Physiological Optics in Fishes

Abstract

The optical system of fish eyes, as present in many shallow-water species, is adapted to hydrodynamic constraints and wide-angle color vision. The cornea has little refractive power in water, and hence the crystalline lens is the dominating refractive element. Fish lenses are gradient-index lenses that often have multiple focal lengths in monochromatic light (multifocal lenses). In light containing a spectrum of wavelengths, a different range of wavelengths is focused on the retina by each focal length, such that well-focused color images are created. Adjustment of focus (accommodation) occurs by movements of the lens in teleosts and chondrichthyans, while it is unclear whether lungfishes and lampreys are capable of accommodation. Various fish eyes have fascinating optical adaptations to visual requirements of their photic environments.