The Teleost Fuse

A while back, I discussed the group of fish known as the Holostei, the gars and bowfin. The Holostei constitute one branch of the clade Neopterygii which includes the majority of living ray-finned fishes. However, their success in the modern environment pales in comparison to that of their sister group, the Teleostei.

Siemensichthys macrocephalus, an early teleost of uncertain affinities, copyright Ghedoghedo.

Teleosts are such a major component of ray-finned fishes that it is simpler to list those members of the modern fauna that do not belong to this clade: the aforementioned gars and bowfin, sturgeons and paddlefish, and the bichirs of Africa. Everything else belongs to the great teleost radiation, representing about 96% of all modern fishes. The earliest fishes generally recognised as teleosts come from marine deposits of the Late Triassic in the form of the Pholidophoridae of Europe. The earliest known members of the crown group are from the Late Jurassic (Nelson et al. 2016). Teleosts have been recognised as an apomorphy-defined clade; the crown clade has been dubbed the Teleocephala. Among the features that have been used to define the Teleostei are the presence of a mobile premaxilla. In my previous post, I explained how the mobile maxilla of neopterygians including bowfins improved feeding by creating suction when the mouth was opened. Having both the maxilla and premaxilla mobile enhances this process further. In some of the most advanced teleosts, such as dories and ponyfish, the connection between the jaws and the cranium is entirely comprised of soft, flexible tissue, allowing the jaw apparatus as a whole to be catapulted towards unwary prey. Other features that have been highlighted include a strongly ossified caudal skeleton with long uroneural spines derived from the neural arches of the vertebrae, and the lower lobe of the caudal fin supported by two plate-like hypural bones articulating with a single vertebral centrum (Bond 1996).

Leptolepis coryphaenoides, one of the earliest teleosts with cycloid scales, copyright Daderot.

Of course, not all these features necessarily appeared in lock with each other. A phylogenetic analysis of basal teleosts by Arratia (2013) identified the aforementioned features of the caudal skeleton as absent in some of the basalmost teleosts. The condition of the premaxilla is ambiguous in Prohalecites, the earliest stem-group teleost from the Middle-Late Triassic boundary. It appears to be absent in the Aspidorhynchiformes and Pachycormiformes, Mesozoic orders that are currently regarded as on the teleost stem but not part of the Teleostei. However, as was found with the mobile maxilla in gars, one can't help wondering whether this character has been affected by the uniquely derived upper jaw morphologies in these orders. Other features identified by Arratia (2013) as supporting the Teleostei clade include the presence of two supramaxillary bones, a suborbital bone between the posterior margin of the posterodorsal infraorbitals and the anterior margin of the opercular apparatus (subsequently lost in the teleost crown group), and accessory suborbital bones ventrolateral to the postorbital region of the skull roof.

The earliest teleosts in the Pholidophoridae and other basal lineages retained the heavy ganoid scales of thick bone that may still be seen in modern Teleostei. Lighter, thinner cycloid scales first appear with the Early Jurassic Leptolepis coryphaenoides (Arratia 2013) and are the basal scale type for the teleost crown group (in some derived subgroups, the scales would become further modified or even lost). The greater mobility permitted by these lighter scales may have been another significant factor in the teleost explosion. By the Cretaceous period, stem-teleosts had radiated into a variety of specialised forms such as the gigantic predatory Ichthyodectiformes (of which Xiphactinus grew up to four metres in length) and the deep-finned Araripichthys. The three major subgroups of the crown Teleostei—the Elopomorpha, Osteoglossomorpha and Clupeocephala—had diverged from each other by the end of the Jurassic. The stem-teleosts would disappear with the end of the Mesozoic; the crown teleosts would dominates the world's waters from that time on.

REFERENCES

Arratia, G. 2013. Morphology, taxonomy, and phylogeny of Triassic pholidophorid fishes (Actinopterygii, Teleostei). Journal of Vertebrate Paleontology 33 (6 Suppl.): 1–138.

Nelson, J. S., T. C. Grande & M. V. H. Wilson. 2016. Fishes of the World 5^th ed. Wiley.

Steatocranus gibbiceps, the Rapid River Bumphead

The cichlid fishes of the Great Lakes of Africa are rightly renowned as one of the world's most spectacular species radiations. Hundreds of species, occupying a wide range of ecological niches, have evolved in what is, geologically speaking, a short period of time. However, cichlids in Africa are not a phenomenon of the Great Lakes alone and many interesting species may be found in other parts of the continent, some of them belonging to local radiations of their own. Consider, for instance, the Congo River rapids endemic Steatocranus gibbiceps.

Male Steatocranus gibbiceps, copyright Polypterus.

The Congo is one of the largest African rivers with a drainage basin covering one-eighth of the continent (Schwarzer et al. 2011). Downstream of Kinshasa, the river gets funneled into an intermittently deep, narrow channel for a distance of some 300 km before broadening as it approaches the sea. The result is the world's longest stretch of river rapids. Many fish species are found only in this unique region of fast-flowing waters, among them multiple species of the cichlid genus Steatocranus including S. gibbiceps. The genus as a whole is restricted to the Congo basin; a single species previously recognised from the Volta River has since been transferred to its own genus (Weiss et al. 2019). The names Steatocranus and gibbiceps both basically mean the same thing: 'fat head', in reference to a fleshy swelling atop the fish's noggin. The exact size of this swelling varies between individuals, being most prominent in large males. Vernacular names given to Steatocranus species generally reflect this feature, such as bumphead cichlid or buffalo-head cichlid. Half a dozen species have been named within Steatocranus with several more being recognised but not yet formally described, most of them belonging to the radiation within the rapids. Schwarzer et al. (2012) found evidence for extensive historical cross-breeding between species and suggested that hybridisation may have been a significant factor in the genus' diversification.

