Eels
| Eels | ||||||||||||
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Young American eels ( Anguilla rostrata ) |
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| Systematics | ||||||||||||
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| Scientific name of the family | ||||||||||||
| Anguillidae | ||||||||||||
| Rafinesque , 1810 | ||||||||||||
| Scientific name of the genus | ||||||||||||
| Anguilla | ||||||||||||
| Cabinet , 1798 |
The eels ( Anguilla , Anguillidae, from Latin anguilla "eel", diminutive of anguis "snake"), also called freshwater eels , are a genus of bony fish and a family belonging to the order of the eel-like (Anguilliformes). They are migratory fish that migrate downstream , spend their adult life in freshwater and migrate into the sea to spawn .
distribution
Eels are found with one species each in Europe (the most well-known in the German-speaking area, the European eel , Anguilla anguilla ), in North America east of the Appalachian Mountains and in Japan and coastal China. There is also one species each in the rivers of southeastern Australia and New Zealand. In addition to these five species that live in temperate or subtropical latitudes, there are another 15 species that live in the tropics of South and Southeast Asia, in New Guinea , in eastern Africa and in the west Australia occur. These eels only colonize rivers at the mouth of which the sea drops directly into the depths, never rivers that flow into wide shallow seas. Eels are absent in the cold polar regions, in the southern Atlantic, in the eastern Pacific and the adjoining mainland (western and central North America, western, northern and central Africa, Central and South America).
features
Eels grow to a length of half a meter to two meters and have 100 to 119 vertebrae with only poorly developed appendages. Their elongated, serpentine shape is characteristic. The body is cylindrical and round in cross section, only in the rear third, after the anus, does it flatten laterally. The sideline on the head and body is fully developed. The dorsal , caudal and anal fins have grown together to form a continuous fin edge. All eels lack ventral fins , but the pectoral fins are well developed.
Fin formula : dorsal 245–275, caudal about 10, anal 205–225, pectoral 17–20.
head
The head is not separated from the body, the transition can only be seen through the position of the crescent-shaped, narrow gill openings, the lower jaw protrudes slightly. The narrow gill openings close so well that eels can survive out of the water for long periods of time without the gills drying out. The roof of the skull is mainly formed by the parietal and " squamosum " (pteroticum), the upper jaw by the maxillary and palatine . A premaxillary (intermaxillary bone) is missing. The branchiostegal rays are long and curved around the bones of the gill cover in an arc that is open in front and above . The Branchiostegal skins are broad. The size and position of the eyes are variable. Eels have two pairs of nostrils, one pair, which acts as an outflow opening, is directly in front of the eyes, the other, the inflow opening, is directly above the upper lip and has the shape of membranous tubes.
The jaws and the ploughshare are occupied by short, pointed teeth that are arranged in bands. In addition, the pharynx bones are covered with even smaller pharyngeal teeth . The tongue is fleshy and toothless.
Overall, the head is well suited for rooting thanks to its solid bones and strong muscles. Depending on the diet, the head can have a very different shape within a species (pointed head eel / broad head eel).
Integument
The elongated, up to 2 mm long cycloid scales of the eels are embedded below the glandular mucous membrane in the subcutaneous tissue (corium). The scales lie next to each other and do not overlap. Scales, the longitudinal directions of which are parallel to one another, stand together in small fields and are replaced by other scale fields in which the longitudinal directions of the scales are perpendicular to that of the preceding scales. The unicellular mucous and protein glands produce a very slippery and tough mucus that coats the entire body of the eel.
Internal organs
The heart of the eel is just behind the gill openings. As is typical of carnivorous animals, the intestines are short. It has no appendages, the stomach is not sharply demarcated from the esophagus and goes into the intestine via two pyloric valves . The elongated, spindle-shaped swim bladder takes up 30 to 50% of the abdominal cavity and is connected in front with the esophagus through the pneumatic duct . The gonads extend as long, narrow bands along the entire body cavity to behind the anus. They lie dorsally , next to the bowel and swim bladder. There are no closed fallopian tubes , the spermatic ducts open into the urinary bladder .
Reproduction
All eels spend their adult lives in freshwater and return to the sea to reproduce. Some species cover thousands of kilometers. After leaving fresh water, they no longer eat and die after releasing their eggs or sperm. The European and American eel spawn in the Sargasso Sea south of the Bermuda Islands between 20 ° and 30 ° north latitude and 80 ° and 50 ° west longitude , the Japanese eel in the western North Pacific south of Japan near Guam and the Australian short-fin eel and the New Zealand eel in the central Pacific between the Bismarck Archipelago and Fiji . The well-known spawning areas of the Southeast Asian eel species are close to the coast at depths of less than 200 meters. Eels that migrate to their spawning areas in deeper water, probably between 400 and 500 meters, get darker skin and greatly enlarged eyes.
