ( Cenomanian ) of Lebanon . These early eels retain primitive traits such as pelvic fins and thus do not appear to be closely related to any extant taxa. Body fossils of modern eels do not appear until the Eocene , although otoliths assignable to extant eel families and even some genera have been recovered from

1335-502: Is a highly coordinated behavior achieved by the dorsal rotation of the dermatocranium , lateral expansion of the suspensorium, and the depression of the lower jaw and hyoid. Suction feeding leads to successful prey capture through rapid movements creating a drop in pressure in the buccal cavity causing the water in front of the mouth to rush into the oral cavity, entrapping the prey in this flow. This mode of feeding has two main phases: expansion and compression. The expansion phase involves

1424-638: Is a hypothesis that they excrete ions in bile salts. Hagfish musculature differs from jawed vertebrates in that they have neither a horizontal septum nor a vertical septum, which in jawed vertebrates are junctions of connective tissue that separate the hypaxial musculature and epaxial musculature . They do, however, have true myomeres and myosepta like all vertebrates. The mechanics of their craniofacial muscles in feeding have been investigated, revealing advantages and disadvantages of their dental plate. In particular, hagfish muscles have increased force and gape size compared to similar-sized jawed vertebrates, but lack

1513-713: Is a traditional Māori food in New Zealand . In Italian cuisine , eels from the Valli di Comacchio , a swampy zone along the Adriatic coast, are especially prized, along with freshwater eels of Bolsena Lake and pond eels from Cabras, Sardinia . In northern Germany , the Netherlands , the Czech Republic , Poland , Denmark , and Sweden , smoked eel is considered a delicacy . Elvers, often fried, were once

1602-599: Is an adjective form of an older word, ελλυ, meaning "snake", which is directly comparable to Hittite ellu-essar- "snake pit". This myth likely came to Greece via Anatolia. In the Hittite version of the myth, the dragon is called Illuyanka : the illuy- part is cognate to the word illa , and the -anka part is cognate to angu , a word for "snake". Since the words for "snake" (and similarly shaped animals) are often subject to taboo in many Indo-European (and non-Indo-European) languages, no unambiguous Proto-Indo-European form of

1691-469: Is attached to the body only along the center ridge of the back and at the slime glands, and is filled with close to a third of the body's blood volume, giving the impression of a blood-filled sack. It is assumed this is an adaptation to survive predator attacks. The Atlantic hagfish, representative of the subfamily Myxininae, and the Pacific hagfish, representative of the subfamily Eptatretinae, differ in that

1780-413: Is composed of a cartilage base bearing two teeth-covered plates (dental plates) articulated with a series of large cartilage shafts. The nasal capsule is considerably expanded in hagfish, comprising a fibrous sheath lined with cartilage rings. In contrast to lampreys, the braincase is noncartilaginous. The role of their branchial arches is still highly speculative, as hagfish embryos undergo a caudal shift of

1869-434: Is debated, but is probably in lateral adduction . These are contrasting methods for the removal of food particles from a water flow: for example, by the gill rakers of fish, the baleen of whales , or the ostia of sponges . In filter feeding , the water flow is primarily generated by the organism itself, for example by creating a pressure gradient, by active swimming, or by ciliary movements. In suspension feeding,

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1958-436: Is known to reproduce and migrate annually. While polychaete marine worms on or near the sea floor are a major food source, 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 prey from the inside. Hagfish have the ability to absorb dissolved organic matter across the skin and gill, which may be an adaptation to

2047-521: Is naked and covers the body like a loosely fitting sock. They are generally a dull pink color and look quite worm-like. They have cartilaginous skulls (although the part surrounding the brain is composed primarily of a fibrous sheath) and tooth-like structures composed of keratin . Colors depend on the species, ranging from pink to blue-grey, and black or white spots may be present. Eyes are simple eyespots, not lensed eyes that can resolve images. Hagfish have no true fins and have six or eight barbels around

2136-400: Is now considered that the hagfish neuroanatomy is similar to that of lampreys. A common feature of both cyclostomes is the absence of myelin in neurons. The brain of a hagfish has specific parts similar to the brains of other vertebrates. The dorsal and ventral muscles located towards the side of the hagfish body are connected to spinal nerves . The spinal nerves that connect to the muscles of

