Fish eye out of water: epithelial surface projections on aerial and aquatic corneas of the ‘four eyed fish’ Anableps anableps

Background: Vertebrate corneas feature a variety of microprojections, to which a tear
film adheres. These microprojections are formed by folds in epithelial cell membranes,
which increase surface area, stabilise the tear film and enhance movement of nutritional
and waste products across cell membranes. Differences in corneal microprojections
among vertebrates have been correlated with habitat and differ markedly between terrestrial
and aquatic species.
Methods: This study investigated epithelial microprojections of both the aerial (dorsal)
and aquatic (ventral) corneal surfaces of the ‘four-eyed fish’ Anableps anableps using
scanning electron microscopy.
Results: The central region of the dorsal cornea, which projects above the water, had a
density of 16,387 3,995 cells per mm2, while the central region of the ventral cornea
(underwater) had a density of 22,428 6,387 cells per mm2, a difference that suggests an
environmental adaptation along the two visual axes. Both corneal surfaces were found to
possess microridges rather than microvilli or microplicae characteristic of terrestrial/
aerial vertebrates. Microridges were 142 9 nm wide and did not differ (p = 0.757)
between dorsal and ventral corneas. Microridges were consistently separated by a distance
of 369 9 nm across both corneas.
Conclusion: Dorsal-ventral differences in corneal epithelial cell density in Anableps anableps
suggest a difference in osmotic pressure of the two corneas. The modest differences
in the microprojections indicate that the need to secure the tear film underlying each
optical axis is of prime importance, due to the likelihood that a persistent layer of water
normally covers both dorsal and ventral corneal surfaces and that maintaining a transparent
optical pathway for vision is critical for a species prone to predation from both
above and below the water’s surface.