Fish have very different visual systems compared to humans. The range and perception of color varies greatly between fish species based on the habitats they live in and evolutionary adaptations. While humans rely heavily on color vision, many fish have limited color perception or see color differently than we do. Understanding what colors fish see can provide insight into how they hunt for food, avoid predators, choose mates, and interact with their environment.
The structure and function of fish eyes
Fish eyes come in a variety of shapes and sizes. Fish species that live in open water tend to have eyes on the sides of their head for a wide field of view. Bottom dwelling fish usually have eyes on top of their heads. The size of the eye and properties of the lens also vary, influencing visual acuity and light sensitivity.
But all fish eyes work in essentially the same way. Light enters through the pupil and lens. In the retina, photoreceptor cells called rods and cones convert light into signals to the brain. Cones provide color vision, while rods detect brightness, motion, and shapes. The relative amount of rods versus cones depends on if a fish is diurnal or nocturnal. Diurnal fish active in daytime have a higher density of cones. Nocturnal fish have more rods to enhance dim light sensitivity.
Cone cells give fish color vision
Cone cells contain light-sensitive pigments that respond to different wavelengths of light. Humans have three cone types – short, medium, and long. This trichromatic vision allows us to see the range of colors in the visible light spectrum. Many fish have fewer or more cone cell types with different spectral sensitivities. This gives them different color vision capabilities.
Some fish species are dichromats with two cone types, like goldfish. Others are tetrachromats with four cone types or pentachromats with five. A few fish may even have more than five spectral classes of cones. More cone cell types expands the range of wavelengths and color differences a fish can detect. However, having more cones does not necessarily mean a fish has better color vision compared to species with fewer cones. It depends on how the cone cell signals are processed in the brain.
Spectral ranges fish can detect
The cone cells of fish are sensitive to wavelengths from the near ultraviolet (300-400 nm) to the far red end of the spectrum (700-800 nm). Many fish have cones sensitive to UV wavelengths below what humans can see (400-700 nm). Certain damselfish may have up to 12 different UV cone receptors. This allows them to see UV patterns on corals and possibly for communication.
Fish can also detect far red and infrared wavelengths. Snakehead fish have receptors at 700-800 nm to help them hunt in murky waters by sensing the body heat of prey. Many fish absorb far red wavelengths for vision in deep waters where red light still penetrates. So while fish have a wider overall spectral range, they may not necessarily see more colors. Much of the UV or far red sensitivity is for enhancing contrast rather than additional hues.
Differences in color vision between fish
The ability to discriminate color varies widely between different fish species. Here are some examples:
Salmon: Salmon have rods and four spectral classes of cones. They use color vision to navigate during migration and locate food. Salmon can see red, green, and blue wavelengths similarly to humans but also detect ultraviolet. This helps them spot zooplankton that absorb UV light against darker backgrounds.
Goldfish: Goldfish only have cones sensitive to red and green light. They have poorer color vision and cannot distinguish between blues and yellows. Goldfish rely more on monochromatic vision to track motion and see contrast. Their limited color vision is sufficient in turbid freshwater.
Tetras: Small tetras living in the Amazon have five types of cones and superior color vision. They use their complex spectral sensitivity to find fruits and plants in vegetated areas. Male tetras also flash iridescent UV colors to attract females.
Seahorses: Seahorses have a single long-wavelength sensitivity cone giving them monochromatic yellowish vision. They rely entirely on brightness contrast and movement for prey capture and navigation.
Trout: Trout inhabit clearer waters and have four cone types with overlapping sensitivities. This gives them an expanded range of color vision and ability to discriminate hue, especially in the red-UV range. Trout distinguish colored foods and respond to colored fishing lures.
How fish use color vision
The exact use of color vision depends on the fish species and their ecological niche. Here are some common functions:
Finding food: Many fish use color to spot fruits, plants, coral, or prey animals against varied backgrounds. Certain wavelengths may indicate food sources.
Avoiding predators: Fish see contrasting colors to identify predators, especially those ambushing from above or below. Some fish camouflage with surroundings.
Navigation: Reef fish discern colored corals, while migratory fish may use color cues for orientation and finding spawning sites.
Communication: Bright colors are used to attract mates and defend territories. Some fish change color to signal aggression or camouflage.
Sensitivity: UV, red, or infrared sensitivity may help fish in dark or murky waters detect objects from better contrast.
How water impacts fish color vision
The aquatic environment also influences what colors fish can see through selective absorption and attenuation of light. Longer wavelengths like reds and oranges are absorbed rapidly, while blues and greens penetrate deeper. Water conditions vary in different habitats:
Clear ocean: Blue is most intense, while reds fade at 5-10 m depths. Fish favor blue cones in clear waters.
Turbid lakes: Suspended particles scatter light, attenuating all wavelengths. Fish detect brightness and contrast, not specific colors.
Deeps seas: Only bluish bioluminescence penetrates beyond 200 m. Vision adapted to faint blue light.
Freshwaters: Broad spectrum but green/yellow/red scattered most. Fish have complex color vision.
Blackwaters: Tannins cause severe red light loss, fish shift to violet/UV sensitivities.
So fish color vision adapts to both the available spectrum and needs of particular environments.
Conclusions
– Fish have very diverse color vision capabilities depending on their cone cells. Some see only shades while others perceive complex hues.
– Many fish see in ultraviolet and infrared wavelengths invisible to humans. But this expands sensitivity more than additional colors.
– Color vision helps fish find food and mates and avoid predators. Functions vary by species and habitat.
– Water color selectively absorbs and scatters light, influencing what colors fish need to see. Environments shape fish visual systems.
– While fish perceive color differently than humans, their unique visual systems are still finely tuned to their aquatic worlds.