2225-455: Is separate from the reproductive tract, and the proximal tubule of the nephron is also connected with the coelom , providing 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. The hagfish embryo can develop for as long as 11 months before hatching, which

2314-400: Is shorter in comparison to other jawless vertebrates. Not much was known about hagfish embryology until recently, when husbandry advances enabled considerable insight into the group's evolutionary development. New insights into the evolution of neural crest cells , support the consensus that all vertebrates share these cells, which might be regulated by a common subset of genes. Their genome has

2403-542: Is used as a metaphor for Nazi atrocities, and the sight of eels being killed by a fisherman triggers the madness of the protagonist's mother. Sinister implications of eels fishing are also referenced in Jo Nesbø 's Cockroaches , the second book of the Harry Hole detective series. The book's background includes a Norwegian village where eels in the nearby sea are rumored to feed on the corpses of drowned humans, making

2492-471: Is used to clear large particles from the pharynx, a function also partly taking place through the nasopharyngeal canal. In other species, the coalescence of the gill openings is less complete, and in Bdellostoma , each pouch opens separately to the outside, as in lampreys. The unidirectional water flow passing the gills is produced by rolling and unrolling velar folds located inside a chamber developed from

2581-666: The Campanian and Maastrichtian , indicating some level of diversification among the extant groups prior to the Cretaceous-Paleogene extinction , which is also supported by phylogenetic divergence estimates. One of these otolith taxa, the mud-dwelling Pythonichthys arkansasensis , appears to have thrived in the aftermath of the K-Pg extinction, based on its abundance. Taxonomy based on Eschmeyer's Catalog of Fishes : Order Anguilliformes In some classifications,

2670-481: The European eel , Japanese eel , and American eel to its seafood red list. Japan consumes more than 70% of the global eel catch. The English name "eel" descends from Old English ǣl , Common Germanic *ēlaz . Also from the common Germanic are West Frisian iel , Dutch aal , German Aal , and Icelandic áll . Katz (1998) identifies a number of Indo-European cognates, among them

2759-878: The Hyperoartia than to the Gnathostomata—i.e., that modern jawless fishes form a clade called the Cyclostomata . The argument is that if the Cyclostomata are indeed monophyletic, Vertebrata would return to its old content ( Gnathostomata + Cyclostomata) and the name Craniata, being superfluous, would become a junior synonym. Nowadays, molecular data are almost unanimously in consensus of cyclostome monophyly, with more recent work being directed at shared microRNAs between cyclostomes and gnathostomes. The current classification supported by molecular analyses (which show that lampreys and hagfishes are sister taxa), as well as

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2848-788: The Northeast . Freshwater eels, known as Kusia in Assamese , are eaten with curry, often with herbs. The European eel and other freshwater eels are mostly eaten in Europe and the United States , and is considered critically endangered. A traditional east London food is jellied eels , although the demand has significantly declined since World War II. The Spanish cuisine delicacy angulas consists of elver (young eels) sautéed in olive oil with garlic ; elvers usually reach prices of up to 1000 euro per kg. New Zealand longfin eel

2937-456: The Paleozoic period. An experiment used an estimation of synonymous and nonsynonymous substitutions for nucleotides and supplemented that data with pre-existing data into a clock that would calculate divergence times for the taxons Myxine and Eptatretus . This data found that the lineage diverged around 93–28 Mya; however, later studies have found even earlier divergence times within

3026-487: The caudal fin, forming a single ribbon running along much of the length of the animal. Eels swim by generating waves that travel the length of their bodies. They can swim backward by reversing the direction of the wave. Most eels live in the shallow waters of the ocean and burrow into sand, mud, or amongst rocks. Most eel species are nocturnal , and thus are rarely seen. Sometimes, they are seen living together in holes or "eel pits". Some eels also live in deeper water on

3115-533: The order Anguilliformes ( / æ ŋ ˈ ɡ w ɪ l ɪ f ɔːr m iː z / ), which consists of eight suborders, 20 families , 164 genera , and about 1000 species . Eels undergo considerable development from the early larval stage to the eventual adult stage and are usually predators . The term "eel" is also used for some other eel-shaped fish, such as electric eels (genus Electrophorus ), swamp eels (order Synbranchiformes ), and deep-sea spiny eels (family Notacanthidae). However, these other clades , with

