Loaches

European spined loach Cobitis taenia, from here.

The spined loaches of the Cobitidae are a family of small freshwater fishes found across Eurasia, with a single species (Cobitis maroccana) making it to the northern tip of Africa. A recent catalogue of the family by Kottelat (2012) recognised twenty-one genera in the families, though phylogenetic studies suggest that some reshuffling may be necessary: the Chinese Paramisgurnus dabryanus, for instance, may be nested within the genus Misgurnus, while the Sino-Japanese genus Niwaella may be a polyphyletic grouping of elongate species adapted to fast-flowing mountain streams (Šlechtová et al. 2008).

Eel loach Pangio anguillaris, photographed by Thomas Frank.
As a whole, loaches are more or less worm-like fishes that feed by benthic scavenging. Most species are small, less than ten centimetres long, though the Thai Acantopsis spectabilis gets up to around 15 centimetres (Kottelat 2012), and the weather fish Misgurnus anguillicaudatus reaches about 25 cm. Phylogenetically, the family was divided by Šlechtová et al. (2007) into two groups, a 'northern clade' containing the northern Eurasian species in the genera Cobitis, Misgurnus and related taxa, and a paraphyletic 'southern group' containing the remaining southern and south-east Asian species. The ranges of the northern and southern subdivisions overlap in northern Vietnem, but otherwise the two groups are geographically disjunct. A potential morphological synapomorphy of the northern clade is a horizontal ossified structure, called the 'scale of Canestrini', on the second ray of the male's pectoral fin, but if so this character has been lost in some subtaxa such as the western Eurasian genus Sabanejewia.

Weather fish Misgurnus anguillicaudatus, photographed by Emma Turner.
One interesting detail about the northern spined loaches is the existence in various localities of natural polyploid populations: such polyploids have been identified among European Cobitis species, and in the Japanese Misgurnus anguillicaudatus. These mostly triploid (sometimes tetraploid) populations of loaches reproduce clonally, but are always found in association with a sexually-reproducing diploid population. This is because the parthenogenetic females are what is referred to as 'sperm parasites'. The parthenogenetic females still mate with sexual males, not to be fertilised but in order that the act of mating will stimulate egg production. In external appearance, these polyploids are generally indistinguishable from their co-existing diploid associates. European polyploid Cobitis are believed to have arisen through hybridisation between closely related sexual species, possibly through male sperm fertilising an unreduced diploid egg.

Protocobitis typhlops, from Kottelat (2012).

Oh yes, and there are cave-dwelling loaches out there: two Chinese species, placed in the genus Protocobitis, are blind species collected from groundwater. How they relate to the above-ground species remains unknown.

REFERENCES

Šlechtová, V., J. Bohlen & A. Perdices. 2008. Molecular phylogeny of the freshwater fish family Cobitidae (Cypriniformes: Teleostei): delimitation of genera, mitochondrial introgression and evolution of sexual dimorphism. Molecular Phylogenetics and Evolution 47: 812-831.

Kottelat, M. 2012. Conspectus cobitidum: an inventory of the loaches of the world (Teleostei: Cypriniformes: Cobitoidei). Raffles Bulletin of Zoology, Supplement 26: 1-199.

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.

A Bizarre New Shark

Live goblin shark Mitsukurina owstoni, from here.

It's a bit unusual for me to be posting anything on a Sunday, but I've just received notice of something so incredibly cool that I couldn't wait to tell you all about it. A new paper has just come out describing a truly remarkable new species of shark:

Takahashi, N., & N. Yuasa. 2012. First recorded use of weaponised light by an elasmobranch. National Daiei Journal 7: 17-87.

The new species, Neomitsukurina nodai, is most closely related to the unusual goblin shark Mitsukurina owstoni, and the resemblance between the two is clearly visible in the head region:

Photo of the new shark species from Takahashi & Yuasa.