Steatocranus gibbiceps is the largest species in this genus of moderately-sized fishes, growing up to about nine centimetres in length (Roberts & Stewart 1976). Its fast-current habitat is reflected in its slender body form. It is olive brown in coloration with the scales being light in colour at the centre and darker around the margins. Steatocranus gibbiceps is most clearly distinguished from other described species in its genus by its teeth: the front teeth of both the upper and lower jaws are conspicuously large and truncate. It also has a shorter gut than its congeners. This species appears to be specialised in feeding on freshwater snails which it scoops up and swallows whole, though it will take a broader range of food in captivity. Other species of Steatocranus mostly feed on algae.

Steatocranus species are not buoyant and tend to sit at the bottom of the water (Chase Klinesteker describes their behaviour as 'hopping around the bottom like a goby'). They escape the current by spending time in the hollows and crevices among rocks. Breeding happens within such hollows with dedicated pairs forming and females affixing their eggs to the rocks. Like many other cichlids, Steatocranus gibbiceps are dedicated parents after the eggs hatch. Tending of the fry is mostly the responsibility of the female while the male patrols the territory on the watch for danger. In this way, the baby bumpheads are given the best possible start at life.

REFERENCES

Roberts, T. R., & D. J. Stewart. 1976. An ecological and systematic survey of fishes in the rapids of the lower Zaïre or Congo River. Bulletin of the Museum of Comparative Zoology 147 (6): 239–317.

Schwarzer, J., B. Misof, S. N. Ifuta & U. K. Schliewen. 2011. Time and origin of cichlid colonization of the lower Congo rapids. PLoS One 6 (7): e22380.

Schwarzer, J., B. Misof & U. K. Schliewen. 2012. Speciation within genomic networks: a case study based on Steatocranus cichlids of the lower Congo rapids. Journal of Evolutionary Biology 25: 138–148.

Weiss, J. D., F. D. B. Schedel, A. I. Zamba, E. J. W. M. N. Vreven & U. K. Schliewen. 2019. Paragobiocichla, a new genus name for Gobiochromis irvinei Trewavas, 1943 (Teleostei, Cichlidae). Spixiana 42 (1): 133–139.

Protacanthopterygii: A Brief History of a Vague Idea

There are some taxon names whose concepts are rock-solid, that have been universally recognised since their inception almost without variation. There are some taxon names that are coined, potentially linger through one or two subsequent uses, then disappear into the mists of history never to be used again. And then there are some taxon names that are used regularly but whose actual concept shifts wildly over time: names that seem to be used not so much for their own sake as because authors seem to think they need to be in there somewhere. Witness today's subject, the Protacanthopterygii.

Brown salmon Salmo trutta, photographed by Eric Engbretson, about as close to a definitive 'protacanthopterygian' as you're going to get.

The Protacanthopterygii has widely been recognised as a major group of ray-finned fishes since the name was established by Greenwood et al. (1966). Using the modern parlance, Greenwood et al.'s Protacanthopterygii was an explicitly paraphyletic group of euteleost fishes that could be recognised as branching off the lineage leading to the Acanthopterygii and Paracanthopterygii but lacked the full suite of characteristics of the latter group. As such, many of the characters listed by Greenwood et al. as diagnostic of the Protacanthopterygii were expressed in the form of trends: "widespread trend toward the development of premaxillary processes", for instance, or "hyoid and branchiostegal skeleton approaching paracanthopterygian and acanthopterygian form". We also get a number of references to majority rather than universal features: "glossohyal teeth usually prominent", or "few species with opercular spines or serrations". Greenwood et al. included the bulk of their Protacanthopterygii in the order Salmoniformes, but recognised this order in a much broader sense than modern authors. As well as the Salmonidae itself, their Salmoniformes included taxa that would now be placed in the orders Galaxiiformes, Esociformes, Myctophiformes, Aulopiformes and Stomiiformes, among others. Greenwood et al.'s Protacanthopterygii was also supposed to include the orders Cetomimiformes, Gonorynchiformes and Ctenothrissiformes. Their concept of Cetomimiformes is now recognised as polyphyletic and neither Cetomimiformes and Gonorynchiformes include any taxa closely related to Salmonidae; the case of Ctenothrissiformes has been discussed on this site previously.

Northern pike Esox lucius, copyright Jik jik.

In the intervening years, of course, the philosophy of systematics has shifted to prioritising the recognition of monophyletic taxa, requiring the dissolution of the original Protacanthopterygii. Unfortunately, calculating basal euteleost relationships has not proven an easy task. As a result, authors have differed considerably on exactly which fishes should be regarded as 'protacanthopterygians'. About the only constant factor in all circumscriptions of the taxon has been the inclusion of the Salmonidae, the salmons, trouts and the like. Indeed, the most extreme restriction of the Protacanthopterygii would treat it as including this family alone.

Recent molecular studies have agreed on the recognition of a clade uniting the Salmonidae with the Esociformes. The Esociformes is a small order of a bit over a dozen species of freshwater fish found in the Holarctic region, uniting the pikes of the genus Esox with the mudminnows of the Umbridae. Betancur-R et al. (2017) recognised Protacanthopterygii as the name for a clade uniting the Salmonidae, Esociformes, Argentiniformes (a marine order including herring smelts, barreleyes and the like) and Galaxiidae (whitebaits). However, other studies have not supported this clade.

Spotted galaxias Galaxias truttaceus, copyright Nathan Litjens, an Australian member of the whitebait family. Though galaxiids are rather salmon-like in overall appearance, it remains an open question whether this resemblance indicates any sort of direct relationship or just a shared hold-over from some ancestral neoteleost.

Considering the difficulty in defining it, one might question why the concept of a 'Protacanthopterygii' persists at all. Really, there doesn't seem to be much reason for it other than that the Greenwood et al. (1966) classification was long the base standard for teleost classifications, leaving subsequent authors loathe to discard any taxon recognised therein lightly. It might, in theory, be possible to rescue the Protacanthopterygii concept by phylogenetic definition: for instance, as those species more closely related to Salmo than Perca (indeed, I would not be surprised to learn this has already been done). But considering that the uncertain composition of the resulting clade would reduce the practicality of its recognition, I don't think I would be weeping too much if someone would just take the Protacanthopterygii concept out the back and shoot it.

