European eels Anguilla anguilla, copyright Dmitriy Konstantinov.

Belongs within: Anguilliformes.

The Anguilloidei are a group of eels in which the premaxillaries are fused with the mesethmoid to form a tooth-bearing bone at the front of the upper jaw (Bond 1996). They include the typical eels of the genus Anguilla, species of which spend the greater part of their post-larval life cycle in fresh water before returning to the oceans to breed.

The surprisingly mysterious eels
Published 30 April 2012
European eel Anguilla anguilla, photographed by Ron Offermans.

The eels are, without a doubt, one of the more distinctive groups of bony fishes, with their elongate snake-like bodies and linearised fins. And among the eels, perhaps the most familiar to many people are the freshwater eels of the genus Anguilla. Being able to wriggle across land on damp nights, eels can be found in a wide variety of water bodies, even small and isolated ones (such as cattle troughs). But the very familiarity of the freshwater eels disguises what are, in some ways, very poorly known animals.

First off, though, I have to provide something of a correction. Way back in 2007, in one of my earliest posts at this site, I made the comment that the deep sea gulper eels were ‘not real eels’, on the basis that they were placed in a separate order Saccopharyngiformes from the true eels of the Anguilliformes (referred to in many older texts as the Apodes, the ‘legless ones’—which is a bit of a funny feature to be focusing on when talking about a fish). Witness the misleading nature of non-phylogenetic classifications! For, as turns out, phylogenetic studies have demonstrated that gulpers are indeed ‘real eels’, with Saccopharyngiformes well-nested among the Anguilliformes (Inoue et al. 2010). Their previous separation was due not to phylogenetic distinctiveness, but just to their individual wierdness.

New Zealand long-finned eel Anguilla dieffenbachii, photographed by Gusmonkeyboy. This species is known to grow surprisingly large: the largest on record being about 24 kg (so sayeth Wikipedia). It is generally believed that such giants are females that have, for some reason, failed to develop to reproductive maturity and instead remain as juveniles.

Anywho, back to Anguilla. This genus includes some fifteen species, most of which are found around the Pacific, with four species around the Indian Ocean and two around the North Atlantic (Lecomte-Finiger 2003). Contrary to one of the opening statements in the just-quoted review, Anguilla species are not the only freshwater eels: the Indo-Pacific moray Gymnothorax polyuranodon also enters fresh water* (Ebner et al. 2011). All freshwater eels also return to the sea to breed; this is referred to as a catadromous life-cycle (as opposed to an anadromous life-cycle as found in salmon, where the fish spend part of their lives in the sea and return to fresh water to breed**). It wasn’t until the 1990s that it was discovered that some Anguilla eels spend their entire lives in the sea, and never enter fresh water (Tsukamoto et al. 1998).

*Just to confuse matters, there are also the pantropical freshwater swamp eels and spiny eels. Despite the name (and despite their superficial appearance), these are members of the percomorph radiation.

**I mention this because personally I can never remember which is which.

Marbled eel Anguilla marmorata, in the evidently excited hands of Seishi Hagihara (the eel, presumably, was somewhat less impressed). This is the only species to be found in both the Indian and Pacific Oceans.

Where the eels go once they return to the sea was long an unknown, and it wasn’t until the Danish biologist Johannes Schmidt traced the leptocephalus larvae of the European eel Anguilla anguilla across the Atlantic in the early 1920s that it was realised that they travel all the way across the Atlantic to the Sargasso Sea, close to North America. Even now the breeding locations are known for only three of the fifteen Anguilla species: the European eel Anguilla anguilla and the American eel A. rostrata both breed in the Sargasso Sea, and the Japanese eel A. japonica breeds in the Marianas Trench. Molecular dating suggests that the two Sargasso species diverged between 3.8 and 1.9 million years ago, and it has still not been established how the species became distinct. Certainly such a date would be far too recent for the once-popular suggestion that they might be the descendants of an ancestral population divided by the widening of the Atlantic. There is also evidence of a hybrid zone between the two species: eels collected from Iceland, though predominantly belonging to the European species, have been shown to have 2-4% derivation from the American species.

Polynesian long-finned eel Anguilla megastoma, from Bernhard Höller. The eel in the photo is estimated to be about 12 kg in weight. Both this species and A. marmorata are found in French Polynesia: A. marmorata is found in downstream, low-gradient parts of rivers while A. megastoma is found in upstream, higher-gradient stretches. A third species in the region, A. obscura, prefers still estuaries (Lecomte-Finiger 2003).