Nevertheless, it possesses several remarkable differences. First there is the distinctive fin array, somewhat more extensive than that found in most shark species. The denticles in the skin are much reduced, giving the body an almost rubbery appearance. Furthermore, in a remarkable case of life imitating art, Neomitsukurina differs in its jaw morphology. The vast majority of depictions of goblin sharks show it with protruding jaws but, as can be seen in the photo at the top of the post, this is not the usual appearance of this species: the jaws are generally only protruded when the shark is picking up food. In Neomitsukurina, however, the jaws are seemingly permanently protruded, and the upper jaw has been modified into a sharpened beak. The most interesting distinction of all, however, is the presence of a massively enlarged photophore on the underside of the rostrum, above the jaws:

Close-up of the head of Neomitsukurina nodai, from Takahashi & Yuasa.

The photophore contains a unique lens structure that focuses the light it produces. So strongly focused is the light, in fact, that it can be used in prey capture by the shark. Through a mechanism not yet fully understood, but possibly a shock reaction to its brightness, the light causes potential prey animals to become stunned, after which they can be easily picked off. Preliminary observations of Neomitsukurina suggest that it may be willing to take on quite large prey: even turtles have not proven immune to stunning, though the shark did not always immediately ingest stunned prey animals. Neomitsukurina has also been observed gliding above the surface of the water through the use of its enlarged pectoral fins.

It might be wondered how such a distinctive and mobile predator eluded discovery until the present, but Neomitsukurina's strict nocturnality might have something to do with it. It is also worth noting that sightings of what may, in hindsight, have been Neomitsukurina have been described in the past (a particularly famous sighting occurred in 1971, near the island of Niemonjima), but attempts to follow up such records have so far only collected other animals such as sea bass.

Just When You Thought It Was Safe

Smalltooth cookiecutter shark Isistius brasiliensis, photographed by Joshua Lambus.

Sometimes, you can get pretty much everything you need to know from the title of an article alone. To whit:

First documented attack on a live human by a cookiecutter shark (Squaliformes, Dalatiidae: Isistius sp.)

The article itself is in a journal I don't have access to, but I can read the abstract: the person attacked was a long-distance swimmer in Hawaii and was bitten twice. The bite was treated with skin grafts, but still took nine months to finish healing.

Cookiecutter sharks are one of the more fascinatingly evil fish out there. They are small, as sharks go (up to about 50 cm, tops) but have proportionately oversized teeth that are arranged in a tight, single-row array that can be protruded outwards to take a neat plug out of the flesh of a larger animal: hence the name of 'cookiecutter'. The effectiveness of the cutting tooth row is maintained by being replaced all at once, rather than individual teeth being replaced piecemeal as in other sharks. Cookiecutters are rarely encountered by humans as they are generally deep sea fish, living below the light zone, but like many mesopelagic animals they appear to migrate closer to the surface at night (Papastamatiou et al. 2010). Bioluminescent photophores behind the head have been suggested to function as a lure, drawing larger fish, dolphins, etc. into range of an ambush. Cookiecutters have very catholic tastes, and evidence of bites has been recorded from just about any decent-sized pelagic animal. They will even bite the external insulation on submarines.

Fish with cookiecutter bites, from Rick Macpherson (who, it turns out, covered this event when it was first happened).

Given their lack of pickiness, it is hardly surprising that a cookiecutter would take a bite out of a human. Of course, humans very rarely venture into the pelagic environment in which cookiecutters can be found. The very fact that the Hawaii indicent is the first confirmed attack indicates how extremely rare this would be expected to be. The Wikipedia page on cookiecutters refers to possible attacks on shipwreck survivors (though the source page linked to does not provide citations for such reports), and the body of a drowned fisherman was recovered in Hawaii with cookiecutter bites. But unless you happen to be swimming in the open ocean at night, your chances of being bitten by a cookiecutter are low.

REFERENCE

Papastamatiou, Y. P., B. M. Wetherbee, J. O’Sullivan, G. D. Goodmanlowe & C. G. Lowe. 2010. Foraging ecology of cookiecutter sharks (Isistius brasiliensis) on pelagic fishes in Hawaii, inferred from prey bite wounds. Environmental Biology of Fishes 88 (4): 361-368.

Who Left All this Fish Lying Around (Taxon of the Week: Neopterygii)

Two species of the swordfish-like Cretaceous pachycormid Protosphyraena. This genus was not even closely related to the modern swordfish (contra Wikipedia), and represents a case of convergence. Reconstruction by Dmitry Bogdanov.