REFERENCES

Betancur-R., R., E. O. Wiley, G. Arratia, A. Acero, N. Bailly, M. Miya, G. Lecointre & G. Ortí. 2017. Phylogenetic classification of bony fishes. BMC Evolutionary Biology 17: 162.

Greenwood, P. H., D. E. Rosen, S. H. Weitzman & G. S. Myers. 1966. Phyletic studies of teleostean fishes, with a provisional classification of living forms. Bulletin of the American Museum of Natural History 131 (4): 339–456.

A Parasitic Eel?

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.

REFERENCE

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.

Many Kinds of Herring

The original herring: Baltic herrings Clupea harengus membras, copyright Riku Lumiaro.

The subject of today's post is something that I'm sure that you've all encountered at one time or another. It's a group of animals that features highly in the world's food supply. Some of you may be grat fans of these animals and seek them out on a regular basis; others may not be so enthused. They go by many names: herring, sardines, sprats, shad... but all are members of the fish family Clupeidae.

For the most part, clupeids are a prime example of what I think of as 'fishy' fish: that is, fish that look exactly how the majority of people imagine a fish to look (as opposed, say, to some of those deep-sea jobs that are all teeth and poor muscle tone). They are most diverse in marine waters of the continental shelf though many spend part or all of their lives in fresh water. Most form schools, sometimes very large ones; it is this tendency to congregate that makes them such an important part of the food chain for humans and other predators. The clupeids themselves are mostly micro-predators, feeding on minute plankton. Most are medium-sized to small fish with large species getting up to a couple of feet in length*. Conversely, species of the south-east Asian freshwater genus Sundasalanx (on which more below) reach maturity at only 15 mm in length.

*Bond (1996) makes the remarkable statement that "Palonia castelnaudi, a freshwater herring of South America, reaches at least 1.5 m (Dr. Barry Chernoff, personal communication)". Not only have I been unable to find another reference to a clupeid of this size, I have been unable to confirm the existence of a species of this name. The same reference gives a maximum length for the Chirocentridae as 3.5 m; a quick search online suggests the correct figure is less than a third of that.

Another commercially significant species: sardines Sardina pilchardus, photographed by Alessandro Duci.

The exact circumscription of the Clupeidae has varied over time. It is the largest family in a clade called the Clupeoidei which is well defined by characters such as a reduction in the lateral line and the presence of the recessus lateralis, a channel running through the pterotic bone between the swim bladder and the inner ear. Other families within the Clupeoidei are the Engraulidae (anchovies), Pristigasteridae (ilishas) and Chirocentridae (wolf herrings). While each of the other families is fairly distinctive, the Clupeidae lack clear uniting features of their own and have tended to be defined as 'the rest'. Historically, some authors have united some or all of the other families within the Clupeidae, or recognised clupeid subgroups as their own additional families.

It therefore would not have come as too much of a surprise when a molecular phylogenetic analysis of the Clupeoidei by Lavoué et al. (2013) did not identify the Clupeidae as a monophyletic group. Instead, both the Pristigasteridae and Chirocentridae were nested within the Clupeidae. What is more, not one of the five subfamilies currently recognised within the clupeids was monophyletic either. Instead, Lavoué et al. found six distinct sublineages within the clupeids; each of these was individually well supported but the broader relationships between them were not. Four of these potentially formed a clade that may correspond to a restricted Clupeidae. However, members of the 'Dussumieriinae' (which differ from other clupeids in the shape of their pelvic scutes) formed two external lineages: one was potentially the sister group to all other clupeoids except the Engraulidae whereas the round herring genus Etrumeus was weakly placed as sister to the Chirocentridae. To the best of my knowledge, no-one has yet suggested a formal reclassification of the clupeoids as a result of such studies, but it seems likely that we will either see the Clupeidae expanded to include the chirocentrids and pristigasterids, or restricted to exclude the dussumieriines. Again, either one of these options would align with alternative classifications used in the past.

The paedomorphic Sundasalanx microps, copyright Michael Lo.

Also of note in recent studies on clupeid phylogeny is the position of the south-east Asian freshwater genus Sundasalanx. When first described in 1981, this genus was not recognised as a clupeid or even as a clupeoid. Instead, it was originally placed in the fish order Osmeriformes, the smelts, together with another fish genus Salanx. Members of these two genera are indeed similar in appearance: they are tiny and transparent, looking overall like whitebait but never growing into a larger adult. However, a study of the morphology of Sundasalanx in 1997 lead to the conclusion that the shared features of Salanx and Sundasalanx were actually convergences resulting from both exhibiting paedomorphy, becoming reproductively mature while still effectively in the larval stage. A relationship of Sundasalanx to the clupeoids was suggested instead and this was later corroborated by molecular analyses (Ishiguro et al. 2005). In fact, Sundasalanx is nested well within the Clupeidae, even in the family's restricted sense. Recent years have seen something of a surge in descriptions of paedomorphic fish (many of which were previously mistaken for juveniles of related taxa). Lavoué et al. (2008) recorded another paedomorphic clupeoid from marine waters of south-east Asia that the identified by molecular analysis as related to the dussumieriines, but to the best of my knowledge this species remains unnamed.

REFERENCES

Bond, C. E. 1996. Biology of Fishes 2^nd ed. Saunders College Publishing.

Ishiguro, N. B., M. Miya, J. G. Inoue & M. Nishida. 2005. Sundasalanx (Sundasalangidae) is a progenetic clupeiform, not a closely-related group of salangids (Osmeriformes): mitogenomic evidence. Journal of Fish Biology 67: 561–569.

Lavoué, S., M. Miya, A. Kawaguchi, T. Yoshino & M. Nishida. 2008. The phylogenetic position of an undescribed paedomorphic clupeiform taxon: mitogenomic evidence. Ichthyol. Res. 55: 328–334.

