Bizarre Ceratioid Anglerfish of the Thaumatichthyidae and their Sideways Mouths

Lasiognathus amphirhamphus, photot by Theodore Pietsch, under Creative Commons Attribution-NonCommercial License - Version 3.0
Lasiognathus amphirhamphus, photo by Theodore Pietsch, under Creative Commons Attribution-NonCommercial License – Version 3.0

If you spend long enough searching the literature for strange animals, you’ll find them in spades, and it’s really a shame that most people will never know just how bizarre animals really get. Take for instance, the latest news on the Triassic marine reptile Atopodentatus unicus, which in itself is a fantastic cautionary tale for paleontological reconstruction. Originally flagged up as a zipperfaced filter feeder with interlocking teeth on both sides of downturned premaxillae (Cheng et. al 2014), recent fossils have suggested a somewhat less strange morphology, in which the maxillae do as they’re meant to and bear downward-facing teeth that have been interpreted as algae-scrapers (Chun et. al 2016). While Atopodentatus remains fascinating, not only because of its uncertain place in the sauropterygian tree and its apparent feeding habits, not to mention its apparent convergence on both Nigersaurus and baleen whales, this new information regarding its mouth is a bit sad. It’s sad because there are now once again only two genera of vertebrates – at least that I’m aware of – with sideways mouths.

Bony fish are almost endlessly diverse, so it stands to reason that if any vertebrate is going to win an award for the strangest mouth, it’ll probably be one of them, and probably a teleost. Anyone passingly familiar with feeding biomechanics will have seen fantastic videos of truly spectacular fish feeding, including the slingjaw wrasse, Epibulus insidiator, shooting its face off in the general direction of prey and sucking it in with stunning speed. But wrasses are only the tip of the iceberg when it comes to craniokinesis (And having taken a vertebrate anatomy course with Dr. Mark Westneat, an expert on reef fishes and the biomechanics thereof, I’ve glimpsed that rabbit hole) and there are a ton of weird fish out there, including the subject of today’s post, the Thaumatichthyidae.

By Theodore Pietsch, under Creative Commons Attribution-NonCommercial License - Version 3.0
By Theodore Pietsch, under Creative Commons Attribution-NonCommercial License – Version 3.0

I first learned of the oddity, or rather, flat out insanity, of the Thaumatichthyidae while researching my final paper for Dr. Michael Coates’ Chordate Anatomy and Evolution class the spring quarter of my first year as an undergraduate. I was quite interested in the evolution of fangs and tusks – let’s say elongate maxillary teeth – as these structures pop up all over the place not just within mammals and para/stem-mammals (Felids like Smilodon fatalis and other Machairodontines, with Thylacosmilus atrox and the Thylacosmilines playing the same strategy in the marsupials, Gorgonopsians and Anomodonts and various Therocephalians ~250 million years earlier, Suidae, Cervidae, Otobenidae, Hippopotomidae, Delphinoidea [Otobenocetops, the strange walrus-mimicking(!?) dolphin, kind of a cheat since the teeth are premaxiallary], Cercopithecid primates… this was off the top of my head and honestly it might be easier to compile a list of mammal groups that don’t have extremely enlarged canines.) but also within dinosaurs (Heterodontosauridae, though again

the upper “fang” teeth occur in the premaxilla – but they’re in the right position) and of course, snakes (Viperidae). Obviously, a term paper had to be quite a bit more focused than an evolutionary history of “fangs” so I chose to focus on animals with some sort of apparatus in which they can actually move their elongate teeth independently of an actual dorsoventral biting action. This narrowed down the list quite a bit, as the only tetrapods that do this are viperids and Muntjacs (yes, really, although it’s pretty lame when compared with the terrifyingly elegant seven-bar linkage of Viperids. I’ll have to write about Muntiacus in the future.). But then there were the real weirdos: fish.

