Shortbelly eel Dysomma anguillare, copyright Kim In Young.

Belongs within: Anguilliformes.
Contains: Congroidea, Ophichthidae.

A parasitic eel?
Published 24 April 2017

The following post was inspired by an e-mail that I was sent recently by Sebastian Marquez. He told me about a friend of his catching a trevally when fishing, then cutting it open to find a snake eel inside the body cavity (but outside the stomach), wrapped around the trevally’s internal organs. According to Sebastian, the lead suspicion for what had happened was that the eel had somehow burst out of the trevally’s stomach before it was caught, and he wanted to know if I’d ever heard of anything similar. I didn’t have an explanation for him, but his story did get me thinking about the snub-nosed eel.

Snub-nosed eel Simenchelys parasitica, from Jordan (1907).

The snub-nose eel Simenchelys parasitica is a small deep-sea eel, about 20 to 35 centimetres long. It has attracted note by being found a number of times burrowed into the body cavity of larger fishes with perhaps the most renowned case being two juveniles that were found nested inside the heart of a mako shark. This lead to the description of S. parasitica as an endoparasite (hence the species name). However, acceptance of this tag has been far from universal. The snub-nosed eel has been caught free-living more regularly than it has been found in other fish and because of its deep-sea habitat it has never been observed in life. An alternative suggestion has been that Simenchelys is normally a scavenger; because many of its recorded ‘hosts’ have been collected through non-targeted methods such as trawls, it is not impossible that the snub-nosed eels may have burrowed into their body cavity after they were already deceased.

It was with this conundrum in mind that the cranial anatomy of the snub-nosed eel was described by Eagderi et al. (2016). The jaws of Simenchelys are relatively short and muscular (hence its ‘snub nose’). It also has teeth arranged in such a way that they form an even cutting edge (in contrast to the more spaced and uneven teeth of other eels). Eadgeri et al. came to the conclusion that the snub-nosed eel probably feeds by biting out plugs of flesh, in a similar manner to a cookie-cutter shark. Simenchelys also resembles a cookie-cutter in having large, fleshy lips that are probably used to form a seal between jaws and food source. A large hyoid (‘tongue’) apparatus probably works to provide suction to maintain the seal. The snub-nosed eel may also rotate while biting, a behaviour known from both cookie-cutters and other eels.

So is Simenchelys a parasite? It is probably not a habitual endoparasite, lacking as it does any clear adaptations to the endoparasitic lifestyle. There are fish that could be described as ectoparasites, in that they habitually feed on live animals larger than themselves in a manner that does not normally lead to the host’s death. The cookie-cutter is one such fish; another is the candiru Vandellia cirrhosa, a small freshwater catfish from the Amazon basin that feeds on blood from the gills of other fish. It is possible that the snub-nosed eel could have a similar lifestyle to one of these. However, recorded evidence of its habits is even more consistent with scavengers such as hagfish and the candiru-açu Cetopsis candiru (another South American catfish) that tear flesh from the submerged bodies of dead animals, and may often burrow their way into the corpse’s body cavity as they do so.

Of course, the two modes of feeding are not mutually exclusive. The only difference between predator and parasite in this scenario is whether the attacked animal is alive or dead, and the thing about flesh-feeders is that they’re not always picky. A habitual scavenger may easily choose the opportunity to take a nibble from a still-living host, especially is said host is in some way incapacited (as a result of being swept up by a trawl, for instance). The snub-nosed eel may not be a habitual parasite, but it may be an opportunistic one.