Lavoué, S., M. Miya, P. Musikasinthorn, W.-J. Chen & M. Nishida. 2013. Mitogenomic evidence for an Indo-west Pacific origin of the Clupeoidei (Teleostei: Clupeiformes). PLoS ONE 8(2): e56485. doi:10.1371/journal.pone.0056485.

Cichlids are Not the Only Radiation

The Congo River catfish Chrysichthys brevibarbis, copyright John P. Sullivan.

With their long barbels around the mouth and lack of scales, the catfish of the Siluriformes are one of most instantly recognisable groups of fishes. They are also one of the more diverse, with close to 3000 species and including a third of the world's freshwater fishes (Diogo & Peng 2010). Within the catfish, the Claroteidae are a distinctly African group of thirteen genera divided between two subfamilies, the Claroteinae and Auchenoglanididae. They are characterised by a moderately elongate body with a distinct adipose fin, and strong spines in the dorsal and pectoral fins (Geerinckx et al. 2003). Distinctive features of the Claroteinae include the presence of a toothplate on the palate. The Auchenoglanidinae have a rounded caudal fin and the anterior nostrils moved to the anteroventral side of the upper lip (Geerinckx et al. 2004). For a long time, the claroteids were included in the catfish family Bagridae before being raised to the level of their own family in 1991. A molecular phylogenetic analysis of the Siluriformes by Sullivan et al. (2006) placed the claroteids within a clade of African catfish that they somewhat whimsically labelled as 'Big Africa'. The Bagridae, meanwhile, were placed within 'Big Asia' (though one true bagrid genus, Bagrus, does occur in Africa). Sullivan et al. (2006) questioned claroteid monophyly, finding Auchenoglanidinae to be sister to a clade grouping the Claroteinae with the family Schilbidae, but other morphological studies have found claroteids as a monophyletic unit (Diogo & Peng 2010).

Lake Tanganyika catfish Lophiobagrus brevispinis, from tanganyikacichlide.nl.

The Claroteinae are notable for having undergone something of an adaptive radiation in one of African Great Lakes, Tanganyika. Though not as dramatic as the famous radiation of cichlids in the same lake, the Tanganyikan claroteines comprise over a dozen species divided between four genera (Bailey & Stewart 1984; Hardman 2008). Seven of these are placed in the genus Chrysichthys which has a wide distribution around Africa; the other three genera are unique to the lake. Molecular phylogeny indicates that the majority of Tanganyikan claroteines represent a single colonisation of the lake; only Chrysichthys brachynema has colonised Lake Tanganyika independently (Peart et al 2014). This indicates that the genus Chrysichthys as currently defined is non-monophyletic (something that had previously been suggested on morphological grounds) but any consequent reclassification is yet to occur. The species of Chrysichthys are mostly larger than the endemic Tanganyikan genera, ranging from 19 to 77 cm within Tanganyika (species elsewhere in Africa may reach up to 1.5 m). Of the endemic genera, the monotypic Bathybagrus tetranema is about 15 cm in length but the other two genera Phyllonemus and Lophiobagrus are even smaller, less than 10 cm in length. Bathybagrus and Lophiobagrus also both have reduced subcutaneous eyes. In Bathybagrus, this possibly reflects their occurrence at greater depths than other Tanganyika fish, occurring down to 80 m (nowhere near the depths reached by Lake Baikal sculpins but still impressive enough in the low-oxygen depths of a tropical lake). Lophiobagrus species are specialised to live in the gaps between rocky rubble on the lake bottom. The species of this genus have also been observed secreting a toxic mucus that can be fatal to other fish; this mucus is believed to be secreted from enlarged glands behind the pectoral fins.

Subcutaneous eyes are also found in two claroteines outside Tanganyika: the species Amarginops platus and Rheoglanis dendrophorus, both found in the Upper Congo (Hardman 2008). These two species are specialised for life in river rapids.

REFERENCES

Bailey, R. M., & D. J. Stewart. 1984. Bagrid catfishes from Lake Tanganyika, with a key and descriptions of new taxa. Miscellaneous Publication, Museum of Zoology, University of Michigan 168: 1–41.

Diogo, R., & Z. Peng. 2009. State of the art of siluriform higher-level phylogeny. In: Grande, T., F. Poyato-Ariza & R. Diogo (eds) Gonorynchiformes and Ostariophysan Relationships: A Comprehensive Review pp. 465–515. Science Publishers.

Geerinckx, T., D. Adriaens, G. G. Teugels & W. Verraes. 2003. Taxonomic evaluation and redescription of Anaspidoglanis akiri (Risch, 1987) (Siluriformes: Claroteidae). Cybium 27 (1): 17–25.

Geerinckx, T., D. Adriaens, G. G. Teugels & W. Verraes. 2004. A systematic revision of the African catfish genus Parauchenoglanis (Siluriformes: Claroteidae). Journal of Natural History 38: 775–803.

Hardman, M. 2008. New species of catfish genus Chrysichthys from Lake Tanganyika (Siluriformes: Claroteidae). Copeia 2008 (1): 43–56.

Peart, C. R., R. Bills, M. Wilkinson & J. J. Day. 2014. Nocturnal claroteine catfishes reveal dual colonisation but a single radiation in Lake Tanganyika. Molecular Phylogenetics and Evolution 73: 119–128.

Sullivan, J. P., J. G. Lundberg & M. Hardman. 2006. A phylogenetic analysis of the major groups of catfishes (Teleostei: Siluriformes) using rag1 and rag2 nuclear gene sequences. Molecular Phylogenetics and Evolution 41: 636–662.

Shock Me like an Electric Eel

Electric eel Electrophorus electricus, photographed by Stefan Köder.

