Animal vision varies across species, offering a unique perspective on their surroundings. Fish perceive their underwater world through visual systems adapted to aquatic environments. This adaptation means their experience of light and color differs significantly from our own.
How Fish Perceive Light
Fish eyes share similarities with those of land vertebrates, featuring a cornea, pupil, and lens. Fish have a more spherical lens, which helps them focus light effectively in water. Light enters the eye and is focused onto the retina, which contains specialized light-sensing cells called photoreceptors.
Photoreceptors are of two types: rods and cones. Rod cells are highly sensitive to dim light, providing black-and-white vision in low-light conditions. Cone cells are responsible for color vision and require brighter light. Different cone cells are sensitive to specific wavelengths, allowing perception of various colors. The ratio of rods to cones varies depending on a fish’s habitat and daily activity.
The Unseen Spectrum for Fish
Fish have limitations in perceiving certain colors, especially those at the longer wavelength end of the visible spectrum. Red and orange light are quickly absorbed by water, particularly beyond shallow depths. Consequently, colors like red appear gray or black to most fish in deeper waters, as no red light is available to reflect. For example, red light is largely absorbed within the upper 10 meters of clear ocean water, and yellow light disappears before 100 meters.
Many fish possess the capacity to see ultraviolet (UV) light, which humans lack. UV light has shorter wavelengths than visible violet light and penetrates water effectively. UV vision is enabled by specific cone cells sensitive to these wavelengths. Fish use UV vision for various purposes, including detecting zooplankton, recognizing other fish of their species, and identifying prey. Juvenile brown trout, for instance, use UV vision to find zooplankton in shallow waters.
Diversity in Fish Visual Acuity
Fish vision shows significant diversity, influenced by habitat, species, and evolutionary pressures. Fish living in shallow, clear waters often have a broader range of color perception, sometimes being tetrachromatic, meaning they have four types of cone cells compared to human three. This allows them to see colors, including UV, that humans cannot. Some shallow-water fish use four to six cone classes to accurately reconstruct color signals from aquatic objects.
Conversely, fish inhabiting deep-sea environments or murky waters have eyes adapted for low-light conditions rather than color perception. Deep-sea fish possess a retina dominated by rod cells to maximize light sensitivity, often lacking cone cells sensitive to red and UV light due to the absence of these wavelengths at great depths. Their vision shifts towards blue-green light, the predominant wavelength available in deeper waters.