The Surprisingly Mysterious Eels

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).

REFERENCES

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.

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.

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

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

It's a Moray Friday

Okay, so I'm only doing this to beat Rick MacPherson. A recent publication in Zootaxa (Smith et al., 2008) has given us a new species of moray eel, Gymnothorax baranesi.



The new species has only been collected so far from a small area of the Gulf of Aqaba in the northern Red Sea, in moderately deep water. The picture above (from the paper) shows the male holotype, which is not quite mature but at ~86 cm long is quite big enough to not want to mess with. Full adults would probably be even larger. Gymnothorax baranesi* is distinguished from closely related species by details of the colour pattern and the arrangement of teeth.

*The authors don't suggest a vernacular name, but being a vertebrate someone's probably going to insist that it needs one. I suppose the main options are Baranes' Moray after the species name (named for Albert Baranes, a researcher on fishes of the Red Sea), or Aqaba Moray after the locality.

The discovery of this species highlights just how little we know about the world's biodiversity in another way, too. As the authors note in the paper, "The fact that the new species was collected in an area that has been well studied... for many years (and in fact, directly in front of a major marine laboratory) indicates how much we still have to learn..."

REFERENCE

Smith, D. G., E. Brokovich & S. Einbinder. 2008. Gymnothorax baranesi, a new moray eel (Anguilliformes: Muraenidae) from the Red Sea. Zootaxa 1678: 63-68.

More Really Ugly Fish

I've noted some examples before (in posts here and here) from 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 (the image above comes from Animal Diversity Web).

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 below in a photo from Wikipedia. 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).



There are four families of Saccopharyngiformes. The most distinctive family is the bobtail snipe eels of the Cyematidae (image above of Cyema atrum from Animal Biodiversity Web again). 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 (the subject of the picture at the top of this post) 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.



The most bizarre of all the Saccopharyngiformes (and that's saying something) are undoubtedly the one-jawed eels of Monognathus, shown above in an image stolen from Smith (2002). 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.

REFERENCES

Smith, D. G. 2002. Families Cyematidae, Saccopharyngidae, Eurypharyngidae, Monognathidae. In: FAO Species Identification Guides for Fishery Purposes, The Living Marine Resources of the Western Central Atlantic, Vol. 2.