The electric eel Electrophorus electricus is one of those animals that seem to border on the mythical. Most people will have come across some sort of reference to their existence, and may even have seen some sort of intended depiction of one in cartoon form. However, said depiction will probably bear little if any resemblance to a real-life electric eel. Most commonly, it will look more like a standard Anguilla eel, to which true electric eels are not close relatives. Instead, electric eels belong to a uniquely South and Central American group of fish, the Gymnotiformes.

The Gymnotiformes, commonly known as the Neotropical knife-fishes, are more closely related to catfish than they are to anguillid eels. They are characterised by an elongate body form, lacking the dorsal fin of other fish. The anus has been moved forward relative to other fish: in some gymnotiforms, the anus is actually in front of the pectoral fins, just behind the head. The anal fin that runs behind the anus has become greatly elongated, and instead of swimming by undulating the body from side to side like other fish, gymnotiforms swim by undulating the anal fin alone while the main body remains more or less rigid. This unusual swimming style is directly related to another distinctive feature of the gymnotiforms: their production of an electrical field. Many fish are able to passively sense electrical fields in the water: gymnotiforms take the next step and generate their own electrical field, which they use to sense their surrounding environment (Albert & Crampton 2005). As a result, they can live and hunt effectively at night and in turbid waters with poor visibility. They can also use their electrical fields for communication, signalling their moods and identities to other fish. The connection between electricity generation and swimming style is that, if gymnotiforms swam in the manner of other fish, their changes in body aspect would create changes in the shape of their electrical field. Holding the body more or less rigid means that the electrical field also remains constant, and any distortions must be caused by something external. Another group of fishes found in Africa and Asia that also navigates by electricity, the Notopteridae, has evolved a very similar appearance and swimming style to the gymnotiforms (and are also known as knife-fishes), but are entirely unrelated phylogenetically.

Tiger knife-fish Gymnotus tigre, from Trix.

The electric eel is something of an outlier among gymnotiforms. For a start, it's a monster: electric eels can be over two metres in length, while other gymnotiforms are all much smaller. The electric eel has also had a Susan Storm-style upgrade, and weaponised its electrosensory system. Electric eels can produce up to 600 volts of electricity, allowing them to stun reasonably large prey. The closest relatives of the electric eel are the banded knife-fishes of the genus Gymnotus; both are predators of fish and other aquatic animals. Males of at least some Gymnotus species and the electric eel build nests that the females lay their eggs into; males of Gymnotus carapo have been recorded to mouth-brood larvae.

The apteronotid Sternarchorhynchus mesensis, from here.

The remaining gymnotiforms were placed by Albert (2001) in a clade called the Sternopygoidei; these taxa have a smaller gape and feed on correspondingly smaller prey (some are planktivores). Two families, the Hypopomidae and Rhamphichthyidae, are united by the lack of teeth in the oral jaws; rhamphichthyids also have a very long and tubular snout. The other sternopygoids are placed in the families Sternopygidae and Apteronotidae; a distinctive feature uniting these two families is that they produce a wave- or tone-type electrical field instead of the pulse-type electrical field of other gymnotiforms. Pulse-type species produce discrete pulses of electricity at a lower frequency, while wave-type species produce a continuous series of electrical discharges at a much higher frequency (Albert 2001). While Albert (2001) regarded the pulse-type electrical field as ancestral for the gymnotiforms and the wave-type field as derived, other authors have preferred the opposite scenario. Sternopygids retain well developed eyes, in contrast to the reduced eyes of other gymnotiforms, while apteronotids are the only gymnotiforms to retain a caudal (tail) fin. If the wave-type families form a derived clade, then either these features were lost independently in the other families, or they represent reversals to an ancestral type.

One final thing to note is that the gymnotiforms have been going through something of a taxonomic boom, with many new species described in recent years. Albert & Crampton (2005) estimated that the total number of species out there could be nearly twice the 135 that had been named so far. In South America, it turns out, the streams are alive with the buzz of electricity.

REFERENCES

Albert, J. S. 2001. Species diversity and phylogenetic systematics of American knifefishes (Gymnotiformes, Teleostei). Miscellaneous Publications, Museum of Zoology, University of Michigan 190: 1-129.

Albert, J. S., & W. G. R. Crampton. 2005. Diversity and phylogeny of Neotropical electric fishes (Gymnotiformes). In: Bullock, T. H., C. D. Hopkins, A. N. Popper & R. R. Fay (eds) Electroreception, pp. 360-409. Springer: New York.

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.

Into the Labyrinth

Climbing perch Anabas testudineus emerging from water, as illustrated by Richard Lydekker.

Amongst the unholy mess that is the Percomorpha, one group that has long been recognised is the labyrinth fishes of the Anabantoidei. The anabantoids are a group of freshwater fishes found in southern Asia and Africa (but not Madagascar) that get their vernacular name from their possession of a distinctive respiratory organ called the labyrinth. This organ, found in a cavity above the gills, is derived from part of the first gill arch; the bone has become expanded and much-folded, and is covered with a layer of respiratory epithelium. So long as the gills do not actually dry out, the labyrinth allows these fish to take in oxygen directly from the air, and they can survive in warm, low-oxygen waters. They can even survive for limited periods entirely out of water (a feature that has helped make some of the larger species popular food fish, due to the greater ease of keeping them fresh in a tropical environment). Recent phylogenetic studies (e.g. Li et al. 2009) have agreed in placing labyrinth fishes as related to a number of other freshwater Indo-Australian fishes, such as the snakeheads of the Channidae and the swamp eels of the Synbranchidae, many of which are also tolerant of air-breathing.

Kissing gouramis Helostoma temminckii, from Peter Bus.

Labyrinth fishes can be divided between three families (Rüber et al. 2006). One of these contains a single species, the kissing gourami Helostoma temminckii of south-east Asia. Kissing gouramis are primarily specialised filter feeders, though they may also graze on algae or insects. The vernacular name refers to their enlarged lips, making them look permanently puckered up. Kissing gouramis even 'kiss', pressing their smackers against one another, though this is regarded as an act not of affection but of aggression (kind of like a 1930s Hollywood melodrama) as the fish push against one another.