3204-432: The sand eel , which can protrude their mouth by several centimeters. This is usually done to extend the striking range of suction feeding, and the retraction of the jaw after protrusion can also help retrieval once the prey has been swallowed. Another example of protrusion is seen in dragonfly larvae (nymphs), which have hydraulic lower mandibles that can extend rapidly, protruding forward to catch prey and bring it to

3293-668: The slender giant moray , which reaches 4 m (13 ft). Eels begin life as flat and transparent larvae , called leptocephali . Eel larvae drift in the sea's surface waters, feeding on marine snow , small particles that float in the water. Eel larvae then metamorphose into glass eels and become elvers before finally seeking out their juvenile and adult habitats. Some individuals of anguillid elvers remains in brackish and marine areas close to coastlines, but most of them enter freshwater where they travel upstream and are forced to climb up obstructions, such as weirs , dam walls, and natural waterfalls. Gertrude Elizabeth Blood found that

3382-518: The Old English word for "hedgehog", which is igil (meaning "snake eater"), and perhaps in the egi- of Old High German egidehsa "wall lizard". According to this theory, the name Bellerophon ( Βελλεροφόντης , attested in a variant Ἐλλεροφόντης in Eustathius of Thessalonica ) is also related, translating to "the slayer of the serpent" ( ahihán ). In this theory, the ελλερο-

3471-650: The Paleobiology Database: Freshwater eels ( unagi ) and marine eels ( conger eel , anago ) are commonly used in Japanese cuisine ; foods such as unadon and unajū are popular, but expensive. Eels are also very popular in Chinese cuisine , and are prepared in many different ways. Hong Kong eel prices have often reached 1000 HKD (128.86 US Dollars) per kg, and once exceeded 5000 HKD per kg. In India , eels are popularly eaten in

3560-406: The ability to capture large elusive prey with more chances of failure—large gape—or to capture smaller elusive prey with greater success—smaller gape. A predator with a small mouth aperture can generate strong suction force compared to an individual with a wider gape. This was demonstrated by Wainwright et al. (2007) by comparing the feeding success of the bluegill sunfish, Lepomis macrochirus , and

3649-465: The ability to have a faster jaw opening when the jaw lengths are longer. The third main tradeoff within suction feeding occurs with the incorporation of ram feeding with suction feeding behaviors. Ram feeding involves movement of the predator with its mouth open to engulf the prey. Most species use ram feeding combined with suction feeding to increase the chances of capturing elusive prey by swimming towards their prey while using suction to draw prey into

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3738-478: The bottom, a single trawler's catch could contain several dozen or even hundreds of hagfish as bycatch, and all the other struggling, captive sea life make easy prey for them. The digestive tract of the hagfish is unique among the chordates because the food in the gut is enclosed in a permeable membrane, analogous to the peritrophic matrix of insects. They are also able to absorb nutrients directly through their skin. Hagfish have also been observed actively hunting

3827-492: The continental shelves and over the slopes deep as 4,000 m (13,000 ft). Only members of the Anguilla regularly inhabit fresh water, but they, too, return to the sea to breed. The heaviest true eel is the European conger . The maximum size of this species has been reported as reaching a length of 3 m (10 ft) and a weight of 110 kg (240 lb). Other eels are longer, but do not weigh as much, such as

3916-461: The direction of water into the mouth of the predator. With use of ram, predators are able to change the flow of water around the mouth and focus the flow of water into the mouth. But with too much ram, a bow wave is created in front of the predator which can push the prey away from the predator's body. The mouth aperture and RSI represent the overall tradeoff between having a large gape with lower accuracy but being able to capture larger prey vs. having

4005-405: The eating of these eels verge on cannibalism. Aquatic feeding mechanisms Most underwater predators combine more than one of these basic principles. For example, a typical generalized predator, such as the cod , combines suction with some amount of protrusion and pivot feeding. Suction feeding is a method of ingesting a prey item in fluids by sucking the prey into the predator's mouth. It