Thaumatichthyidae, a family of Ceratioid anglerfish consisting of just two genera, Thaumatichthys and Lasiognathus, with nine species spread between them after a recently described new species bumped Lasiognathus up to six species (Pietsch and Sutton 2015). A lot of news outlets reporting on the new Lasiognathus, which made the SUNY College of Environmental Science and Forestry’s list of  the top ten new species in 2015, (unsurprisingly) completely missed the whole point of the animal, which is that its mouth is beyond imagination. Both Thaumatichthys and Lasiognathus have large, hinged premaxillaries bearing a plethora of long, thin, hooked teeth. The premaxillae are hinged on the ethmoid cartilage that is their most secure connection to the rest of the cranium, and can swivel up and down in the transverse plane. This results in a sort of sideways grabbing mechanism that’s hypothesized to be involved in trapping or skewering prey underneath the protruding upper jaw of the fish. This, along with the fact that these species are usually only documented from deepwater trawls, points towards a benthic sit-and-wait feeding strategy, which is pretty similar to that of other anglerfish, which do much the same thing in the water column.

While Lasiognathus would be strange enough alone, it’s Thaumatichthys that really ratchets up the strange factor. Instead of holding its esca (the bait organ that contains bioluminescent symbiont bacteria to lure in prey) out above its head on a modified fin ray like other anglerfish, it actually has an esca on the roof of its mouth (Bertelsen and Struhsaker 1977). This particular adaptation strikes me as both obvious and quite extraordinary. It’s clear that placing such a lure inside of the mouth rather than dangling out above it is advantageous to a bottom-dweller with this fishing apparatus – other anglerfish presumably keep it out on a rod to ensure as wide a viewing angle as possible for prey items, but if you’re specialized for snatching fishes up off the benthos, there’s no point in advertising your presence to the water column at large. Luckily, enough growth stages of Thaumatichthys have been recovered to understand the developmental history of this interesting escal position. The ilicium begins in a reasonable spot between the eyes in larva and moves forward as the larva grows larger, eventually embedding itself in the membranous tissue between premaxillaries at the time of metamorphosis, leading to its placement at the anterior of the mouth (Bertelsen and Struhsaker 1977).

Reproduced directly from Bertelsen and Struhsaker 1977.
Reproduced directly from Bertelsen and Struhsaker 1977. le: lateral ethmoid pa: palatine

Bertelsen and Struhsaker’s coverage of Thaumatichthys (1977) is quite comprehensive, and it was one of the first descriptive papers I ever read, so it’s something of a favorite of mine. Section IV covers the jaw apparatus and it’s quite an interesting read. They theorize that the maxillaries are pulled back and outwards by the levator maxillae superioris muscle, which transfers force into the premaxillaries through premaxillary lignaments, which also serve to hold the trap in the closed position by attaching the articular processes of each premaxillary to the distal tip of the opposite. They suggest that the force is transferred “into an up and outward pull in the articular processes of the premaxillaries in such a way that these long processes, like a pair of handles, will turn the jaws downwards into the closed position.” This is a rather awkward kind of swivel and it’s quite hard to get your head around, not in the least because it doesn’t seem to make sense that the premaxillaries could be pulled outwards and upwards as there doesn’t appear to be much on their medial side to pull on. But this is apparently what you can do when you reinvent the jaw entirely, starting from scratch.

 

Sadly, the evolutionary story of the shift from a dorsoventral bite (like that of, I must stress, almost all the other vertebrates) to a transverse bite is almost completely opaque, and probably will be forevermore unless transitional fossils pop up. The phylogeny of Ceratioid anglerfish obtained by Pietsch and Orr (2007) groups them with the Oneirodidae, which they note even in the abstract of the paper was completely responsible for imperfect resolution of the tree. Oneirodidae is a large and poorly understood group in the first place, so it wouldn’t be too surprising if the Thaumatichthyidae end up inside them or somewhere else entirely, as the sister-relation favored by Pietsch and Orr is not well-supported at all, with less than 40% bootstrap support. The possibility of Thaumatichthyidae within the Oneirodidae is upheld by mitogenomics, as Miya et. al found Lasiognathus to be deeply nested within Oneirodidae in their mitogenomic phylogeny of the Ceratioids – although, frustratingly, Thaumatichthys wound up outside the group again. Given the good morphological support for the monophyly of Thaumatichthyidae (Pietsch and Orr 2007), it’s unlikely that Miya et al’s paraphyletic grouping of Thaumatichthys and Lasiognathus is correct (and rather stunningly, their time-calibrated phylogeny separates the genera by ~90 million years), and it seems most parsimonious to just look at the whole Thaumatichthyidae/Oneirodidae group as a sort of blur for now unless you’re a specialist.