A rather unfortunate Cape kurper Sandelia capensis, photographed by Darryl Lampert.

The climbing perches of the Anabantidae include the south Asian Anabas and the African Ctenopominae. These short-bodied carnivores have serrated edges to their gill covers that the Asian species use to pull themselves over land when travelling between water bodies (imagine lying on your stomach and pulling yourself along with your chin). You can see video of some climbing perch Anabas testudineus emerging from water here.

Giant gourami Osphronemus goramy, photographed by E. Naus.

The most diverse subgroup of the Anabantoidei is the gouramis of the Osphronemidae, another south Asian group. The largest of the Osphronemidae, the giant gourami Osphronemus goramy, grows up to 70 cm, but most species are quite a bit smaller. A number of gourami species (as well as the kissing gourami) are popular aquarium fishes; the most popular by far is the Siamese fighting fish Betta splendens, males of which have been bred to exhibit much longer and more ornamental fins than found in the wild. The gouramis are generally omnivorous, with species varying in the extent to which they prefer plant or animal food. The most specialised carnivore of the Osphronemidae is the pikehead Luciocephalus pulcher, a small but elongate species that has been described as having the most protrusible mouth of any fish (and that, by the way, is no small claim). You can see the pikehead in action below:

The pikehead is so divergent from other labyrinth fishes that past authors have regarded it as its own family, possibly the sister taxon to all other anabantoids, or even questioned whether it was a labyrinth fish at all. However, as confirmed by Rüber et al. (2006), Luciocephalus is not only a true anabantoid but nested well within the Osphronemidae as sister to the chocolate gouramis of the genus Sphaerichthys. These and two other genera, Ctenops and Parasphaerichthys, form what is known as the 'spiral egg' clade, named after the presence of spiraling ridges on the egg leading to the micropyle, that have been suggested to act as guides for the sperm.

Siamese fighting fish Betta splendens mating below a bubble-nest, photographed by Stephen & John Downer.

The anabantoids are also known for the bubble-nests constructed by a number of species, in which the eggs are contained within a floating nest of bubbles that is guarded by the male parent (both parents in the Ceylonese combtail Belontia signata). Bubble-nesting has evolved at least twice among the anabantoids: once in the Osphronemidae, and once in the ctenopomine genus Microctenopoma (other anabantids and Helostoma are free spawners that do not construct nests or guard their eggs; the ctenopomine Sandelia capensis digs a nest in the bottom substrate) (Rüber et al. 2006). Though bubble-nesting is probably the ancestral behaviour for Osphronemidae, it has been modified in a number of sublineages. Osphronemus species build submerged nests from vegetation, while members of the 'spiral egg' clade (except Parasphaerichthys) and a number of Betta species are mouthbrooders. Usually the male broods the fry in these species, but the female is the brooder in a couple of Sphaerichthys species.

REFERENCES

Li, B. A. Dettaï, C. Cruaud, A. Couloux, M. Desoutter-Meniger & G. Lecointre. 2009. RNF213, a new nuclear marker for acanthomorph phylogeny. Molecular Phylogenetics and Evolution 50: 345-363.

Rüber, L., R. Britz & R. Zardoya. 2006. Molecular phylogenetics and evolutionary diversification of labyrinth fishes (Perciformes: Anabantoidei). Systematic Biology 55 (3): 374-397.

Empire of the Sunfish

Do you remember when this particular nightmare was vomited forth from the jaws of pop culture hell?


Yes, this was the execrable Billy the Bass, just one more reason we can all be glad that the 90s aren't around any more. But what was it supposed to be?

Smallmouth bass Micropterus dolomieu, photographed by Eric Engbretson.

The bass and sunfishes of the family Centrarchidae are a group of more than thirty species of freshwater fish mostly native to North America east of the Rocky Mountains. A single species, the Sacramento perch Archoplites interruptus, is native to northern California. The family was more widely distributed in the past: the Oligocene–Miocene genera Plioparchus and Boreocentrarchus hail from Alaska, Oregon and the Dakotas (Near & Koppelman 2009). They will also be much more widely distributed in the future: species of the genera Lepomis and Micropterus have been introduced to numerous places around the world as sportfish. The centrarchids are all carnivorous, though the nature of their prey varies from zooplankton to insects to other fish.

White crappie Pomoxis annularis, photographed by D. Ross Robertson.

The molecular analysis of the Centrarchidae by Near et al. (2005) identified the mud sunfish Acantharchus pomotis as sister to all other centrarchids, contrary to its previous inclusion in the subfamily Centrarchinae with other centrarchids possessing more than three spines in the anal fin (Near & Koppelman 2009). Instead, the two genera whose species possess only three anal spines, Lepomis and Micropterus, form a clade that is sister to the remaining 'centrarchine' genera. These are the aforementioned Archoplites, the flier Centrarchus macropterus, the banded sunfishes Enneacanthus, the rock basses Ambloplites and the somewhat unfortunately named crappies of the genus Pomoxis. These are mostly deep-bodied feeders on small invertebrates, though the larger species may also take small fish. Archoplites is a more dedicated piscivore. This latter species is also notable for having less elaborate mating behaviour than other centrarchids: in contrast to the elaborate courtship rituals and nests of other centrarchids, Archoplites males do little more than use the tail fin to dig a small depression (Berra 2007). One can't resist wondering if Archoplites' lax behaviour is connected with its geographic isolation from other species.

Pumpkinseed Lepomis gibbosus, photographed by Cliff.

The genera Micropterus and Lepomis are each more diverse than the centrarchine genera. The black basses of the genus Micropterus are relatively long-bodied compared to other centrarchids, and are all piscivores. Lepomis, with twelve species, is the most diverse centrarchid genus both numerically and ecologically; as well as numerous insectivorous species, it contains the piscivorous warmouth Lepomis gulosus, the specialised planktivorous bluegill L. macrochirus, and two molluscivorous species, the redear sunfish L. microlophus and the pumpkinseed L.gibbosus. Phylogenetic relationships within Lepomis indicate a certain dynamism of ecology as well: a number of species pairs can be identified connecting large and small species, while the two molluscivores are not immediate relatives within the genus (Near et al. 2005).