4094-442: The eel fisheries at Ballisodare were greatly improved by the hanging of loosely plaited grass ladders over barriers, enabling elvers to ascend more easily. Several sets of classifications of eels exist; some, such as FishBase which divide eels into 20 families, whereas other classification systems such as ITIS and Systema Naturae 2000 include additional eel families, which are noted below. Genomic studies indicate that there

4183-481: The eggs are laid in burrow beneath the sand, and the slime produced by the hagfish is used to hold the eggs in a small area. It is worth noting that no direct evidence has been found to support any of these hypotheses. Hagfish do not have a larval stage, in contrast to lampreys . Hagfish have a mesonephric kidney and are often neotenic of their pronephric kidney . The kidney(s) are drained via mesonephric/ archinephric duct . Unlike many other vertebrates, this duct

4272-400: The exception of deep-sea spiny eels, whose order Notacanthiformes is the sister clade to true eels, evolved their eel-like shapes independently from the true eels. As a main rule, most eels are marine. Exceptions are the catadromous genus Anguilla and the freshwater moray , which spend most of their life in freshwater, the anadromous rice-paddy eel , which spawns in freshwater, and

4361-483: The eyes are partly covered by the trunk musculature. Paleontological evidence suggests, however, that the hagfish eye is not plesiomorphic but rather degenerative, as fossils from the Carboniferous have revealed hagfish-like vertebrates with complex eyes. This would suggest that ancestrally Myxini possessed complex eyes. Hagfish are known to have one of the lowest blood pressures among the vertebrates. One of

4450-444: The fact that hagfishes do, in fact, have rudimentary vertebrae, which places hagfishes in Cyclostomata. Hagfish are in the group Cyclostomata which includes jawless fish. The group Cyclostomata is characterized by two significant characteristics; keratinous tooth plates and movement of postotic myomeres to the orbitals. According to fossil record, hagfish and lampreys have been estimated to have diverged from one another during

4539-918: The family Cyematidae of bobtail snipe eels is included in the Anguilliformes, but in the FishBase system that family is included in the order Saccopharyngiformes . The electric eel of South America is not a true eel but is a South American knifefish more closely related to the carps and catfishes . Phylogeny based on Johnson et al. 2012. Protanguillidae Synaphobranchidae Heterenchelyidae Myrocongridae Muraenidae [REDACTED] Chlopsidae Derichthyidae Nettastomatidae Congridae [REDACTED] Ophichthidae Muraenesocidae Moringuidae Eurypharyngidae Saccopharyngidae [REDACTED] Monognathidae Cyematidae [REDACTED] Nemichthyidae Serrivomeridae Anguillidae [REDACTED] Based on

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4628-425: The freshwater snake eel Stictorhinus . Eels are elongated fish, ranging in length from 5 cm (2 in) in the one-jawed eel ( Monognathus ahlstromi ) to 4 m (13 ft) in the slender giant moray . Adults range in weight from 30 g (1 oz) to well over 25 kg (55 lb). They possess no pelvic fins , and many species also lack pectoral fins . The dorsal and anal fins are fused with

4717-565: The group, with Myxine and Eptatretus diverging during the Triassic and the ancestor of Rubicundus diverging during other extant hagfishes during the Permian . Hagfish are excluded from the subphylum Gnathostomata because of morphological characteristics including the hagfish arched tongue. Hagfish embryos have characteristics of gnathostomes and may be plesiomorphic; however, these characteristics drastically change morphologically as

4806-452: The hagfish heart was not innervated (as the hearts of jawed vertebrates are), but further investigation revealed that the hagfish does have a true innervated heart. The hagfish circulatory system also includes multiple accessory pumps throughout the body, which are considered auxiliary "hearts". Hagfish are the only known vertebrates with osmoregulation isosmotic to their external environment. Their renal function remains poorly described. There

4895-571: The hagfish matures. The following hagfish and lamprey phylogeny is an adaptation based on the 2019 work of Miyashita et al. † Haikouella † Haikouichthys † Myllokunmingia † Metaspriggina Gnathostomata (jawed fish) † Cornovichthys † Achanarella † Ciderius † Birkeniida † Lasanius † Euphanerops † Jamoytius † Pipiscius † Conodonta (conodonts) † Myxinikela † Tethymyxine tapirostrum Rubicundus eos Rubicundus lopheliae Eel see text Eels are ray-finned fish belonging to

