The four-eyed fish (Anableps) revisited

Updated December 17, 2020
with the addition of Fundulus to the LRT, Anableps moves away from Amia and Anguilla (Fig. 1). Here’s the updated portion of the cladogram:

Figure 1. Subset of the LRT focusing on the ray fin only clade of bony fish. Fundulus (yellow) is the new taxon. It attracted Anableps. Various convergent eel-like taxa are shown in baby blue.

Figure x. Subset of the LRT focusing on the ray fin only clade of bony fish. Fundulus (yellow) is the new taxon. It attracted Anableps. Various convergent eel-like taxa are shown in baby blue.

Figure 4. Amia, Anableps and Anguilla are related to one another in the LRT.

Figure 1. Amia, Anableps and Anguilla are now related to one another in the LRT.

This increasingly odd crocodile-mimic,
Anableps (Figs. 1-3), now nests between primitive bowfins and eels (Fig. 1) in the large reptile tree (LRT, 1655+ taxa; subset Fig. 3). The posterior placement of the pelvic fins is a clue to this fish’s primitive status. The ‘apparent’ lack of cheek bones was a problem that nested Anableps elsewhere earlier.

Figure 3. The four-eyed fish, Anableps, from three data sources. Compare to Fundulus in figure 4.

Figure 3. The four-eyed fish, Anableps, from three data sources. Compare to Fundulus in figure 4.

Previous data (Fig. 1) from Gregory 1933
was just supplemented by incomplete CT scans from Michel 2015 (Fig. 2). I needed this data to understand the vestigial and displaced cheekbones of Anableps that produced the current scores in the LRT. The readily apparent elevation of the orbit was accompanied by a similar, but until now overlooked elevation and shrinkage of the jugal (cyan) and postorbital (amber).

Figure 3. The four-eyed fish, Anableps, from three data sources. Compare to Fundulus in figure 4.

Figure 4. Skull of Fundulus from Gregory 1938. Compare to Anableps in figure 3.

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 shors where they beach themselves to eat. Traditionally Anableps is a member of the (as yet untested) guppy family. Here it nests between the bowfin, Amia, and the American eel, Anguilla (below). Note the elevation of the jugal and postorbital, along with the elevated orbit. The naris is dorsal with an incurrent anterior tubular, pendant one near the mouth and the excurrent one near the orbit, as in eels. The fossil record is as yet unknown. Females are much larger than males. Internal fertilization (with a modified tubular anal fin) leads to live birth (viviparity) of up to 14 young.


References
Gregory WK 1933. Fish skulls. A study of the evolution of natural mechanisms. American Philosophical Society 23(2) 1–481.
Michel KB, Aerts P, Gibb AC and Van Wassenbergh S 2015. Functional morphology and kinematics of terrestrial feeding in the largescale foureyes (Anableps anableps). The Company of Biologists Ltd. Journal of Experimental Biology (2015) 218, 2951-2960 doi:10.1242/jeb.124644

 

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