REFERENCES

Berra, T. M. 2007. Freshwater Fish Distribution. University of Chicago Press.

Near, T. J., D. I. Bolnick & P. C. Wainwright. 2005. Fossil calibrations and molecular divergence time estimates in centrarchid fishes (Teleostei: Centrarchidae). Evolution 59 (8): 1768-1782.

Near, T. J., & J. B. Koppelman. 2009. Species diversity, phylogeny and phylogeography of the Centrarchidae. In: Cooke, S. J., & D. P. Philipp (eds) Centrarchid Fishes: Diversity, biology and conservation, pp. 1-38. Blackwell Publishing.

The Live-Bearing Brotulas

Black brotula Stygnobrotula latebricola, photographed by Thomas W. Doeppner.

The subject of today's post is the Bythitidae, a family of mostly marine fishes referred to as the live-bearing brotulas. Bythitids belong to the Ophidiiformes, a group of more or less elongate fishes with long soft dorsal and anal fins. They differ from most other ophidiiforms in that the males have an external intromittent organ and they are mostly live-bearers rather than egg-layers (though at least one species, Didymothallus criniceps, is potentially an egg-layer: Schwarzhans & Møller 2007). Bythitids do share these features with the deep-water Aphyonidae, which are however particularly elongate, lack scales and a swim bladder, and have loose translucent skin in contrast to the firm skin of bythitids (Nielsen et al. 1999).

Bahamian cave fish Lucifuga spelaeotes, photographed by Joe Dougherty.

Bythitids are often thought of as deep-water fishes, but there is also a reasonable diversity of them in shallower habitats such as coral reefs. The shallower-living species are mostly very cryptic in their habits and may be only rarely encountered; deeper-water species may occupy more open habitats or be found in association with hydrothermal vents. Some species of the genera Lucifuga and Ogilbia are found in freshwater caves in the Caribbean (Lucifuga species), the Yucatan (Ogilbia pearsei) and the Galapagos (O. galapagosensis); other species are found in marine caves such as the 'blue holes' of the Bahamas. New species of bythitid continue to be described at a reasonable rate of knots (over 100 species have been described in the last ten years alone). They vary in size from small (Microbrotula species are about four centimetres in length) to very large (Cataetyx laticeps reaches over 75 cmm; the Fishes of Australia website states that bythitids grow up to 2 m, but I haven't been able to find which species this refers to).

Yellow cuskeel Dinematichthys iluocoeteoides, from here.

Because of their cryptic habits, the lifestyles of most bythitids remain poorly known. They are predators of invertebrates and other fish. The few identified larvae have been collected in the epipelagic zone (Nielsen et al. 1999) but bythitids are believed to have relatively low fecundity rates (presumably as only small numbers of embryos have been found in gravid females). Reef-dwelling species, as far as is known, have only small ranges, and many may be endangered by habitat degradation.

REFERENCES

Nielsen, J. G., D. M. Cohen, D. F. Markle & C. R. Robins. 1999. FAO species catalogue. Volume 18. Ophidiiform fishes of the world. An annotated and illustrated catalogue of pearl-fishes, cusk-eels, brotulas and other ophidiiform fishes known to date. FAO Fisheries Synopsis 125 (18): I–XI + 1–178.

Schwarzhans, W., & P. R. Møller. 2007. Review of the Dinematichthyini (Teleostei: Bythitidae) of the Indo-west Pacific. Part III. Beaglichthys, Brosmolus, Monothrix and eight new genera with description of 20 new species. The Beagle, Records of the Museums and Art Galleries of the Northern Territory 23: 29-110.

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.

Hunters in the Deep Sea

Longnose lancetfish Alepisaurus ferox, photographed by Paulo De Oliveira.

For today's post, I'm going to tackle the Alepisauroidei. The exact scope of this clade of fishes has changed a bit between authors; here, I'm focusing on the restricted sense used by Sato & Nakabo (2002). In contrast, Davis & Fielitz (2010) used 'Alepisauroidei' in a broader sense that combined the Alepisauroidei, Chlorophthalmoidei and Giganturoidei of the former authors; if I have to refer to this larger clade, it'll be as 'Alepisauroidei sensu lato'.

Tedious definitionising aside, the Alepisauroidei sensu stricto include the living families Scopelarchidae, Evermannellidae, Alepisauridae and Paralepididae. All members of these families are predators in the mesopelagic zone of the ocean, below the level of the light. Members of the Alepisauridae (lancetfishes) and Paralepididae (barracudinas) are elongate, reaching lengths of over a metre in the case of the lancetfishes. Many mesopelagic fish migrate closer to the surface at night, and Bond (1996) refers to lancetfishes being caught by anglers standing on the shore during spring in the Pacific Northwest of North America. The predatory nature of the alepisauroids, as well as something of their general appearance, can be inferred from the common names given to many of them: as well as the barracudinas already mentioned, there are the sabretooth fishes of the Evermannellidae, and the daggertooth Anotopterus pharao.

An array of alepisauroids, from here. Species shown are: (Paralepididae) (1) Stemonosudis rothschildi, (2) Lestidium atlanticum, (3) Lestrolepis intermedia, (Evermannellidae) (4) Coccorella atlantica, (5) Evermannella indica, (Scopelarchidae) (6) Scopelarchus analis.