But this uncertainty won’t keep me from a bit of speculation as to the evolution of the jaw mechanism. Like viperids, the Ceratioid skull is pretty diffuse, and honestly quite confusing to look at – especially if you’re used to proper, solidly-built crania like those of mammals. But since they’re fish – and small, resource-limited deep-sea fish at that – the anglers have a lot less bone in general than snakes do. (This is one of my main complaints about fish skeletons; there’s really not enough bone on them at all, and for a lot for them it’s mostly ropy cartilage that’s holding the whole thing together. It makes them very hard to get a handle on, and if I had my druthers they’d all just work out their cranial issues and make a proper skull.) This sort of osteology allows for a lot of free movement of individual bones, which can easily be confirmed just by playing with a fish head; you can move different parts all around the place quite easily, and the feeding apparatus of something like an anglerfish is going to be able to easily warp and bend as it closes around or possibly on a prey item. All you really need to go from a loose arrangement of bones to a flexing apparatus is the development of associated muscles, and the jaw musculature of Thaumatichthys doesn’t appear to be very complex, primarily driven by the levator maxillae superioris.

It’s a shame that so little is known about these animals, and none have been seen capturing prey – I remember reading a few years ago that there was some footage of a Thaumatichthyid out there but have been unable to locate it. This also means that it’s difficult to find pictures of the fish, especially ones which are under a sharing license. Luckily, the Tree of Life maintains a small gallery of pictures of three actual specimens which, while not a lot to go on, are still better than nothing.

After what seems like only a few hours and what’s rapidly approaching 2,000 words thanks to my bloated sentences, there’s finally something out there to document my interest in these fish. And in the process, I’ve determined to write about muntjacs and a couple of other interesting animals in the future. I’m finally understanding how Darren Naish manages this.


Refs.

Bertelsen, E.and P. J. Struhsaker. 1977. The ceratioid fishes of the genus Thaumatichthys: osteology, relationships, distribution, and biology. Galathea Report 14:740.

Bertelsen, E., and T. W. Pietsch. 1996. Revision of the Ceratioid Anglerfish Genus Lasiognathus (Lophiiformes: Thaumatichthyidae), with the Description of a New Species. Copeia 1996:401.

Carnevale, G., T. W. Pietsch, G. T. Takeuchi, and R. W. Huddleston. 2008. Fossil Ceratioid Anglerfishes (Teleostei: Lophiiformes) from the Miocene of the Los Angeles Basin, California. Journal of Paleontology 82:996–1008.

Cheng, L., X.-H. Chen, Q.-H. Shang, and X.-C. Wu. 2014. A new marine reptile from the Triassic of China, with a highly specialized feeding adaptation. Naturwissenschaften 101:251–259.

Chun, L., O. Rieppel, C. Long, and N. C. Fraser. 2016. The earliest herbivorous marine reptile and its remarkable jaw apparatus. Science Advances 2.

Miya, M., T. W. Pietsch, J. W. Orr, R. J. Arnold, T. P. Satoh, A. M. Shedlock, H.-C. Ho, M. Shimazaki, M. Yabe, and M. Nishida. 2010. Evolutionary history of anglerfishes (Teleostei: Lophiiformes): a mitogenomic perspective. BMC Evol Biol BMC Evolutionary Biology 10:58.

Pietsch, T. W., and J. W. Orr. 2007. Phylogenetic Relationships of Deep-sea Anglerfishes of the Suborder Ceratioidei (Teleostei: Lophiiformes) Based on Morphology. Copeia 2007:1–34.

Pietsch, T. W., and T. T. Sutton. 2015. A New Species of the Ceratioid Anglerfish GenusLasiognathusRegan (Lophiiformes: Oneirodidae) from the Northern Gulf of Mexico. Copeia 103:429–432.
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