Hagfish
| Hagfish | ||||||||||||
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Schleimaal Eptatretus stoutii |
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| Myxinidae | ||||||||||||
| Rafinesque , 1815 |
Schleimaale or Inger (Myxinidae) are a family of vertebrates that, together with the lampreys (Petromyzontida), represent the recent representatives of the jawless round- mouthed mouth (Cyclostomata). Around 82 species are known. Hagfish are found on all coasts of the sea at depths of 30 to 2000 m.
features
External morphology
Hagfish usually reach a body length of 35 to 60 cm. Smaller species are the two Eptatretus species off the coast of Taiwan with a length of 20 to 30 cm, as well as Myxine kuoi and Myxine pequenoi , which do not grow larger than 18 cm. Eptatretus carlhubbsi off the coasts of Hawaii and Guam reaches up to 100 cm. They are elongated, worm-shaped, scaly, and usually hardly pigmented. Accordingly, their skin color varies - due to the capillary network under the skin for skin breathing - mostly between white and pink. Some species are also dark pink to red-brown in color. The tail section is flattened and has a fin edge that extends from the back to the belly and includes the rear end. The anus lies directly in front of the ventral (ventral) fin edge.
The front end is mainly recognizable by the tentacles located there . These are three pairs of large appendages and a small pair of lip tentacles that are flexible and very sensitive to the touch. The nostril is also at the very front, while the mouth is below the front end. The eyes cannot be seen because of a layer of skin covering them. They lack the lens and iris and are accordingly severely restricted in their function. However, the hagfish have light-sensitive receptors on their skin , so that they can at least perceive quantities of light. The main senses, however, are the sense of smell and touch .
The mucus is formed by numerous mucous cells in the skin of the animals, which are distributed over the entire body. The main mucous cells lie laterally in two longitudinal rows slightly belly below the midline. They are sunk into the subcutaneous tissue and are squeezed out by separate muscles within the skin. The pores are clearly visible, with the North Atlantic hagfish Myxine glutinosa there are more than a hundred pores on both sides.
anatomy
As a basal taxon of vertebrates , the hagfish have skeletal elements , which, however, are limited to a few cartilaginous structures in the head area and cartilage threads along the cellular notochord . The brain is covered almost exclusively by connective tissue and rests on the brain stem on the incipient notochord. The connective tissue goes forward into the cartilaginous nasal capsule and the nasal tube. Various clasp-like cartilages form a rudimentary head skeleton underneath. Particularly noticeable are the subnasal cartilage below the nasal passage and the large tongue cartilage, which is important for food intake. The palatal tooth is located on a palatine cartilage above the oral cavity. Other structures are the tentacle cartilage, which extends into the tentacles, the pituitary cartilage below the brain and the coronoid cartilage on the side of the oral cavity. The velar cartilage also extends to the rear to stabilize the cleft gill .
Instead of a vertically articulated jaw as in the jaw mouths (Gnathostomata), the hagfish have a couple of horizontally movable structures with tooth-like protrusions to tear off their food. These are horn teeth that sit on the front end of the tongue cartilage and form a rasp apparatus.
The central nervous system of the hagfish consists of the brain and the spinal cord lying above the notochord , which is band-shaped in them. The latter has different types of motor neurons and interneurons , which, unlike the higher vertebrates, are not arranged segmentally. Another difference is the formation of the spinal nerves : the dorsal and ventral branches only unite in the periphery and not near the spinal cord as in other vertebrates. In the brain, the olfactory system dominates with a very large olfactory bulb , while other areas of the brain are significantly smaller. A cerebellum is completely absent from the hagfish. The reduction in cranial nerve pairs to seven is also noticeable ; cranial nerves III, IV, and VI are absent because external eye muscles are absent. The lower cranial nerves XI and XII only appear in terrestrial vertebrates .
As already shown, the olfactory system of the hagfish is very well developed and represents the central sensory system. Hagfish do not have a lateral line organ that is typical for most fish ; In the head region of the Eptatretus species, however, there are sensory organs that could have a similar function. The tentacles also carry taste buds and mechanoreceptors . The eyes probably only perceive differences in brightness; other light-sensing cells are suspected to be in the animals' skin.