4984-511: The hagfish to avoid suffocation. Because of the mucus, few marine predators target the hagfish. Other predators of hagfish are varieties of birds or mammals. Free-swimming hagfish also slime when agitated, and later clear the mucus using 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. Hagfish thread keratin ( EsTKα and EsTKγ; Q90501 and Q90502 ),

5073-412: The initial opening of the jaws to capture prey. These movements during the expansion phase are similar across all suction feeders with the kinesis of the skull leading to slight variations. During the compression phase the jaws close and water is compressed out of the gills. Though suction feeding can be seen across fish species, those with more cranial kinesis show an increase in suction potential as

5162-408: The interior of the body cavity of larger carcasses. A healthy larger sea creature likely would be able to outfight or outswim this sort of assault. 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

5251-429: The largemouth bass, Micropterus salmoides . L. macrochirus has a smaller gape and was found to have higher accuracy with higher flow velocity and acceleration while M. salmoides has a larger gape with lower accuracy and lower flow velocity and acceleration. However, with the larger gape the largemouth bass were able to capture larger elusive prey. Using ram feeding in combination with suction feeding can also influence

5340-528: The latter has muscle fibers embedded in the skin. The resting position of the Pacific hagfish also tends to be coiled, while that of the Atlantic hagfish is stretched. Hagfish can exude copious quantities of a milky and fibrous slime or mucus , from specialized slime glands. When released in seawater, the slime expands to 10,000 times its original size in 0.4 seconds. This slime that hagfish excrete has very thin fibers that make it more durable and retentive than

5429-464: The mid- Cretaceous around 100 million years ago. Hagfish are typically about 50 cm (19.7 in) in length. The largest-known species is Eptatretus goliath , with a specimen recorded at 127 cm (4 ft 2 in), while Myxine kuoi and Myxine pequenoi seem to reach no more than 18 cm (7.1 in). Some have been seen as small as 4 cm (1.6 in). Hagfish have elongated, eel-like bodies, and paddle-like tails. The skin

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5518-506: The most primitive types of fluid balance found in animals is among these creatures; whenever a rise in extracellular fluid occurs, the blood pressure rises and this, in turn, is sensed by the kidney, which excretes excess fluid. They also have the highest blood volume to body mass of any chordate, with 17 ml of blood per 100 g of mass. The hagfish circulatory system has been of considerable interest to evolutionary biologists and present day readers of physiology. Some observers first believed that

5607-419: 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 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. Its skin

5696-451: The mouth opening in such fishes, the criteria for pure suction feeding includes little or no bodily movement towards their prey. The morphologies and behaviors during suction feeding have led to three main proposed tradeoffs that determine the success of prey capture: the rate of jaw opening and closing, the mobility of the bony elements in the skull, and the ratio of ram to suction feeding behavior. The first two qualifications center around

5785-516: The mouth. This diversity in relative use is quantified using the Ram Suction Index (RSI) that calculates the ratio of use for ram and suction during prey capture. The RSI ratio can be influenced by the morphology of the predator and by the elusiveness of the prey. Ram feeding and suction feeding are on opposite sides of the feeding spectrum, where extreme ram feeding is when a predator swims over an immobile prey item with open jaws to engulf

5874-520: The nasohypophyseal tract, and is operated by a complex set of muscles inserting into cartilages of the neurocranium, assisted by peristaltic contractions of the gill pouches and their ducts. Hagfish also have a well-developed dermal capillary network that supplies the skin with oxygen when the animal is buried in anoxic mud, as well as a high tolerance for both hypoxia and anoxia, with a well-developed anaerobic metabolism. The skin has also been suggested to be capable of cutaneous respiration . The origins of

5963-488: The only other living group of jawless fish, the lampreys , was long the subject of controversy, genetic evidence suggests that hagfish and lampreys are more closely related to each other than to jawed vertebrates , thus forming the clade Cyclostomi . The oldest-known stem group hagfish are known from the Late Carboniferous , around 310 million years ago, with modern representatives first being recorded in

6052-412: The openings have coalesced, with canals running backwards from each opening under the skin, uniting to form a common aperture on the ventral side known as the branchial opening. The esophagus is also connected to the left branchial opening, which is therefore larger than the right one, through a pharyngocutaneous duct (esophageocutaneous duct), which has no respiratory tissue. This pharyngocutaneous duct

