Morphological and Histological Adaptations for Retinal Structure in Eyes of the Amphibious Fish, Alticus kirkii and the Non-Amphibious Fish, Gambusia affinis

The current research aimed to investigate the adaptations in eye morphology and the structure of retinal photoreceptors in amphibious fish (Alticus kirkii) and non-amphibious fish (Gambusia affinis) to understand the visual adaptations of A. kirkii upon being out of water. The morphological and histological features of the eyes of both species were adressed. Results showed that A. kirkii exhibits several visual adaptations for seeing in air. Morphologically, the eyes of A. kirkii are bulging and positioned on retractable stalks at the top of the head, unlike those of G. affinis. The ratio of axial length to head length is greater in A. kirkii than in G. affinis. Histologically, the cornea of A. kirkii is more curved than that of G. affinis, with a crooked internal layer that is more curled in A. kirkii and reduced in G. affinis. The lens is larger, slightly flattened, and more stable in A. kirkii compared to G. affinis. The retina is thicker in A. kirkii and consists of 10 layers. In A. kirkii, the external layer of pigmented epithelium is densely packed in the peripheral region of the photoreceptor layer, whereas in G. affinis, it is composed of epithelial cells in the peripheral region but is less thick. The visual cell layer includes simple and double cones; A. kirkii has a greater abundance and larger size of cones compared to G. affinis. The outer nuclear layer, which contains the visual cell bodies, is thicker in A. kirkii than in G. affinis. The outer plexiform layer is narrower in A. kirkii, with higher densities of both bipolar and horizontal cells compared to G. affinis. The inner nuclear layer shows a heterogeneous cell composition and is more crowded in A. kirkii than in G. affinis. Additionally, disparities in the thickness of the inner limiting membrane and nerve fiber layer affect the transmission of signals from the retina to the brain.