The Neopterygii, or "new fins" (not, as it is often translated, "new wings") are one of the most successful clades of fishes today. One particular subgroup of the Neopterygii, the teleosts, includes almost all the living ray-finned fishes. However, just to be difficult, I decided that the most appropriate tack for a post on Neopterygii was to leave the teleosts in all their diversity for another time, and focus on the non-teleost neopterygians. This, as it turns out, was a mistake. The non-teleost neopterygians seem, to a fish, to be almost universally ignored, and most of what there is out there was covered by Toby White almost seven years ago. Nevertheless, I'll see what I can do.

The origins of the Neopterygii date back to sometime in the Permian (Hurley et al., 2007). Compared to earlier actinopterygians, the ancestors of Neopterygii lost their clavicle, beginning a trend of lightening and strengthening their skeletons, while at the same time reducing the weight of their scales. Early fish had been heavily armoured arrangements, but like the origins of the modern military, neopterygians were to trade in their clunky plate armour for something a bit more like a bullet-proof jacket*.

*Something that has almost nothing to do with the main post, but which struck me when I was thinking about it yesterday evening: When one looks at the living vertebrates only, it is easy to imagine that there was a progressive development of the bony skeleton - at the base of the tree, we have the living cartilaginous fishes and jawless fishes with little or no ossification, followed by the bony fishes and the tetrapods mostly with full skeletons. The fossil record, however, indicates that things were a little more complicated - early fishes such as placoderms had extensive skeletons, and the modern unossified fishes are actually the descendants of vertebrates that lost most of their skeletons. However, the original vertebrate bony skeleton did differ from the modern bony skeleton in one major regard - it was on the outside. Early fish had great coverings of bony armour, but little ossified interior skeleton. So over the course of evolution, vertebrates have gone from having their skeletons on the outside and meaty parts in the middle, to have the meaty parts on the outside and the skeletons in the middle. In other words, vertebrates have effectively been turned inside out.


Longnose gar, Lepisosteus osseus, one of the few living non-teleost neopterygians. Photo from here.

There are few living groups of non-teleost neopterygians - in fact, there's only two, both restricted to fresh waters of North America. One group, the Halecostomi, is represented in the modern fauna by only a single species, the bowfin, Amia calva. As Toby has noted before me, perhaps the single most remarkable feature of the bowfin is that it has absolutely nothing remarkable about it whatsoever. Amiid fishes go all the way back to the Jurassic, and don't look too much different from each other in all that time. The other living group, the American gars of the family Lepisosteidae, are entirely a different matter - gigantic carnivorous fish, with long beaks and sharp teeth. The largest gars can be over two metres long, and according to this site Rafinesque referred to gars up to twelve feet long. They also lay eggs that are toxic to humans. Unfortunately, it looks like American gars don't have green bones, despite common rumour - the green-boned "garfish" is a quite different, marine fish (Belone) nestled well within the teleosts.


Bowfin, Amia calva, the other survivor. Photo from here.

Relationships between the neopterygian clades are almost completely obscure - while features of the jaw musculature support a relationship between Amia and teleosts to the exclusion of gars, other authors have supported an Amia-Lepisosteidae clade that excludes teleosts. Hurley et al. (2007) found the latter result in a morphological analysis, but the former in a molecular analysis. While a number of fossil groups of non-teleost neopterygians are known, few authors seem to have plugged them into a phylogenetic analysis except for Hurley et al. (2007) and Arratia (2001) (the latter of which I don't have access to). A number of authors have supported a relationship between the gars and the extinct Semionotiformes (Olsen & McCune, 1991), while the Pachycormiformes and Aspidorhynchiformes seem likely to be stem-teleosts. Finally, the Dapediidae and Pycnodontiformes were found by Hurley et al. (2007) to form a third clade in a polytomy with the Amia-Lepisosteidae clade and the teleosts.


The pycnodontiform Coelodus costai. Photo by Giovanni Dall'Orto.

Some of these were decidedly odd fishes. The Pycnodontiformes were deep-bodied fish, about as tall as they were long. They had strong teeth, and would have fed on shellfish. The Pachycormiformes, mostly pelagic hunters, are best known through the monster Leedsichthys, a gigantic filter feeder growing to lengths over ten metres, which is probably the largest known ray-finned fish.