All fifteen Anguilla species were included in the phylogenetic analysis of Anguilliformes by Inoue et al. (2010). This analysis supported a relationship of Anguilla with a clade of mesopelagic eels containing the Serrivomeridae (sawtooth eels) and Nemichthyidae (snipe eels). Sister to all of these were our old friends the gulpers. The (admittedly limited) available evidence about the habits of Anguilla during the marine phase of their life suggests that these three lineages may form a single ancestrally pelagic clade, contrasting with the near-bottom habits of most other eels (members of the Derichthyidae, the longneck eels, represent an independent origin of pelagism).

Systematics of Anguilloidei
<==Anguilloidei [Anguilloidea]
    |--Milananguilla [Milananguillidae]P93
    |    `--M. lehmani Blot 1978P93
    |    |--Anguilloides branchiostegalis (Eastman 1905)P93
    |    `--Veronanguilla ruffoi Blot 1978P93
    |    |--Heterenchelys [incl. Pythonichthys]B96
    |    |--PanturichthysB96
    |    `--Heterenchelyidarum richardsi Nolf 1988P93
    |--Moringuidae [Ratabouridae]IM01
    |    |  i. s.: ‘Leptocephalus’ tuberculatus Castle 1965C65
    |    |--Neoconger tuberculatus (Castle 1965)S99b
    |    |--Stilbiscus Jordan & Bollman 1889 [incl. Anguillichthys Mowbray 1927]C65
    |    |    `--S. bahamensis (Mowbray 1927)C65
    |    |--AphthalmichthysM58
    |    |    |--A. bicolor (Kaup 1856) [=Moringua bicolor]M58
    |    |    |--A. floresiana (Weber & de Beaufort 1916) [=Moringua floresiana]M58
    |    |    |--A. macrocephalus Bleeker 1863 [=Moringua macrocephalus]M58
    |    |    `--A. macrochir (Bleeker 1855) [=Moringua macrochir]M58
    |    `--Moringua Gray 1831TP86
    |         |--*M. raitaborua Ham-Buch. 1822S62
    |         |--M. abbreviata (Bleeker 1863)TP86 [=Aphthalmichthys abbreviatusM58]
    |         |--‘Leptocephalus’ diptychus Eigenmann & Kennedy 1900C65
    |         |--M. edwardsiIM01
    |         |--M. javanica (Kaup 1856)TP86 [=Aphthalmichthys javanicusM58]
    |         |--M. lumbricoideaJR10
    |         |--M. microchir Bleeker 1853S62 [=Aphthalmichthys microchirM58; incl. M. ferruginea Bliss 1883S62]
    |         `--M. raitaborua Hamilton-Buchanan 1822C65
         |--Eoanguilla leptoptera (Agassiz 1835)P93
              |--A. amboinensisJR10
              |--A. anguilla (Linnaeus 1758)SE08
              |--A. annosa Stinton 1975P93
              |--A. australis Richardson 1841S99a
              |--A. bengalensisCS77
              |--A. bicolor McClelland 1844S99a
              |    |--A. b. bicolorAC90
              |    `--A. b. pacifica Schmidt 1932AC90
              |--A. celebesensis Kaup 1856 (see below for synonymy)M58
              |--A. chrysypaD56
              |--A. dieffenbachiiWH02
              |--A. ignotaG88
              |--A. interioris Whitley 1938S99a
              |--A. japonica Temminck & Schlegel 1846S99a
              |--A. malgumora Schlegel in Kaup 1846 [incl. A. borneensis]S99a
              |--A. manillensisJR10
              |--A. marmorata Quoy & Gaimard 1824AC90
              |--A. mauritiania Bennett 1831 [=Muraena mauritiania; incl. A. labiata Weber 1908]M58
              |--A. megastoma Kaup 1856S99a
              |--A. obscura Günther 1871M58
              |--A. pachyuraG88
              |--A. reinhardtii Steindachner 1867M58
              |--A. rostrata (LeSueur 1817)ST02 [incl. A. texana Günther 1870F16]
              |--A. spengeli Weber 1912M58
              `--A. vulgarisB96
Inorganic: Anguilla japonica minilorientalis Okamura 1987O87

Anguilla celebesensis Kaup 1856 [=Muraena celebesensis; incl. A. aneitensis Macleay 1880, A. otaheitensis Günther 1910]M58

*Type species of generic name indicated


[AC90] Allen, G. R., & D. Coates. 1990. An ichthyological survey of the Sepik River, Papua New Guinea. Records of the Western Australian Museum Supplement 34: 31–116.

[B96] Bond, C. E. 1996. Biology of Fishes 2nd ed. Saunders College Publishing: Fort Worth.

[C65] Castle, P. H. J. 1965. Moringuid leptocephali in Australasian waters. Transactions of the Royal Society of New Zealand, Zoology 7 (7): 125–133.