The respiration is via gills , end their gill slits in gills bags. In the Myxinidae the gill slits all lead to a single opening, while in the Eptatretinae they open individually to the outside. The water flow of the respiratory water is created by muscle contraction from the nostril via the anterior intestine ( gill intestine ) and through the gill pockets to the outside. The blood vessel system mainly consists of a muscular heart with a clear separation into a main chamber (ventricle) and an antechamber (atrium). This lies behind the gills and pumps the blood into the aorta ventralis (abdominal aorta), from which it is supplied with oxygen via the gill pockets and passes into the aorta dorsalis (back aorta). All parts of the body are supplied via other arteries , and the blood returns to the heart via veins . The heart's pumping capacity is supported by a number of smaller, muscular areas (cardinal, caudal and portal hearts).
The intestine is stretched and leads to the anus without grinding . The front area is designed as a gill intestine and carries the respiratory water. The digestive enzymes are released from the intestinal wall and the liver , a pancreas and a separate stomach do not exist.
distribution and habitat
With the exception of the Red Sea , the Arctic and the Antarctic Ocean, hagfish are distributed worldwide and live on the sea floor at depths of 30 to 2000 m. Their distribution depends on various abiotic factors, above all the water temperature and salinity , i.e. the salt content of the water. The water temperature must not exceed 20 ° C, the optimal temperature is around 10 ° C. That is why the animals are only found in the temperate to cold sea areas at shallower depths of around 30 m. In the warm water areas of the tropics and subtropics, however, they live at much greater depths.
In the coastal regions of the northern Atlantic, Myxine glutinosa occurs, a population of which was also carried off to the coast of South Africa. In contrast, species of the genus Eptatretus dominate in the Pacific and tropical Atlantic .
Way of life
Habitat use and nutrition
Most of the time, hagfish live buried in the sediment of the sea floor, in which they first dig their rump through meandering movements and then, slowing down, use their head to dig. They are accordingly vertical in the ground; the head is facing the tube opening at the tip of a silt cone. Populations in shallower, still light-flooded sea areas are mostly nocturnal.
Their food consists on the one hand of soil organisms such as small molluscs , worms, protozoa and bacteria, and on the other hand of the carrion of fish lying on the seabed and other larger animals. They use these carcasses very effectively. Mostly they use body openings such as the gills or the mouth to penetrate the inside of the body. However, they also attack living, injured and defenseless animals - for example fish that are caught in ground nets - thereby contributing to considerable economic damage for fishermen. The hagfish have developed a special technique for tearing off pieces of food: The extremely agile animals form a knot and pull their head through the resulting knot. This now presses on the surface and forms an abutment when ripping off and tearing off loot. In the same way, the animals can also strip off mucus residues from the front half of their bodies.
Both as sediment burrowers and as carrion eaters , the hagfish play an important ecological role in the marine areas they colonize. They collectively skeletonize a huge whale carcass that has sunk to the bottom over the course of several months.
Defense with slime
When hagfish feel threatened, they release a secretion from their mucous cells that suddenly binds large amounts of water as mucus and clogs the mouth and gills of predators. In most cases, the attackers expel the slime and the eel. The Schleimaal itself forms a knot with its body to shed the mucus. The secretion consists of up to 15 cm long protein filaments wound up in balls, which when in contact with water wind up explosively and mucin . The slime is considered to be the most efficient binding agent for water in nature; 1 gram of secretion is sufficient to form approx. 10 liters of mucus, which corresponds to a mass fraction of 0.01%. Another special feature is that no energy input is necessary for slime formation, which is why cold sea water can be bound without any problem. However, the mucus is very short-lived and not particularly stable, which is why one speaks of mucus and refrains from being called a hydrogel .
Reproduction and development
The hagfish are hermaphrodites , which means that each individual produces both female and male germ cells . The different sex cells probably mature at different times to avoid self-fertilization . The formation of the germ cells takes place in a single gonad in the rear part of the body of the animals; the products are released into the body cavity and pass through an exit to the outside.
The eggs are fertilized in open water. The clutch usually consists of several long oval eggs with a large proportion of yolk , connected by adhesive filaments . After fertilization, a discoid furrow occurs , during which a germinal disc forms on the yolk of the eggs and grows into an embryo . After development is complete, young animals hatch from the eggs, which already resemble the adult hagfish; There are no larval stages .
Tribal history
The oldest known fossils , which can be assigned to the hagfish or very closely related animal groups, come from the Upper Carboniferous . It is about 7 cm long Myxinikola siroka from the USA, which with its tentacles at the mouth opening and the proven horny teeth showed very typical characteristics of the animals. The Myllokunmingia fengjioana from China from the lower Cambrian is classified in the close relationship of the hagfish . This and also the classification of different animal groups in the further development of the tribe towards the more modern cranial animals such as the arandaspidida or the astraspidida from the middle to upper Ordovician suggest that the hagfish go back to forms that developed in the Cambrian about 545 million years ago to have.
