Have you ever really thought about how we are surrounded by air but cannot see it? Sometimes, when it is very windy, we can feel it, but mostly it is simply there. How about fish – how do they relate to the element that surrounds them?
Can fish see water? No, they don’t. In the same way, our brain is trained to ignore constant sensory input that would otherwise distract us, fish filter out the sight, smells, and sound of water because being aware of all this would lead to constant overstimulation which would leave them unable to cope with their environment. If a new sensory input is introduced to their environment, like the smells or sounds that might come with a predator, a
The reason why we are not able to see air is that everything that has the same or a lower refractive index as air is invisible to us. This is why we also cannot see, for example, helium (it has a lower refractive index than air) but can see water, which has a higher refractive index.
A refractive index is the measurement of the speed with which light travels through a medium. The bigger the refractive index, the slower the light travels. This means that light travels more slowly through water than it does through air, and it travels even more quickly through helium.
When you think about it, it is not even water as such that even we humans see, it is the reflections and refractions of light. You can try this yourself: If you fill a glass with completely clear water up to its rim and look at it through its side, you will not be able to recognize that there is water in the glass unless you look at its surface where light is reflected.
Perfectly clear water is found very rarely in nature, though, so what we often see when we see the water is the debris that is floating in it or its general opaque murkiness. When you are diving, you are also less aware of the water that is right in front of you, unless you look upwards and see the light reflecting on its surface, or a beam of light crosses your field of vision.
Do All Fish See The Same Things?
No, because different species of fish live in a lot of different environments, their abilities to see have to differ greatly.
As mentioned above, vision, in general, depends on light. You might already know by experience that the amount of light decreases the deeper into the water you dive. But instead of everything just gradually getting darker, various colors disappear at different depths.
Red light is absorbed pretty quickly, within just about 30 feet. Shortly after, yellow and orange light disappears, then the green light. Blue light reaches down to about 650 feet! This is why some deep-sea creatures only have the cones in their eyes that are necessary to receive blue light.
To be able to receive more colors would be a waste at the depth they are living in. If you have seen the kids’ movie Finding Nemo, you might remember the scene in which Marlin, a red and white clownfish, encounters a deep-sea anglerfish. Now you know that anglerfish would not be able to see the actual colors of the clownfish!
Other deep-sea creatures like the hagfish do not even have eyes. The hagfish only have light-sensitive cells on its head that can differentiate between light and dark.
Can Fish See Colors?
Fish that swim closer to the surface or in relatively shallow bodies of water can see a variety of colors, though. This is why there are so many colors of fishing lures available – a lot of anglers swear on changing their colors depending on the season, weather, or whether you are fishing from the shore or from a boat.
In general, fish have a much wider field of vision than humans, but they are not good at seeing details, except for certain predatory fish like the brown trout. Fish are better evolved at noticing movement and contrast within a wide range, rather than focusing on something that is directly in front of them.