[CS77] Cramp, S., & K. E. L. Simmons (eds) 1977. Handbook of the Birds of Europe, the Middle East and North Africa: The Birds of the Western Palaearctic vol. 1. Ostrich to Ducks. Oxford University Press: Oxford.

[D56] Dawes, B. 1956. The Trematoda with special reference to British and other European forms. University Press: Cambridge.

Ebner, B. C., B. Kroll, P. Godfrey, P. A. Thuesen, T. Vallance, B. Pusey, G. R. Allen, T. S. Rayner & C. N. Perna. 2011. Is the elusive Gymnothorax polyuranodon really a freshwater moray? Journal of Fish Biology 79 (1): 70–79.

[F16] Fowler, H. W. 1916. The fishes of Trinidad, Grenada, and St. Lucia, British West Indies. Proceedings of the Academy of Natural Sciences of Philadelphia 67 (3): 520–548.

[G88] Gray, J. 1988. Evolution of the freshwater ecosystem: the fossil record. Palaeogeography, Palaeoclimatology, Palaeoecology 62: 1–214.

[IM01] Inoue, J. G., M. Miya, K. Tsukamoto & M. Nishida. 2001. Complete mitochondrial DNA sequence of Conger myriaster (Teleostei: Anguilliformes): novel gene order for vertebrate mitochondrial genomes and the phylogenetic implications for anguilliform families. Journal of Molecular Evolution 52: 311–320.

Inoue, J. G., M. Miya. M. J. Miller, T. Sado, R. Hanel, K. Hatooka, J. Aoyama, Y. Minegishi, M. Nishida & K. Tsukamoto. 2010. Deep-ocean origin of the freshwater eels. Biology Letters 6: 363–366.

[JR10] Jordan, D. S., & R. E. Richardson. 1910. Check-list of the species of fishes known from the Philippine archipelago. Bureau of Printing: Manila.

Lecomte-Finiger, R. 2003. The genus Anguilla Schrank, 1798: current state of knowledge and questions. Reviews in Fish Biology and Fisheries 13: 265–279.

[M58] Munro, I. S. R. 1958. The fishes of the New Guinea region: a check-list of the fishes of New Guinea incorporating records of species collected by the Fisheries Survey Vessel “Fairwind” during the years 1948 to 1950. Papua and New Guinea Agricultural Journal 10 (4): 97–369 (reprinted 1958. Territory of Papua and New Guinea Fisheries Bulletin no. 1).

[O87] Okamura, C. 1987. New facts: Homo and all Vertebrata were born simultaneously in the former Paleozoic in Japan. Original Report of the Okamura Fossil Laboratory 15: 347–573.

[P93] Patterson, C. 1993. Osteichthyes: Teleostei. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 621–656. Chapman & Hall: London.

[SE08] Sevcsik, A., & T. Erös. 2008. A revised catalogue of freshwater fishes of Hungary and the neighbouring countries in the Hungarian Natural History Museum (Pisces). Annales Historico-Naturales Musei Nationalis Hungarici 100: 331–383.

[S99a] Smith, D. G. 1999a. Anguillidae. Freshwater eels. In: Carpenter, K. E., & V. H. Niem (eds) The Living Marine Resources of the Western Central Pacific vol. 3. Batoid fishes, chimaeras and bony fishes part 1 (Elopidae to Linophrynidae) pp. 1630–1636. FAO: Rome.

[S99b] Smith, D. G. 1999b. Moringuidae. Spaghetti eels. In: Carpenter, K. E., & V. H. Niem (eds) The Living Marine Resources of the Western Central Pacific vol. 3. Batoid fishes, chimaeras and bony fishes part 1 (Elopidae to Linophrynidae) pp. 1637–1638. FAO: Rome.

[ST02] Smith, D. G., & K. A. Tighe. 2002. Feshwater eels. Family Anguillidae. In: Collette, B. B., & G. Klein-MacPhee (eds) Bigelow and Schroeder’s Fishes of the Gulf of Maine 3rd ed. pp. 92–95. Smithsonian Institute Press: Washington.

[S62] Smith, J. L. B. 1962. Sand-dwelling eels of the western Indian Ocean and the Red Sea. Ichthyological Bulletin 24: 447–466.

[TP86] Thinès, G., & G. Proudlove. 1986. Pisces. In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 709–733. E. J. Brill/Dr W. Backhuys: Leiden.

Tsukamoto, K., I. Nakai & W.-V. Tesch. 1998. Do all freshwater eels migrate? Nature 396: 635–636.

[WH02] Worthy, T. H., & R. N. Holdaway. 2002. The Lost World of the Moa: Prehistoric life of New Zealand. Indiana University Press: Bloomington (Indiana).

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