6141-461: The pharyngeal wall grow individually to reach them. The hagfish eye lacks a lens, extraocular muscles , and the three motor cranial nerves (III, IV, and VI) found in more complex vertebrates, which is significant to the study of the evolution of more complex eyes . A parietal eye is also absent in extant hagfish. Hagfish eyespots, when present, can detect light, but as far as it is known, none can resolve detailed images. In Myxine and Neomyxine ,

6230-412: The posterior pharyngeal pouches; thus, the branchial arches do not support gills. While parts of the hagfish skull are thought to be homologous with lampreys, they are thought to have very few elements homologous with jawed vertebrates. Very little is known about hagfish reproduction. Obtaining embryos and observing reproductive behavior are difficult due to the deep-sea habitat of many hagfish species. In

6319-425: The predator moves forward with its mouth open, engulfing the prey along with the water surrounding it. During ram feeding, the prey remains fixed in space, and the predator moves its jaws past the prey to capture it. The motion of the head may induce a bow wave in the fluid which pushes the prey away from the jaws, but this can be avoided by allowing water to flow through the jaw. This can be accomplished by means of

6408-482: The prey. Extreme suction feeding is demonstrated by sit-and-wait predators that rely on rapid depression of the jaws to capture prey (e.g. frogfish, Antennariidae ). There is wide diversity on how much of each feeding strategy an individual uses, especially when body ram movements are considered. The relative use of ram and suction feeding is species dependent, but it can help determine the accuracy of prey capture. The mouth aperture represents another tradeoff between

6497-476: The protein that make up its slime filaments, is under investigation as an alternative to spider silk for use in applications such as body armor. These alpha-keratin proteins in hagfish slime transform from an α-helical structure to a stiffer β sheet structure when stretched. With combined draw-processing (stretching) and chemical crosslinking, recombinant slime keratin turns into a very strong fiber with an elastic modulus reaching 20 GPa. When in 2017

6586-471: The reason for unequal sex ratios. Hagfish testis are relatively small. Depending on species, females lay from one to 30 tough, yolky eggs. These tend to aggregate due to having Velcro -like tufts at either end. It is unclear how hagfish go about laying eggs, although researchers have proposed three hypotheses based on observations of the low percentage of males and small testis. The hypotheses are that female hagfish lay eggs in small crevices in rock formations,

6675-751: The red bandfish, Cepola haastii , in its burrow, possibly using their slime to suffocate the fish before grasping it with their dental plates and dragging it from the burrow. Originally, Myxine was included by Linnaeus ( 1758 ) in Vermes . The fossil hagfish Myxinikela siroka , from the Late Carboniferous of the United States, is the oldest-known member of the group. It is in some respects more similar to lampreys, but shows key autapomorphies of hagfish. In recent years, hagfish have become of special interest for genetic analysis investigating

6764-409: The relationships among chordates . Their classification as agnathans places hagfish as elementary vertebrates in between invertebrates and gnathostomes . However, discussion has long occurred in scientific literature about whether the hagfish were even invertebrate . Using fossil data, paleontologists posited that lampreys are more closely related to gnathostomes than hagfish. The term "Craniata"

6853-766: The second part of the Latin word for eels, anguilla , attested in its simplex form illa (in a glossary only), and the Greek word for "eel", egkhelys (the second part of which is attested in Hesychius as elyes ). The first compound member, anguis ("snake"), is cognate to other Indo-European words for "snake" (compare Old Irish escung "eel", Old High German unc "snake", Lithuanian angìs , Greek ophis, okhis , Vedic Sanskrit áhi , Avestan aži , Armenian auj, iž , Old Church Slavonic *ǫžь , all from Proto-Indo-European *h₁ogʷʰis ). The word also appears in

6942-488: The situation that results from a highly kinetic skull. Having a highly mobile skull introduces a tradeoff between the ability to have high speed jaw opening (high kinesis) or higher bite transmission (lower kinesis). While there is a more complex relationship between mechanical advantage and the speed of lower jaw depression, there is consensus that species using high-speed attacks have more cranial kinesis compared to species that exhibit low speed attacks. Species that have

