Freshwater Anableps finds a new home with deep sea Gigantura

Weeks of work on ray-fin fish
continue to produce fascinating results. Unfortunately only one or two taxonomic insights are appearing at a time, as readers have witnessed. Convergence is a frustrating problem. Hundreds of mistakes have been corrected with new data similar to today’s new data.

A problematic taxon, Anableps,
the fresh water four-eyed fish (Figs. 1, 3), finally finds a home with a taxon that shares a long list of previously overlooked traits, a deep-sea predator, Gigantura, the telescope fish (Figs. 2, 3). This match only came about because data on the dorsal view of the skull of Anableps became known. This skull architecture appears in only these two taxa. Perhaps, not surprisingly, but rather obviously, both Anableps and Gigantura have eyeballs that extend beyond the skull boundaries.

LCA
Trachinocephalus, a so-called lizard fish (Fig. 3) is a last common ancestor. It prefers shorelines. Lepidogalaxias, a so-called salamander fish (Fig. 3), is another relative one step removed, but has a similar elliptical caudal fin and prefers fresh water. So we’re getting new clues to the ancestry of these highly derived, yet basal bony fish, Anableps and Gigantura.

If Gigantura and Anableps have ever been nested together
in an academic publication, let me know so I can cite it. Otherwise this appears to be an overlooked hypothetical interrelationship.

Let’s look at these two together,
perhaps for the first time.

Dorsal views of the two skulls
(Figs 1, 2) show nearly identical architecture. In the large reptile tree (LRT, 1801+ taxa) no other taxon nests closer to Anableps than Gigantura and it’s even stranger relative, Stylephorus (Fig. 3). Given that semi-related Doliodus (Fig. 3) comes from the Early Devonian, all these taxa have had plenty of time to go their separate ways from a basal bony fish radiation.

Th interrelationship between Gigantura and Anableps
may have gone unnoticed thus far because lateral views of the skulls differ:

1. In Gigantura the occiput is far anterior to the jaw joint
2. In Anableps the occiput is far posterior to the jaw joint
3. In Gigantura the maxilla is greatly reduced below the nasal
4. In Anableps the maxilla is not reduced
5. In Gigantura the nasals are greatly reduced
6. In Anableps the nasals are larger than typical

Konstantidnidis and Johnson 2016 
discussed the homology of the tooth-bearing upper jaw elements. The authors considered the upper jaw composed of the palatine alone “based on topological evidence.” Anableps was not mentioned in their text. Based on comparative anatomy with Anableps, new identities are assigned to certain bones in the Gigantura skull.

1. The ‘palatine’ (Fig. 2) is here a premaxilla.
2. The ‘vomer’ is here a vestigial maxilla, still dorsal to the premaxilla.
3. The leaf-like ‘maxilla’ is here a lacrimal, as in Anableps.
4. The frontals include parietals, perhaps fused

Anableps tetrophthalmus
(originaly Cobitis anableps Linnaeus 1758, Scopolis 1777; 32 cm) is the extant four-eyed fish (aka: cuatro ojos), a surface predator of insects that fall into fresh waters or are preyed upon on shallow shores where Anableps beach themselves to eat. The short lower jaw enables this. Traditionally Anableps is a member of the pupfish (guppy, killifish, topminnow) family. Here Anableps nests with deepsea telescope fish like Gigantura. The naris is anterolateral. Traditionally the fossil record is unknown, but now see figure 3. Females are much larger than males. Internal fertilization (with a modified tubular anal fin) leads to live birth (viviparity) of up to 14 young. The vertebral number is higher than typical for most ray-fin fish.

Gigantura indica 
(Brauer A 1901, Konstantinidis P and Johnson GD 2016; 20 cm standard length, not counting caudal fin) is the extant telescope fish. This small fish can swallow prey larger than itself.

Stylephorus chordatus 
(Shaw, 1791, Regan 1924) is the extant tube-eye or thread-tail. It was considered an oarfish relative, but here nests with Gigantura. Distinct from Gigantura, but convergent with seahorses and oarfish, the tube-eye feeds on tiny plankton sucked in as its tubular mouth enlarges the oral cavity by 40x.

References
Brauer A 1901. Über einige von der Valdivia-Expedition gesammelte Tiefseefische und ihre Augen. Sitzungsberichte der Gesellschaft zur Beförderung der Gesamten Naturwissenschaften zu Marburg 8: 115–130.
Konstantinidis P and Johnson GD 2016. Osteology of the telescope fishes of the genus Gigantura (Brauer, 1901), Teleostei: Aulopiformes. Zoological Journal of the Linnean Society 179(2):338–353.
Michel KB, Aerts P, Gibb AC and Van Wassenberg S 2015. Functional morphology and kinematics of terrestrial feeding in the largescale foureyes (Anableps anableps). Journal of Experimental Biology (2015) 218, 2951-2960 doi:10.1242/jeb.124644
Perez et al. (9 co-authors) 2017. Eye development in the four-eyed fish Anableps anableps: cranial and retinal adaptations to simultaneous aerial and aquatic vision. Proc. R. Soc. B 284: 20170157.  http://dx.doi.org/10.1098/rspb.2017.0157
Scopoli GA 1777. Introductio ad historiam naturalem sistens genera lapidum, plantarum, et animalium. Wolfgang Gerle, Pragae 3-506.
Regan CT 1924. The morphology of the rare oceanic fish, Stylophorus chordatus, Shaw; based on specimens collected in the Atlantic by the “Dana” expeditions, 1920–1922. Proceedings of the Royal Society B 96(674): PDF
Shaw G 1791. Description of the Stylephorus chordatus, a new fish. Transactions of the Linnean Society of London, 2d Ser: Zoology 1:90–92.

wiki/Anableps
wiki/Telescopefish = Gigantura
wiki/Tube-eye = Stylephourus