Alepisauroids show many of the adaptations common among deep-water fishes, such as the presence of bioluminescent organs (absent in Alepisauridae) and thin-walled, distensible stomachs allowing the immediate engulfment of large prey items. In the evermannellid genus Coccorella, the caecum of the intestine has become expanded to the extent that part of it actually extends into the animal's head and can be seen in the base of the oral cavity (Wassersug & Johnson 1976). The members of the family Scopelarchidae are known as pearleyes due to their enlarged, dorsally-directed tubular eyes (also present in the Evermannellidae) that presumably increase their ability to detect the limited light filtering down from above (and, more importantly, from the bioluminescent organs of other mesopelagic animals). Another notable adaptation to the mesopelagic environment present in all alepisauroids is that they are simultaneous hermaphrodites: each individual has fully functional male and female reproductive organs. In an environment where the usual scarcity of food items means that species exist at very low population densities, simultaneous hermaphroditism means that any other individual of your species is a potential mate. Simultaneous hermaphroditism is also found in other members of the Alepisauroidei sensu lato, making it the largest clade of vertebrates utilising this reproductive strategy (Davis & Fielitz 2010).

Specimen of daggertooth Anotopterus vorax, photographed by Peter Marriott.

A comprehensive investigation of the molecular phylogeny of alepisauroids was published by Davis & Fielitz (2010). Evermannellids and scopelarchids were resolved as successive sister groups to the other alepisauroids sensu stricto, suggesting that their tubular eyes may have arisen independently (as corroborated by their absence in the evermannellid genus Odontostomops; alternatively, tubular eyes could have been lost in other alepisauroids). The Alepisauridae were nested within an apparently paraphyletic Paralepididae. The clade as a whole was suggested by molecular dating to have diverged some time in the Early Cretaceous, a result in concordance with the known fossil record.

REFERENCES

Bond, C. E. 1996. Biology of Fishes, 2nd ed. Saunders College Publishing.

Davis, M. P., & C. Fielitz. 2010. Estimating divergence times of lizardfishes and their allies (Euteleostei: Aulopiformes) and the timing of deep-sea adaptations. Molecular Phylogenetics and Evolution 57: 1194-1208.

Sato, T., & T. Nakabo. 2002. Paraulopidae and Paraulopus, a new family and genus of aulopiform fishes with revised relationships within the order. Ichthyological Research 49 (1): 25-46.

Wassersug, R. J., & R. K. Johnson. 1976. A remarkable pyloric caecum in the evermannellid genus Coccorella with notes on gut structure and function in alepisauroid fishes (Pisces, Myctophiformes). Journal of Zoology 179 (2): 273-289.

Pomfrets of the High Seas

The fanfish Pterycombus petersii, photographed off the Kerama Islands by Kazuo Kayama.

The Bramidae, commonly known as pomfrets, are a cosmopolitan family of pelagic fishes, found mostly in the upper layers of the world's oceans. Pomfrets are teardrop- or elliptical-shaped, deep-bodied and strongly-compressed fish with a single long dorsal fin that is ventrally mirrored by (usually slightly shorter) similar-shaped anal fin. Some species are quite large, with about a metre as the maximum recorded length for the family (McEachran & Fechhelm 2006). Thompson (2002) stated that pomfrets feed on other fish and larger invertebrates such as squid, but García & Chong (2002) found that Brama australis fed primarily on crustaceans such as krill.


Pteraclis aesticola, photographed by Kanno Takayuki.

The Bramidae are divided between two subfamilies, Pteraclinae and Braminae, though the monophyly of the latter in particular does not necessarily appear to have been established. Pteraclinae include two genera, the fanfishes Pteraclis and Pterycombus, with particularly large triangular dorsal and anal fins. Despite their unwieldy appearance, these fins can be completely depressed into a special groove formed by modified scales running on either side of the fins, as is being done by the individual in the photo above (if expanded, the fins of Pteraclis are even more expansive than those of Pterycombus, with the dorsal fin extending all the way forward to the snout). Members of the Braminae (the genera Brama, Eumegistus, Taractes, Taractichthys and Xenobrama) have less flamboyant fins with scales running partway along the rays and unable to be depressed (Thompson 2002).


The large bramine Taractes rubescens, from here.

Phylogenetically speaking, the molecular study using by Li et al. (2009) placed the Bramidae among a clade that they referred to as Stromateoidei (though somewhat different from earlier uses of this name), that also included families such as Stromateidae (butterfishes), Scombridae (mackerels), Trichiuridae (cutlassfishes) and Chiasmodontidae (black swallowers). A comparable clade was also recovered by Yagishita et al. (2009) using different molecular markers (Li et al. used nuclear genes; Yagishita et al. used mitochondrial genes; however, Yagishita et al. sampled a smaller number of families than Li et al.). Though morphologically diverse, all families in this clade are primarily pelagic.

REFERENCES

García M., C., & J. Chong. 2002. Composicion de la dieta de Brama australis Valenciennes 1837 en la zona centro-sur de Chile (VIII región) en Otoño 2000 y Verano 2001. Gayana 66 (2): 225-230.

Li, B., A. Dettaï, C. Cruaud, A. Couloux, M. Desoutter-Meniger & G. Lecointre. 2009. RNF213, a new nuclear marker for acanthomorph phylogeny. Molecular Phylogenetics and Evolution 50: 345-363.

McEachran, J. D., & J. D. Fechhelm. 2006. Fishes of the Gulf of Mexico, vol. 2. University of Texas Press.

Thompson, B. A. 2002. Bramidae: pomfrets. In: Carpenter, K. E. (ed.) The Living Marine Resources of the Western Central Atlantic, vol. 3. Bony fishes part 2 (Opistognathidae to Molidae), sea turtles and marine mammals. FAO Species Identification Guide for Fishery Purposes and American Society of Ichthyologists and Herpetologists Special Publication 5. Food and Agriculture Organization of the United Nations: Rome.

Yagishita, N., M. Miya, Y. Yamanoue, S. M. Shirai, K. Nakayama, N. Suzuki, T. P. Satoh, K. Mabuchi, M. Nishida & Tetsuji Nakabo. 2009. Mitogenomic evaluation of the unique facial nerve pattern as a phylogenetic marker within the percifom fishes (Teleostei: Percomorpha). Molecular Phylogenetics and Evolution 53 (1): 258-266.