Anguilliformes

Monognathus jesperseni, from Poulsen et al. (2018).

Belongs within: Elopomorpha.
Contains: Muraenidae, Congroidei, Anguilloidei.

More really ugly fish
Published 22 August 2007
Eurypharynx pelecanoides, from Animal Diversity Web.

Far below the surface of the oceans lies the bizarre world of deep-sea fish, where life gets really ugly—because where there’s no light and no-one can see you, you can really let yourself go. I thought I’d put in a mention of what are arguably among the most bizarre of deep-sea fishes, the Saccopharyngiformes. I can’t recall when I first came across an illustration of these incredible creatures, but they’re not something you readily forget.

Leptocephalus larva, from Wikipedia.

Saccopharyngiformes are deep-sea ‘eels’. They’re not real eels (i.e. they’re not members of the order Anguilliformes), but they are closely related and like true eels are members of the clade Elopomorpha. Elopomorphs are united by a distinct planktonic larval form called a leptocephalus, with a leaf-shaped transparent form shown above. Admittedly, this photo shows a true eel rather than a saccopharyngiform eel, but the general idea’s the same except that saccopharyngiform leptocephali have the unique feature that the myomeres (the muscle blocks) are V-shaped instead of W-shaped. Saccopharyngiformes have greatly elongate jaws, attached to the neurocranium by only a single condyle. Most of the other uniting features of the order are absences: no scales, no pelvic fins, no ribs (see Fishbase for a complete list).

Cyema atrum, from Animal Biodiversity Web.

There are four families of Saccopharyngiformes. The most distinctive family is the bobtail snipe eels of the Cyematidae. Cyematidae are relatively small creatures with a distinctly cut-off appearance. Only two adult species are known but apparently the known diversity of leptocephali attributable to this family suggests the existence of more. The long jaws bend away from each other and so can’t be closed against each other, a feature shared by the unrelated but superficially similar true snipe eels of Nemichthyidae in the Anguilliformes.

The family Saccopharyngidae is the most familiar in the order (relatively speaking, of course), containing the gulper eels. Gulper eels can be extremely long, up to 2m in length, but the greater part of this (2/3 to 4/5 of the length) is taken up by the exceedingly long and filamentous tail. The remainder is dominated by the head – specifically the jaws – giving the appearance that these creatures are all mouth. How exactly that gigantic mouth is propelled by such a slender tail seems somewhat mysterious to me, and I’d love to know just how gulper eels spend their time. The tip of the tail bears an expanded, usually luminescent caudal organ. Is has been suggested that this is used for a lure to attract prey, but without life observations this is mere speculation. Male gulpers have reduced jaws and an enlarged olfactory system relative to females. Eurypharynx pelecanoides, the pelican eel, is similar to the gulpers, but is separated as its own monotypic family. Eurypharynx has an even larger mouth than the Saccopharyngidae, over half the preanal length in the former as opposed to less than 40% in the latter.

Monognathus, from Smith (2002).

The most bizarre of all the Saccopharyngiformes (and that’s saying something) are undoubtedly the one-jawed eels of Monognathus. Monognathus are the deepest-living of all elopomorphs and have been found at depths of 5400m. The head is greatly reduced, and the common name refers to the complete absence of the upper jaw. A single venomous pronged fang sticks forward from the skull where the upper jaw should be – doubtless this is used to impale prey, but as Smith (2002) notes, “ their odd morphology and their near total lack of sense organs make it difficult to imagine how they function and survive in their environment“. Like other Saccopharyngiformes, Monognathus have a distensible abdomen, the posterior part of which oftens extends in a pouch that may go past the anus.

Though fourteen species of Monognathus have been described, only a single mature male specimen has ever been recovered. This specimen differed significantly from females. The lower jaw was almost absent, the fang was blunted, the olfactory organs were greatly enlarged, a layer of spongy tissue covered the head and the dorsal and anal fins were enlongated behing the tail into a notched fin. Obviously the males completely stop feeding on reaching maturity, and become totally dedicated to finding a mate. Their short life-span as a result probably explains why specimens are so rare.

