Can fish see more colors than humans?
Fish inhabit a very different environment from humans, living underwater instead of on land. This means their eyes have evolved differently to allow them to see well underwater. An interesting question is whether the way fish eyes work gives them the ability to see a wider range of colors than humans can.
How do fish eyes differ from human eyes?
Human and fish eyes share some similarities in their basic structure, with a lens that focuses light and a retina containing photoreceptor cells that detect light and send signals to the brain. However, there are some important differences that adapt fish eyes for vision in an aquatic environment.
One key difference is that fish eyes are spherical while human eyes are more oval shaped. The spherical shape improves peripheral vision under water. Fish eyes also have much harder and more curved lenses compared to humans. This curvature helps counteract the refraction that occurs when light passes from water to the eye.
The major difference though is in the photoreceptor cells. Humans have two main types of photoreceptors – rods for low light vision and cones for color vision. Fish retinas contain rods and cones too, but often have more cone cell types than humans. Many fish have four or even five different cone cell types compared to three in humans.
| Photoreceptor Type | Humans | Fish |
| Rods | 1 type | 1 or more types |
| Cones | 3 types | 4 or 5 types |
The extra cone cell types enable fish to detect light of different wavelengths, which would allow them to distinguish more colors.
What are the color detecting cone cells?
The cone cells in human and fish retinas contain photopigments that are sensitive to particular wavelengths of light. When a photon with the right wavelength is absorbed, the photopigment changes shape and sends an electrical signal to the brain.
Humans have three cone types with different photopigments that detect short (blue), medium (green), and long (red) wavelength light. These three cone types allow us to see the range of colors in the visible light spectrum.
Many fish have additional cone types compared to humans. For example:
| Fish Species | Cone Types |
| Goldfish | 4 |
| Trout | 4 |
| Salmon | 4-5 |
| Zebrafish | 4 |
The extra cones allow these fish to detect more wavelengths of light. Zebrafish for example have cones detecting blue, green, red and ultraviolet light. This gives them a wider spectral range than humans, extending further into the ultraviolet.
Do more cone types definitely allow better color vision?
Having more types of cone cells with different photopigments provides fish with the potential to discriminate more colors. However, it does not necessarily mean they have superior color vision compared to humans. Just comparing the number of cone types has limitations.
There are a few other factors that influence how an animal perceives color:
– Overlap between cone sensitivities – There may be overlap in the wavelengths the different cone cells detect. This reduces the number of discriminable colors.
– How the signals from different cones are processed by the brain – The brain integrates and interprets signals from the cones. More complex processing may enable better color discrimination even with fewer cone types.
– Environmental light conditions – Color perception depends on the ambient light conditions. Murky or restricted wavelengths underwater may limit what fish can detect.
So having more cone types doesn’t automatically translate to seeing more colors. How the fish visual system as a whole interprets and processes those extra signals also plays a major role.
Can we test what colors fish can discriminate?
Researchers have used various techniques to try to test the color vision abilities of different fish species:
– **Training experiments** – Fish are trained to discriminate between two colors in order to receive a food reward. The spacing and number of discriminable colors indicates visual capability.
– **Electroretinography** – Measuring electrical responses from cones or other retinal neurons as different wavelengths of light are shown. Subtle differences in response timing and strength can indicate color discrimination.
– **Microspectrophotometry** – Analyzing the absorbance spectra of individual photoreceptor cells. Shows what wavelengths they detect.
– **Sequencing cone opsin genes** – Opsins are the photopigments in cone cells. Sequencing them identifies their light sensitivity.
– **Behavioral assays** – Observing how fish behave with different colored substrates, prey items or markings. Suggests an ability to discriminate colors.
Such tests have demonstrated some species like salmon and goldfish can detect and discriminate colors that humans cannot see, into the ultraviolet range. But overall fish color discrimination is still thought to be fairly comparable to humans rather than superior.
How does the aquatic environment affect color vision?
We have to consider the environment fish have evolved to see in. Light wavelengths in aquatic habitats are restricted compared to air. Red, orange and infrared wavelengths are absorbed strongly by water, meaning fish are unlikely to detect long wavelength colors well.
Water conditions also affect what fish can see. Turbid or murky water scatters and filters out more light. In extremely silty waters, fish may rely largely on other senses like smell rather than vision. Nearer the surface, changing light conditions like rippling can also disrupt color perception.
So while fish may detect some colors humans cannot, the smearing and attenuation of light underwater likely limits how well they actually discriminate colors. Their color vision abilities are adapted to be sufficient rather than superior in the available conditions.
Do fish have better vision than humans in other ways?
While the jury is still out on whether fish definitively see more colors, research has shown their vision can be superior to humans in other aspects:
– **Motion detection** – Fish are very sensitive to movement, useful for spotting both prey and predators. Goldfish have been found to detect movement of only a few degrees of visual angle.
– **Low light** – Many fish see well in dark murky conditions. Some fish retinas are a thousand times more sensitive to light than human eyes.
– **Polarization vision** – Some fish can detect the polarization of light, which is invisible to humans. This aids navigation and prey detection.
So fish visual abilities are impressive and tailored to their environment, just tuned to different attributes than human sight. Having better motion detection and low light vision is likely more essential for fish than an expanded color range.
Conclusion
While fish like salmon and goldfish have more cone cell types than humans, and can see into the ultraviolet, it’s not clear this allows them to see significantly more colors. Other aspects of their visual processing and the restrictions of aquatic environments likely limit advanced color discrimination. Fish have evolved specialized vision to suit their needs, but overall evidence suggests they do not have markedly superior color vision compared to humans. Their sight is tuned to other attributes like motion detection and low light capability. So while fish may see some hues outside our range, their perception of color is probably fairly comparable to our own.