The actual spawning process has not been observed by humans. The eggs do not sink to the bottom, but float in the open water with the help of numerous drops of oil. The pre-leptocephali, the first larval stage , also keep themselves in suspension with drops of oil in the yolk sac . Pre-leptocephali are elongated and slender. They have been caught at depths of 300 to 100 meters, while the subsequent leptocephali, or willow leaf larvae, are found between 50 meters and the surface of the water. The larval time of the tropical eels is short because of the short distance from the spawning grounds and is around two to three months. In the case of the European eel, which has the longest way from the spawning area, the larval period is extended to three years. Leptocephali are willow or bay leaf-shaped, completely transparent and have a remarkably small head. Your body is divided by the chorda dorsalis (a spine has not yet formed) into a dorsal and a ventral part, both of which are almost the same size. The number of muscle segments corresponds exactly to the later number of vertebrae. The fin edge forms gradually from front to back. Leptocephali feed on small plankton . They are phototaxic and move down during the day while they strive to the surface at night. When they reach their maximum size, the transformation to the adult animal begins via the glass eel stage .
External system
The eels belong to the order of the eel-like (Anguilliformes) and, within this order, are most closely related to the deep-sea snipe eels (Nemichthyidae), sawtooth snipe eels (Serrivomeridae) and pelican eel-like (Saccopharyngoidei). The probable family relationships are given in the following simplified cladogram .
| Anguilliformes |
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Internal system
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Relationships within the Anguillidae according to Inoue et al. 2010.
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There are currently 20 species recognized as valid.
- European eel ( Anguilla anguilla Linnaeus , 1758)
- Short-finned eel ( Anguilla australis Richardson , 1841)
- Anguilla bengalensis Gray , 1831
- Anguilla bicolor McClelland , 1844
- Borneo eel ( Anguilla borneensis ) Popta , 1924
- Celebes eel ( Anguilla celebesensis ) Kaup , 1856
- New Zealand longfin eel ( Anguilla dieffenbachii Gray , 1842)
- Anguilla interioris Whitley , 1938
- Japanese eel ( Anguilla japonica Temminck & Schlegel , 1847)
- Anguilla luzonensis Watanabe, Ayoama & Tsukamoto 2009
- Anguilla malgumora Kaup , 1856
- Indo-Pacific eel ( Anguilla marmorata Quoy & Gaimard , 1824)
- Anguilla megastoma Kaup , 1856
- Anguilla mossambica Peters , 1852
- Anguilla nebulosa McClelland , 1844
- Anguilla obscura Günther , 1872
- Anguilla reinhardtii Steindachner , 1867
- American eel ( Anguilla rostrata Le Sueur , 1821)
With Anguilla pachyura from the Miocene of Öhningen (Baden-Württemberg) at least one fossil species is known.
use
All species are used for human consumption and are important edible fish . They are sold as fresh fish, smoked or canned. Captured young eels ( glass eels ) are raised in aquaculture . In the meantime, there has been a lucrative illegal trade in glass eels from Europe to Asia.
swell
literature
- Frederich W. Tesch: The eel. 3. Edition. Ulmer, 1999, ISBN 978-3-8001-4563-8 .
- Joseph S. Nelson : Fishes of the World. John Wiley & Sons, 2006, ISBN 0-471-25031-7 .
- Kurt Fiedler: Textbook of Special Zoology. Volume II, Part 2: Fish, Gustav Fischer Verlag, Jena 1991, ISBN 978-3-334-00338-1 .
- Horst Müller: The eels. A. Ziemsen Verlag, Wittenberg 1975, ISBN 3-89432-823-1 .
- Eugen or : eel . In: Paulys Realencyclopadie der classischen Antiquity Science (RE). Volume I, 1, Stuttgart 1893, Col. 1-4.
Individual evidence
- ↑ Hideki Tanaka, Kazuharu Nomura, Yukinori Kazeto, Shigeho Ijiri, Kazuhiro Hata: Oceanic spawning ecology of freshwater eels in the western North Pacific . In: Nature Communications . tape 2 , February 1, 2011, ISSN 2041-1723 , p. 179 , doi : 10.1038 / ncomms1174 ( nature.com [accessed April 20, 2019]).
- ↑ a b Jun G. Inoue et al. : Deep-ocean origin of the freshwater eels. In: Biol. Lett. 6 (3), 2010, pp. 363-366, doi : 10.1098 / rsbl.2009.0989 .
- ↑ Anguilla on Fishbase.org (English)
- ^ Karl Albert Frickhinger: Fossil Atlas of Fishes. Mergus-Verlag, Melle, 1999, ISBN 3-88244-018-X .
- ↑ The European eel - an endangered species of fish. (PDF; 286 KB) Federal Food Safety and Veterinary Office , February 6, 2019, accessed on December 1, 2019 .
Web links
- Eels on Fishbase.org (English)