Systematics of Anguilliformes
Anguilliformes [Apodes]
    |--Muraenoidei [Muraenoidea]IM01
    |    |--MuraenidaeIM04
    |    |--ParanguillidaeP93
    |    |    |--Paranguilla tigrina (Agassiz 1839)P93 (see below for synonymy)
    |    |    `--Dalpiaziella brevicauda Cadrobbi 1962P93
    |    |--Myroconger [Myrocongridae]B96
    |    |    |--M. compressusB96
    |    |    |--M. gracilisB96
    |    |    |--M. prolixus Castle & Bearez 1995S99b
    |    |    `--M. roustami Arambourg 1967P93
    |    `--Chlopsidae [Xenocongridae]IM01
    |         |--Chilorhinus platyrhynchus (Norman 1922)S99a
    |         |--Xenoconger fryeri Regan 1912S99a
    |         |--Boehlkenchelys longidentata Tighe 1992S99a
    |         |--RobinsiaS99c
    |         |--Whitapodus Blot 1980P93, B80
    |         |    `--*W. breviculus (Agassiz 1839)B80, P93 [=Anguilla breviculaB80, Eomyrus breviculusB80]
    |         `--KaupichthysIM01
    |              |--K. atronasus Schultz 1943S99a
    |              |--K. brachychirus Schultz 1953S99a
    |              |--K. diodontus Schultz 1943S99a
    |              |--K. hyoproroides (Strömman 1896)MM14
    |              `--K. nuchalisB96
    `--+--CongroideiIM04
       `--+--AnguilloideiIM04
          `--Saccopharyngiformes [Saccopharyngoidea]IM04
               |  i. s.: Enchelion [Encheliidae]P93
               |           `--E. montium Hay 1903P93
               |--Cyematidae [Cyematoidei]B96
               |    |--Cyema atrum Günther 1878S02
               |    `--Neocyema erythrosoma Castle 1978S02
               `--SaccopharyngoideiB96
                    |--Eurypharynx [Eurypharyngidae]B96
                    |    `--E. pelecanoides Vaillant 1882S02 (see below for synonymy)
                    |--Saccopharynx [Saccopharyngidae]B96
                    |    |--S. ampullaceus (Harwood 1827)S02
                    |    |--S. harrisoni Beebe 1932S02
                    |    |--S. lavenbergi Nielsen & Bertelsen 1985S99d
                    |    |--S. ramosus Nielsen & Bertelsen 1985S99d
                    |    |--S. schmidti Bertin 1934S99d
                    |    `--S. thalassa Nielsen & Bertelsen 1985S02
                    `--MonognathusS02 [MonognathidaeB96]
                         |--M. ahlstromi Raju 1974S99f
                         |--M. bertini Bertelsen & Nielsen 1987S02
                         |--M. boehlkei Bertelsen & Nielsen 1987S02
                         |--M. brunni Bertin 1936S99f
                         |--M. herringi Bertelsen & Nielsen 1987S02
                         |--M. isaacsi Raju 1974S99f
                         |--M. jesperseni Bertin 1936S02
                         |--M. jesse Raju 1974S99f
                         |--M. nigeli Bertelsen & Nielsen 1987S02
                         |--M. ozawai Bertelsen & Nielsen 1987S99f
                         |--M. rajui Bertelsen & Nielsen 1987S99f
                         |--M. rosenblatti Bertelsen & Nielsen 1987S99f
                         |--M. smithi Bertelsen & Nielsen 1987S99f
                         `--M. taningi Bertin 1936S02
Anguilliformes incertae sedis:
  Thallassenchelys coheniB96
  ProtanguillidaeB-RB13
  Anguillavus [Anguillavidae]P93
    `--A. quadripinnis Hay 1903P93
  Urenchelys Woodward 1900W00 [UrenchelyidaeP93]
    |--*U. hakelensis (Davis 1887) [=Anguilla hakelensis]W00
    |--U. anglicus Woodward 1900W00
    `--U. germanus Hay 1903P93
  Enchelurus anglicus Woodward 1901P93
  Bolcanguilla Blot 1980B80
    `--*B. brachycephala Blot 1980B80
  Gazolapodus Blot 1980B80
    `--*G. homopterus Blot 1980B80

