Eels are ray-finned fish belonging to the order Anguilliformes (), 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), slime eels (class Myxini), and deep-sea spiny eels (family Notacanthidae). However, these other clades, with 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 general 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 the freshwater snake eel Stictorhinus.
Description Eels are elongated fish, ranging in length from in the one-jawed eel (Monognathus ahlstromi) to in the slender giant moray. Adults range in weight from to well over . They possess no pelvic fins, and many species also lack pectoral fins. The dorsal and anal fins are fused with the caudal fin, forming a single ribbon running along much of the length of the animal. Eels swim by generating propulsive waves that travel the length of their bodies. They can swim backward by reversing the direction of the wave.Long Jr, J. H., Shepherd, W., & Root, R. G. (Loot). Manueuverability and reversible propulsion: How eel-like fish swim forward and backward using travelling body waves". In: Proc. Special Session on Bio-Engineering Research Related to Autonomous Underwater Vehicles, 10th Int. Symp. (pp. 118–134).
The heaviest true eel is the European conger. The maximum size of this species has been reported as reaching a length of and a weight of .Conger conger, European conger: fisheries, gamefish, aquarium. Fishbase.org Other eels are longer, but do not weigh as much, such as the slender giant moray, which reaches . Most species of eels are nocturnal; they inhabit shallow waters of the ocean and burrow into sand, mud, or amongst rocks. Sometimes, they are seen living together in holes or "eel pits". Some eels also live in deeper water on the continental shelves and over the slopes deep as . Only members of the Anguilla regularly inhabit fresh water, but they, too, return to the sea to breed.
Life cycle
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 remain 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 the eel fisheries at Ballisodare were greatly improved by the hanging of loosely plaited grass ladders over barriers, enabling elvers to ascend more easily.
Classification
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 is a monophyletic group that originated among the deep-sea eels.
Taxonomy 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 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.
Extant taxa
Taxonomy based on Eschmeyer's Catalog of Fishes:
Order Anguilliformes Suborder Chlopsoidei Family Chlopsidae Rafinesque, 1815 (false morays) Suborder Synaphobranchoidei Family Protanguillidae G. D. Johnson, Ida & Miya, 2011 (primitive cave eels) Family Synaphobranchidae J. Y. Johnson, 1862 (cutthroat eels) Subfamily Simenchelyinae Gill, 1879 (pugnose parasitic eels) Subfamily Ilyophinae D. S. Jordan & Davis, 1891 (arrowtooth eels or mustard eels) Subfamily Synaphobranchinae J. Y. Johnson, 1862 (cutthroat eels) Suborder Anguilloidei Family Moringuidae Gill, 1885 (spaghetti eels) Family Anguillidae Rafinesque, 1810 (freshwater eels) Family Nemichthyidae Kaup. 1859 (snipe eels or threadtail snipe eels) Family Serrivomeridae Trewavas, 1932 (sawtooth eels) Family Cyematidae Regan, 1912 (bobtail eels) Family Monognathidae Trewavas, 1937 (onejaw gulpers) Family Neocyematidae Poulsen, M. J. Miller, Sado, Hanel, Tsukamoto & Miya, 2018 (orange bobtail eels) Family Eurypharyngidae Gill, 1883 (gulper eels or pelican eels) Family Saccopharyngidae Bleeker, 1859 (swallower eels or whiptail gulpers) Suborder Muraenoidei Family Heterenchelyidae Regan, 1912 (mud eels) Family Myrocongridae Gill, 1890 (myroconger eels) Family Muraenidae Rafinesque, 1815 (moray eels) Subfamily Uropterygiinae Fowler, 1925 (tailfin moray eels) Subfamily Muraeninae Rafinesque, 1815 (morays) Suborder Congroidei Family Colocongridae Smith, 1976 (shorttail eels) Family Derichthyidae Gill, 1884 (longneck eels or narrowneck eels) Family Ophichthidae Günther, 1870 (snake eels and worm eels) Subfamily Myrophinae Kaup, 1856 (worm eels) Subfamily Ophichthinae Günther, 1870 (snake eels) Family Muraenesocidae Kaup, 1859 (pike conger eels) Family Nettastomatidae Kaup, 1859 (duckbill eels) Family Congridae Kaup, 1856 (conger eels) Subfamily Congrinae Kaup, 1856 (congers) Subfamily Bathymyrinae Böhlke, 1949 Subfamily Heterocongrinae Günther, 1870 (garden eels)
In some classifications, 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 Phylogeny based on Johnson et al. 2012.
Extinct taxa
Based on the Paleobiology Database:
Genus †Abisaadia Genus †Bolcanguilla Genus †Eomuraena Genus †Eomyrophis Genus †Gazolapodus Genus †Hayenchelys Genus †Luenchelys Genus †Mastygocercus Genus †Micromyrus Genus †Mylomyrus Genus †Palaeomyrus Genus †Parechelus Genus †Proserrivomer Family †Anguillavidae Family †Anguilloididae Family †Libanechelyidae Family †Milananguillidae Family †Paranguillidae Family †Patavichthyidae Family †Proteomyridae Family †Urenchelyidae
Commercial species
Main commercial species Common name Scientific name Maximumlength Commonlength Maximumweight Maximumage Trophiclevel FishBase FAO ITIS IUCN status American eel Anguilla rostrata (Lesueur, 1817) 152 cm 50 cm 7.33 kg 43 years 3.7 Endangered European eel Anguilla anguilla (Linnaeus, 1758) 150 cm 35 cm 6.6 kg 88 years 3.5Anguilla anguilla (Linnaeus, 1758) FAO, Species Fact Sheet. Retrieved 20 May 2012. Critically endangered Japanese eel Anguilla japonica (Temminck & Schlegel, 1846) 150 cm 40 cm 1.89 kg 3.6Anguilla japonica, Temminck & Schlegel, 1846 FAO, Species Fact Sheet. Retrieved May 2012. Endangered Short-finned eel Anguilla australis (Richardson, 1841) 130 cm 45 cm 7.48 kg 32 years 4.1 Near Threatened
Use by humans
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 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.Acou, Anthony, et al. "Assessment of the Quality of European Silver Eels and Tentative Approach to Trace the Origin of Contaminants – A European Overview." The science of the total environment. 743 (2020): n. pag. Web. 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 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 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,"Blood serum of the eel." M. Sato. Nippon Biseibutsugakukai Zasshi (1917), 5 (No. 35), From: Abstracts Bact. 1, 474 (1917)"Hemolytic and toxic properties of certain serums." Wm. J. Keffer, Albert E. Welsh. Mendel Bulletin (1936), 8 76–80. 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.
Sustainable consumption In 2010, Greenpeace International added the European eel, Japanese eel, and American eel to its seafood red list. Japan consumes more than 70% of the global eel catch.
Etymology The English name "eel" descends from Old English , Common Germanic *ēlaz. Also from the common Germanic are West Frisian , Dutch , German , and Icelandic . Katz (1998) identifies a number of Indo-European cognates, among them the second part of the Latin word for eels, , attested in its simplex form illa (in a glossary only), and the Greek word for "eel", enkhelys (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 "eel", Old High German "snake", Lithuanian , 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 the Old English word for "hedgehog", which is (meaning "snake eater"), and perhaps in the egi- of Old High German "wall lizard".