7031-447: The slime by the travelling-knot. Recently, the slime was reported to entrain water in its keratin -like intermediate filaments excreted by gland thread cells , creating a slow-to-dissipate, viscoelastic substance, rather than a simple gel. It has been shown to impair the function of a predator fish's gills . In this case, the hagfish's mucus would clog the predator's gills, disabling their ability to respire. The predator would release

7120-617: The slime excreted by other animals. The fibers are made of proteins and also make the slime flexible. If they are caught by a predator, they can quickly release a large amount of slime to escape. If they remain captured, they can tie themselves in an overhand knot , and work their way from the head to the tail of the animal, scraping off the slime and freeing themselves from their captor. Rheological investigations showed that hagfish slime viscosity increases in elongational flow which favors gill clogging of suction feeding fish , while its viscosity decreases in shear which facilitates scraping off

7209-488: The speed amplification given by jawed vertebrates' muscles, suggesting that jaws are faster acting than hagfish dental plates. The hagfish skeleton comprises the skull, the notochord , and the caudal fin rays. The first diagram of the hagfish endoskeleton was made by Frederick Cole in 1905. In Cole's monograph, he described sections of the skeleton that he termed "pseudo-cartilage", referring to its distinct properties compared to jawed chordates. The lingual apparatus of hagfish

7298-429: The speed of pivot feeding is in a locking mechanism , in which the hyoid arch is folded under the head and is aligned with the urohyal which connects to the shoulder girdle . A four-bar linkage at first locks the head in a ventrally bent position by the alignment of two bars. The release of the locking mechanism jets the head up and moves the mouth toward the prey within 5–10 ms. The trigger mechanism of unlocking

7387-456: The stomach content of several species has revealed a large variety of prey, including polychaetes , shrimp, hermit crabs , cephalopods , brittle stars , bony fishes , sharks, birds, and whale flesh. 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 chunks of flesh from carrion fish or cetaceans, eventually making an opening to permit entry to

7476-410: The top jaw. Pivot feeding is a method to transport the mouth towards the prey by an upward turning of the head, which is pivoting on the neck joint. Pipefish such as sea horses and sea dragons are specialized on this feeding mechanism. With prey capture times of down to 5 ms ( shrimpfish Centriscus scutatus ) this method is used by the fastest feeders in the animal kingdom. The secret of

7565-874: The town is located. The daylight passage in the spring of elvers upstream along the Thames was at one time called "eel fare". The word 'elver' is thought to be a corruption of "eel fare". A famous attraction on the French Polynesian island of Huahine (part of the Society Islands ) is the bridge across a stream hosting three- to six-foot-long eels, deemed sacred by local culture. Eel fishing in Nazi -era Danzig plays an important role in Günter Grass ' novel The Tin Drum . The cruelty of humans to eels

7654-413: The vertebrate nervous system are of considerable interest to evolutionary biologists, and cyclostomes (hagfish and lampreys) are an important group for answering this question. The complexity of the hagfish brain has been an issue of debate since the late 19th century, with some morphologists suggesting that they do not possess a cerebellum , while others suggest that it is continuous with the midbrain . It

7743-429: The wild females outnumber males, with the exact sex-ratio differing depending on the species. E. burgeri , for example, has nearly a 1:1 ratio, while M. glutinosa females are significantly more common than males. Some species of hagfish are sexually undifferentiated before maturation, and possess gonadal tissue for both ovaries and testis. It has been suggested that females develop earlier than males, and that this may be

7832-554: The word for eel can be reconstructed. It may have been *ēl(l)-u- , *ēl(l)-o- , or something similar. The large lake of Almere , which existed in the early Medieval Netherlands , got its name from the eels which lived in its water (the Dutch word for eel is aal or ael , so: " ael mere " = "eel lake"). The name is preserved in the new city of Almere in Flevoland , given in 1984 in memory of this body of water on whose site

7921-465: Was used to refer to animals that had a developed skull, but were not considered true vertebrates. Molecular evidence in the early 1990s first began suggesting that lampreys and hagfish were more closely related to each other than to gnathostomes. The validity of the taxon "Craniata" was further examined by Delarbre et al. (2002) using mtDNA sequence data, concluding the Myxini are more closely related to

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