Visual adaptations for special habitats
Fishes that live at the water’s surface, or that occasionally find themselves totally out of the water, must be able to see in the air. The eyes of mudskippers (Periopthalmidae) are well adapted for aerial vision. A strongly curved cornea and slightly flattened lens permit focusing out of water (Brett1957). This structural adaptation, along with the location of the eyes on retractable stalks on the top of the head, allows these fishes to forage on tidal flats and exposed mangrove roots of the swamps in which they live. The eyes of the surface-dwelling South American “four-eyed fishes”(Anableps, Anablepidae), are adapted to permit simultaneous vision above and below the water (see Brett 1957).Each eye has two pupils (one above and one below the surface of the water), an oblong lens, and a retina that is divided into dorsal and ventral sections. Light entering from above the water’s surface enters the upper pupil, travels through the short axis of the oblong lens, and focuses on the ventral retina. Conversely, light from below the surface enters the lower pupil, travels the long axis of the lens, and is focused on the dorsal retina.
The deep sea is an optically challenging environment –the only light is either dim blue light from above or pointsources of bioluminescence. Deep-sea fishes demonstrate a variety of adaptations that help to optimize vision in these vastareas with little light. The mesopelagic zone (approximately 150–1000 m) has light filtering down from the surface, which diminishes with depth, as well as sources of bioluminescence, so we see great variation in eye designs in fishes of this zone. Adaptations include changes in the size, shape, and orientation of the eyes, as well as changes in visual pigments, in order to maximize the capture and detection of the wavelengths of light reaching these depths(see The deep sea).
Even faint deep blue light from the surface does not reach the bathypelagic zone (>1000 m), where the only light is from bioluminescence. In this zone, small eyes seem to be the answer for a couple of reasons. Small eyes are well suited for detecting point sources of light that are nearby, and therefore within range of bathypelagic fishes, which are weak swimmers due to their watery muscles. In addition, eyes are energetically quite expensive to maintain, and meals in the bathypelagic zone are few and far between– so small eyes are less of a drain on the fish’s overall energy budget.
Some fishes lack functional eyes as a result of degenerative evolution in perpetually dark habitats. The lack of eyesin hagfishes (Myxinidae) is likely a degenerative condition, like the loss of functional eyes among some populations of cave-dwelling fishes. Although some cave fishes lack a cornea, lens, and iris, they still may possess the genes that code for the opsins needed to detect light (Parry et al. 2003).