Eurypharynx pelecanoides Vaillant 1882S02 [incl. Gastrostomus bairdi Gill & Ryder 1883S02, G. pacificus Bean 1904S99e]

Paranguilla tigrina (Agassiz 1839)P93 [=Enchelyopus tigrinusB80; incl. P. tigrina hypsicephala Cadrobbi 1962B80, P. longicauda tigrinoides Cadrobbi 1962B80]

*Type species of generic name indicated

References

Betancur-R., R., R. E. Broughton, E. O. Wiley, K. Carpenter, A. López, C. Li, N. I. Holcroft, D. Arcila, M. Sanciangco, J. C. Cureton, II, F. Zhang, T. Buser, M. A. Campbell, J. A. Ballesteros, A. Roa-Varon, S. Willis, W. C. Borden, T. Rowley, P. C. Reneau, D. J. Hough, G. Lu, T. Grande, G. Arratia & G. Ortí. 2013. The tree of life and a new classification of bony fishes. PLoS Currents Tree of Life April 18 2013. doi: 10.1371/currents.tol.53ba26640df0ccaee75bb165c8c26288.

[B80] Blot, J. 1980. La faune ichthyologique des gisements du Monte Bolca (Province de Vérone, Italie). Catalogue systématique présentant l’état actuel des recherches concernant cette faune. Bulletin du Museum National d’Histoire Naturelle C 2: 339–396.

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

[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.

[IM04] Inoue, J. G., M. Miya, K. Tsukamoto & M. Nishida. 2004. Mitogenomic evidence for the monophyly of elopomorph fishes (Teleostei) and the evolutionary origin of the leptocephalus larva. Molecular Phylogenetics and Evolution 32: 274–286.

[MM14] Moore, G. I., S. M. Morrison, J. B. Hutchins, G. R. Allen & A. Sampey. 2014. Kimberley marine biota. Historical data: fishes. Records of the Western Australian Museum Supplement 84: 161–206.

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

[S99a] Smith, D. G. 1999a. Chlopsidae. False morays. 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. 1639–1640. FAO: Rome.

[S99b] Smith, D. G. 1999b. Myrocongridae. Thin morays. 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. 1641–1642. FAO: Rome.

[S99c] Smith, D. G. 1999c. Synaphobranchidae. Cutthroat 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. 1658–1661. FAO: Rome.

[S99d] Smith, D. G. 1999d. Saccopharyngidae. Swallower 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. 1694. FAO: Rome.

[S99e] Smith, D. G. 1999e. Eurypharyngidae. Gulper 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. 1695. FAO: Rome.

[S99f] Smith, D. G. 1999f. Monognathidae. Monognathids. 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. 1696–1697. FAO: Rome.

[S02] Smith, D. G. 2002. Families Cyematidae, Saccopharyngidae, Eurypharyngidae, Monognathidae. In: Carpenter, K. E. (ed.) FAO Species Identification Guides for Fishery Purposes, The Living Marine Resources of the Wester Central Atlantic vol. 2. Bony fishes part 1 (Acipenseridae to Grammatidae), pp. 757-763. Food and Agriculture Organization of the United Nations: Rome.

[W00] Woodward, A. S. 1900. Evidence of an extinct eel (Urenchelys anglicus, sp. n.) from the English Chalk. Annals and Magazine of Natural History, series 7, 5: 321–323, pl. 9.

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