A process described in January 2019 almost complete and well-preserved fossil of a 100 million year old Schleimaals shows that they already in the Cretaceous period were similar to the modern species and closer to the lampreys are used as the jawed vertebrates .
Contrary to earlier assumptions, the very limited functionality of the eyes of hagfish does not seem to be a result of regression. A study published in 2007 showed that the eyes are more likely to be interpreted as a particularly original (“primitive”) form in the course of the evolution of the eye .
Systematics
External system
The hagfish were and are recently again classified together with the lampreys (Petromyzontidae) in the superclass of the round mouths (Cyclostomata). With the advent of the principles of cladistics in the late 1970s, it was recognized that the round- mouthed taxon must be a paraphyletic taxon and that the lampreys are more closely related to the jaw- mouthed gnathostomata ( cartilaginous fish , bony fish and terrestrial vertebrates ) than with the hagfish. This is based on a number of common features that are said to have only formed after the hagfish split off, including above all the muscular bases of the fins, the nerve- equipped heart, the structure of the spleen and pancreas and various physiological properties.
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Molecular biological studies in recent years show, however, that the round mouths are monophyletic ; d. That is, lampreys and hagfish have a youngest common ancestral form, from which no other group emerges. They share four unique microRNA families and 15 unique paralogies between primitive microRNA families.
The round mouths are said to have developed around 500 million years ago in the Cambrian from a last common ancestor of all vertebrates, which was, however, much more complex than the round mouths. The round mouths then went through a degeneration and lost many of the characteristics typical of vertebrates, the hagfish more, the lampreys less. 300 million year old hagfish fossils are quite similar to modern forms.
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Internal system
The approximately 78 species of hagfish today are divided into three taxa, which in the classical system are classified as subfamilies within the only family Myxinidae of hagfish. The position and number of the mucous pores and horn teeth are used as characteristic features. The main distinguishing feature of the two subfamilies is the number of outwardly visible gill openings. It refers to:
- The Myxininae , with 28 species that are divided into four genera and have only one outer gill opening to which all gill ducts lead.
- the Eptatetrinae , with 50 species in one genus. This has 5 to 16 gill openings, mostly according to the number of gill pockets themselves.
- the Rubicundinae , a subfamily only newly described in 2013 with a genus with 4 species, which is characterized by elongated tubular noses and a reddish color.
Humans and hagfish
Hagfish pose a problem, especially for bottom-net fishermen , as they attack the fish caught in the nets and feed on them. Where they occur in large numbers, they can make the bottom catch almost completely unusable.
However, hagfish do not only play a role as economic pests. In the last 20 years their importance as a leather supplier has grown strongly, and today the so-called eel leather is almost exclusively made from the hides of the hagfish. For this reason, they have become popular catch fish in some coastal areas. Especially on the west coast of North America and the Asian coasts, their stocks have therefore already decreased massively. In particular, the up to 60 cm long Eptatretus atami on the coasts of Japan , Taiwan and South Korea is very popular and endangered accordingly. In Korea, hagfish are also eaten.
In recent years, hagfish have become the focus of interest for genetic analyzes that examine the relationships among chordates. It was also recently discovered that the slime released by hagfish is structurally unique in that it contains tear-resistant, thread-like fibers that have certain chemical similarities to those of spider silk . Researchers are now looking for a possible use of the ingredient in this gel or similar synthetic gels. Potential areas of application would be new biodegradable polymers , gels as fillers and agents to stop bleeding in accident victims and surgery patients.
literature
- Joseph S. Nelson : Fishes of the World. John Wiley & Sons, 2006, ISBN 0-471-25031-7 .
- Gunde Rieger, Wolfgang Maier: Myxinoida, Schleimaale, Inger. In: W. Westheide and R. Rieger: Special Zoology. Part 2: vertebrates or skulls. Spektrum, Munich 2004, ISBN 3-8274-0307-3 .
- Jørgen M. Jørgensen, JP Lomholt, RE Weber, H. Malte (Eds.): The biology of hagfishes . Chapman & Hall, London 1997, ISBN 0-412-78530-7 .
Web links
- Schleimaale on Fishbase.org (English)
- Schleimaale at the Tree of Life Web Project (English)
- Schleimaale at Oceanlink.net ( Memento from October 18, 2007 in the Internet Archive ) (English)
Individual evidence
- ^ J. Dudel, R. Menzel, RF Schmidt: Neuroscience: From Molecule to Cognition. Springer-Verlag, 2013, ISBN 3-64256-497-6 , p. 19 f, ( limited preview in the Google book search).