Systematics of Congroidei
    |  `--+--OphichthidaeIM04
    |     `--Heterocongridae [Heterocongrinae]NE12
    |          |--GorgasiaS99c
    |          |    |--G. barnesi Robison & Lancraft 1984S99c
    |          |    |--G. maculata Klausewitz & Eibl-Eibesfeldt 1959S99c
    |          |    |--G. neocepaea Böhlke 1951S99c
    |          |    |--G. preclara Böhlke & Randall 1981S99c
    |          |    `--G. taiwanensisIM04
    |          `--HeterocongerNE12
    |               |--H. chapmani (Herre 1923)S99c
    |               |--H. cobra Böhlke & Randall 1981S99c
    |               |--H. hassi (Klausewitz & Eibl-Eibesfeldt 1959)S99c
    |               |--H. lentiginosus Böhlke & Randall 1981S99c
    |               |--H. perissodon Böhlke & Randall 1981S99c
    |               |--H. polyzona Bleeker 1868S99c
    |               `--H. taylori Castle & Randall 1995S99c
    `--Synaphobranchidae [Dyssommatidae, Dyssomminidae, Nettodariidae, Synaphobranchoidea]IM04
         |--Simenchelys [Simenchelyidae, Simenchelyinae]S99a
         |    `--S. parasitica Gill 1879S99a
         |--Synaphobranchus [Synaphobranchinae]TS02
         |    |--S. affinis Günther 1877S99a
         |    |--S. bathybius Günther 1877S99a
         |    |--S. brevidorsalis Günther 1887M58
         |    |--S. bruuni (Castle 1964)S99a
         |    |--S. capensis (Barnard 1923)S99a
         |    |--S. infernalisB96
         |    `--S. kaupi Johnson 1862S99a
              |--Dysommina rugosa Ginsburg 1951S99a
              |--Thermobiotes mytelogeiton Geistdorfer 1991S99a
              |    |--I. arx Robins & Robins 1976S99a
              |    `--I. brunneus Gilbert 1891S99a
              |    |--M. abyssalis (Kamohara 1938)S99a
              |    `--M. roseni Mok, Lee & Chan 1991S99a
                   |--D. anguillare Barnard 1923S99a
                   |--D. brevirostre (Facciola 1887)S99a
                   |--D. bucephalus Alcock 1889S99a
                   |--D. dolichosomatum Karrer 1982S99a
                   |--D. fuscoventralis Karrer & Klausewitz 1982S99a
                   |--D. goslinei Robins & Robins 1976S99a
                   |--D. melanurum Chen & Weng 1967S99a
                   |--D. muciparus (Alocock 1891)S99a
                   |--D. opisthoproctus Chen & Mok 1995S99a
                   `--D. polycatodon Karrer 1982S99a
Congroidei incertae sedis:
  Coloconger [Colocongridae]C60
    |--C. japonicus Machida 1984S99b
    |--C. raniceps Alcock 1889S99b
    `--C. scholesi Chan 1967S99b
  Anago anagoIM04
  Xyrias revulsusIM04
  Proteomyrus [Proteomyridae]P93
    `--P. ventralis (Agassiz 1839)P93 [=Anguilla ventralisB80; incl. Eomyrus brevicaudus Cadrobbi 1962B80]
  Parechelus [Parechelidae]P93
    `--P. prangei Casier 1967P93

*Type species of generic name indicated


[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.

[C60] Castle, P. H. J. 1960. Two eels of the genus Pseudoxenomystax from New Zealand waters. Transactions of the Royal Society of New Zealand 88 (3): 463–472.

Eagderi, S., J. Christiaens, M. Boone, P. Jacobs & D. Adriaens. 2016. Functional morphology of the feeding apparatus in Simenchelys parasitica (Simenchelyinae: Synaphobranchidae), an alleged parasitic eel. Copeia 104 (2): 421–439.

[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.

[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 1).

[NE12] Near, T. J., R. I. Eytan, A. Dornburg, K. L. Kuhn, J. A. Moore, M. P. Davis, P. C. Wainwright, M. Friedman & W. L. Smith. 2012. Resolution of ray-finned fish phylogeny and timing of diversification. Proceedings of the National Academy of Sciences of the USA 109 (34): 13698–13703.

[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. 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.

[S99b] Smith, D. G. 1999b. Colocongridae. Colocongrids. 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. 1670. FAO: Rome.

[S99c] Smith, D. G. 1999c. Congridae. Conger 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. 1680–1687. FAO: Rome.

[TS02] Tighe, K. A., & D. G. Smith. 2002. Cutthroat eels. Family Synaphobranchidae. In: Collette, B. B., & G. Klein-MacPhee (eds) Bigelow and Schroeder’s Fishes of the Gulf of Maine 3rd ed. pp. 95–97. Smithsonian Institute Press: Washington.

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