Light is bent, or refracted, when it moves from one medium to another, such as from air into the eye. The stark differences between the aquatic environment of fish and the terrestrial environment of humans have forced the evolution of two distinct visual systems. The fundamental challenge for any eye is to focus light precisely onto the retina, but the medium in which the eye operates determines the necessary optical design. Consequently, the eyes of fish and humans represent two highly specialized solutions to the same problem.
Structural Adaptations for Water Vision
The largest difference in structure is driven by the refractive index of water, which significantly impacts the eye’s primary focusing element. In the human eye, the cornea performs the vast majority of light refraction because of the large difference between the refractive index of air and the fluid within the eye. However, since water has a refractive index very similar to the fluid of the fish eye, the fish cornea is largely ineffective for focusing light and serves mainly as a protective layer. This loss of focusing power necessitates a completely different lens design in fish.
The human lens is relatively flattened and flexible, acting only as a fine-tuning mechanism for focusing light. Fish, in contrast, possess a dense, perfectly spherical lens that protrudes through the iris. This spherical shape maximizes the lens’s refractive power, compensating for the lack of corneal refraction and ensuring light is strongly bent to form a clear image on the retina.
Another structural difference lies in the iris and pupil. The human iris contains muscles that can rapidly contract or dilate, changing the pupil’s size to regulate the amount of light entering the eye. Most species of bony fish, known as teleosts, have an iris that is fixed or nearly immovable, meaning their pupil size is constant. This fixed aperture reflects the generally lower and more stable light levels found in most underwater habitats, reducing the need for rapid light adjustment.
How Fish and Humans Focus Light
The process of accommodation, or changing focus for objects at different distances, is achieved by fundamentally different mechanisms in the two species. Humans use a process that changes the focal length of the lens itself. Ciliary muscles surrounding the human lens contract, which relaxes the tension on the suspensory ligaments. This release of tension allows the naturally elastic human lens to become thicker and rounder, increasing its refractive power to focus on near objects. To focus on distant objects, the ciliary muscles relax, pulling the lens flat again and decreasing its power.
Fish, however, have a rigid, spherical lens that cannot change its shape. Instead of altering the lens curvature, fish adjust focus by moving the entire lens closer to or farther from the retina, similar to how a camera lens focuses. In bony fish, a muscle called the retractor lentis pulls the lens backward toward the retina to focus on distant objects. The lens is often held in a position that naturally focuses on objects close to the fish, meaning many species are considered naturally nearsighted. Cartilaginous fish, like sharks, use a protractor lentis muscle to pull the lens forward, away from the retina, to achieve a near focus.
The Range of Light They Perceive
The sensory capabilities within the retina also show significant divergence, particularly regarding color and light sensitivity. Human color vision is based on trichromacy, meaning we possess three types of cone photoreceptor cells that are sensitive to different wavelengths of light: red, green, and blue. Many fish species, particularly those in shallow water, are tetrachromatic, possessing a fourth type of cone that often extends their color vision into the ultraviolet (UV) spectrum. This ability to perceive UV light provides an ecological advantage, allowing fish to detect UV-reflective plankton or to use UV patterns for communication and species recognition. The ancestral vertebrate was likely a tetrachromat, meaning humans and other mammals lost this capability, while many fish retained it.
The ratio of light-sensitive rods to color-sensitive cones adapts to the habitat’s light level. Humans rely on a combination of rods for low-light vision and cones for detailed color vision in bright light. Fish living in the perpetually dark deep sea often have retinas overwhelmingly dominated by rods, optimizing their ability to detect the minimal light available from bioluminescence. Some fish also possess the specialized ability to detect polarized light, a capability largely absent in humans. Polarized light, which is light waves oscillating in a single plane, is useful for navigation in open water and for breaking the camouflage of prey or predators.