- ↑ Schleimaale on Fishbase.org (English)
- ^ J. Lim, D. Fudge, N. Levy, JM Gosline: Hagfish slime ecomechanics: testing the gill-clogging hypothesis . In: Journal of Experimental Biology . 209, No. 4, February 15, 2006, pp. 702-710. doi : 10.1242 / jeb.02067 .
- ↑ Lukas Böni, Peter Fischer, Lukas Böcker, Simon Kuster, Patrick A. Rühs: Hagfish slime and mucin flow properties and their implications for defense . In: Scientific Reports . 6, No. 1, July 27, 2016. doi : 10.1038 / srep30371 .
- ^ DS Fudge, N. Levy, C. Scott, JM Gosline: Composition, morphology and mechanics of hagfish slime . In: Journal of Experimental Biology . 208, No. 24, December 15, 2005, pp. 4613-4625. doi : 10.1242 / jeb.01963 .
- ↑ Tetsuto Miyashita et al .: Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological-molecular conflict in early vertebrate phylogeny. In: PNAS . Jan., 2019 doi : 10.1073 / pnas.1814794116 .
- ↑ Trevor D. Lamb et al. a .: Evolution of the vertebrate eye: opsins, photoreceptors, retina and eye cup. In: Nature Reviews Neuroscience . 8, 2007, pp. 960-976, doi : 10.1038 / nrn2283 .
- ↑ Volker Storch, Ulrich Welsch: Systematic Zoology. 5th edition, Fischer, 1997, ISBN 978-3-437-25160-3 , pp. 544-548.
- ^ Naoko Takezaki, Felipe Figueroa, Zofia Zaleska-Rutczynska, Jan Klein: Molecular Phylogeny of Early Vertebrates. Monophyly of the Agnathans as Revealed by Sequences of 35 Genes. In: Molecular Biology and Evolution. University Press, Oxford 20 (2): 2003, pp. 287-292, ISSN 0737-4038 .
- ↑ Christiane Delarbre, Cyril Gallut, Veronique Barriel, Philippe Janvier, Gabriel Gachelin: Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: The comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly. In: Molecular phylogenetics and evolution. 22 (2): 2002, pp. 184-192, doi : 10.1006 / mpev.2001.1045 .
- ↑ Shigehiro Kuraku, Kinya G. Ota, & Shigeru Kuratani, S. Blair Hedges: Jawless fishes (Cyclostomata). In: SB Hedges & S. Kumar: Timetree of Life. Oxford University Press, 2009, ISBN 978-0-19-953503-3 , pp. 317-319.
- ↑ Jon Mallatt, Christopher J. Winchell: Ribosomal RNA genes and deuterostome phylogeny revisited: More cyclostomes, elasmobranchs, reptiles, and a brittle star. In: Molecular Phylogenetics and Evolution. Volume 43, Issue 3, 2007, pp. 1005-1022, doi : 10.1016 / j.ympev.2006.11.023 .
- ↑ Jon Mallatt, J. Sullivan: 28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes. In: Mol. Biol. Evol. 15 (12): 1998, pp. 1706-18, doi : 10.1093 / oxfordjournals.molbev.a025897 .
- ↑ Alysha M. Heimberg, Richard Cowper-Sal·lari, Marie Sémon, Philip CJ Donoghue & Kevin J. Peterson: microRNAs reveal the interrelationships of hagfish, lampreys, and gnathostomes and the nature of the ancestral vertebrate. In: PNAS . Vol. 107, no. 45, 2010, pp. 19379-19383, doi : 10.1073 / pnas.1010350107 .
- ↑ a b Philippe Janvier: microRNAs revive old views about jawless vertebrate divergence and evolution. In: PNAS. Volume 107, No. 45, 2010, doi : 10.1073 / pnas.1014583107 .
- ↑ Myxininae on Fishbase.org (English)
- ↑ Eptatetrinae on Fishbase.org (English)
- ↑ a b Bo Fernholm, Michael Norén, Sven O. Kullander , Andrea M. Quattrini, Vincent Zintzen, Clive D. Roberts, Hin-Kiu Mok & Chien-Hsien Kuo: Hagfish phylogeny and taxonomy, with description of the new genus Rubicundus ( Craniata, Myxinidae). In: Journal of Zoological Systematics and Evolutionary Research. 51 (4): 2013, doi : 10.1111 / jzs.12035 .