Figure from McCune (2004), showing a reconstruction of Semionotus, and variation in dorsal spine row morphology and overall body shape in Newark Semionotus.

Perhaps the coolest of all, though, were the Semionotidae. Semionotus wasn't anything much to look at - not spectacularly large (probably about half a foot) and pretty generalised morphologically. During the Mesozoic it was found in freshwater deposits pretty much around the world, so it would have been dirt common. Where things get interesting is when you get to the Late Triassic and Early Jurassic Newark Supergroup of eastern North America. The Newark Supergroup comprises a series of lake deposits, formed by a process of rifting similar to the modern Great Lakes of Africa. And Semionotus was the Newark deposits' cichlid. Within a single lake deposit, a whole series of Semionotus species can be found, varying from long and narrow to deep-bodied and humpbacked (McCune, 2004). And that is very cool - that the incredible African cichlid radiation is not so incredible after all, but represents patterns and processes that were just as active 100 million years ago.

REFERENCES

Arratia, G. 2001. The sister group of Teleostei: consensus and disagreements. Journal of Vertebrate Paleontology 21 (4): 767-773.

Hurley, I. A., R. Lockridge Mueller, K. A. Dunn, E. J. Schmidt, M. Friedman, R. K. Ho, V. E. Prince, Z. Yang, M. G. Thomas & M. I. Coates. 2007. A new time-scale for ray-finned fish evolution. Proceedings of the Royal Society of London Series B 274: 489-498.

McCune, A. R. 2004. Diversity and speciation of semionotid fishes in Mesozoic rift lakes. In Adaptive Speciation (U. Dieckmann, M. Doebeli, J. A. J. Metz & D. Tautz, eds) pp. 362–379. Cambridge University Press.

Olsen, P. E., & A. R. McCune. 1991. Morphology of the Semionotus elegans species group from the Early Jurassic part of the Newark Supergroup of eastern North America with comments on the family Semionotidae (Neopterygii). Journal of Vertebrate Paleontology 11 (3): 269-292.

Scleritome Week: Not just an invert thing

Unfortunately, I can't touch the chancelloriids until tomorrow, but they will be here, I promise.

So far, all the animals I've shown you in relation to Scleritome Week (see here, here and here) have all been definitely in the class of organisms dismissed by the sadly vertebrate-centric as "creepy-crawlies". Nevertheless, the disarticulated scleritome issue is not unique to invertebrates.

The figure at the top of the post (from Valiukevičius & Burrow, 2005) shows scales of Silurian fish of the family Tchunacanthidae. This family was originally described by Karatajute-Talimaa & Smith (2003) as a new order, distinct from all others previously described (while Valiukevičius & Burrow seem a little sceptical of such a high ranking, they do still maintain the family's distinctiveness). The interesting thing for this post is that, so far, tchunacanthids are known only from scales.



Tchunacanthidae are member of the Acanthodii, an extinct class of vertebrates found from the Silurian to the Permian (a couple of examples are shown above in an illustration from here). Acanthodians are sometimes referred as "spiny sharks", a name that probably survives more because it sounds neat than because of its appropriateness for the actual animals. While generally regarded as more closely related to modern bony fishes and tetrapods than actual sharks, acanthodians resembled sharks in having a cartilaginous rather than a bony skeleton. As a result, acanthodian skeletons were rarely fossilised, and usually only the hard mineralised parts survived - teeth, scales and spines. Without the soft tissue holding them together, however, the fossils became disarticulated, just like the sclerites of a scleritome animal. Tchunacanthids are far from being the only family of non-bony fish known only from scattered pieces of armation - articulated specimens (except in those taxa that developed large bony plates) are the exception rather than the rule. Beyond the general features shared by all acanthodians, we are doomed to ignorance about what a living tchunacanthid looked like unless some day we are lucky enough to find one of those rare articulated fossils.

REFERENCES

Karatajute-Talimaa, V., & M. M. Smith. 2003. Early acanthodians from the Lower Silurian of Asia. Transactions of the Royal Society of Edinburgh: Earth Sciences 93: 277-299.

Valiukevičius, J., & J. C. Burrow. 2005. Diversity of tissues in acanthodians with Nostolepis−type histological structure. Acta Palaeontologica Polonica 50 (3): 635-649.

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.