Fish live in water, so they have a special set of organs.
You may have thought that we humans cannot see underwater. Then how do fish see underwater?
The truth is that they do not see as clearly as we do. Water is not a good medium for light. When light travels, its intensity decreases very quickly. Sound travels better, but the light gets lost, so fish can merely see 164-492 ft (50-150 m) in clear water also. While on land, we can see up to a few kilometers on a clear day. In the water, everything gets blurry.
If we talk of muddy water or turbulent waters, the visibility is even less. It shall be only a few centimeters. When we go deeper into water the penetration of light is lesser. So, the fish that live in the deep-sea can barely see anything. They do not have much use for deep-sea fish eyes. In fact, there are species of fish that do not have eyes, or you can say that they are blind.
In the ocean, there are many different types of fish, like fish with bulging eyes, bubble eyes fish, barreleye fish eyes, and cuttle fish eyes. They are all very different. You may not have known there are fish with eyes on one side as well in the ocean. Read on to learn more!
If you enjoyed this article, why not also read about fish eggs or how often do you feed a betta fish here on Kidadl?
How do fish eyes differ from human eyes?
Fish eyes are used to seeing underwater, while we humans see in the air around us. So, the pair of eyes we have must be different.
It is known that the structure of the fish eye is similar to the human eye. But the shape is a little different. There are parts like the cornea, iris (adjustable, lens), visual cells, and a retina having cods and cones. All these parts help make the final image.
While the human eye has a flat lens system, the lens in a fish eye is round. They're protruding outwards. This is the reason for greater peripheral vision. It gives them the ability to catch prey and stay out of the hands of predators.
The lens in the fish eye is sphere-shaped. It is the most functional and important part of the eye. The lens protrudes right into the opening in the pupil. This whole arrangement is different from that of the human eyes. The lens in the human eye is flat and has a position behind the pupil.
The other noted difference is that fish do not cry. There are no tear glands in the fish eye, the lachrymal glands are missing. The fish live in water, so their eyes are always being washed on their own. So, they do not have the utility of the tear glands.
Also, the fish do not have any eyelids. Some people may not agree, as there is evidence of a few fish having their skin from eye cover extend to cover their eyes. While in the case of sharks, it is believed that they have a nictitating membrane. This can be pulled down as well over the eye. When they go out hunting or feeding, this layer protects the eyes of the shark.
Can fish see color?
As we know, water absorbs light. It is quite possible that the entire light is not visible the same way underwater as it is seen above the water.
Light is made up of various wavelengths. The intensity is also different. Violet light has the shortest wavelength, while red light has the longest wavelength. So, the absorption of red light is much greater than that of violet or blue light. When we go down only 3.2 ft (1 m) you will notice that one-fourth of the red light has been absorbed. There will be a loss of intensity.
So, the deeper you go into the ocean, the fewer colors you will see. If you go around 328 ft (100 m) deep, there are chances that there will be no light at all. So, this means that deep-sea fish do not see anything in the loss of light.
A study suggests that stingray fish and sharks cannot see color. They are literally colorblind. They learn to smell blood and catch prey with the help of shapes they see. While the fish close to the surface of the water still has the sensitivity towards light. Fish use touch, taste, and even sound to make out the environment around them. There are not many roles for the eyes. The small fish copy the elders to learn these tricks.
Why do cave fish lose their eyes?
The main reason for any fish losing eyes would be the lack of penetration of light to greater depths. There are chances that the eyes are not used at all.
A few deep-sea researchers believe that an epigenetic mechanism must have taken place. There must have been DNA mutations with time. As in the deep caves, there is no light, hence no role for the eyes. So eventually, the cave fish lost their eyes. They became blind. But this does not mean they just keep banging here and there. They have separate means of finding ways in the dark.
Some researchers also call it pleiotropy. This means there can be more than one reason for such a mutation.
What fish can blink with both eyes?
No fish can blink as they do not have eyelids. Although a few people may say that sharks have the ability to blink.
The skin that is the extension of the skin over the eyes is the extra skin that is working as the eyelids. But these cannot move or cover the eyes. When the sharks go out to catch prey, the layer called the nictitating membrane covers the eye for protection.
Fish Eye Anatomy
The structure of the fish eye is very similar to the eyes of vertebrates.
The light is processed in the following manner. Light enters through the cornea and passes through the pupil. This light eventually falls on the lens where the image is formed. Interestingly, the pupil size of the fish remains the same; it does not get altered.
It has been noted that the eyes of sharks or rays have a muscular iris. This means that the pupil can be dilated or adjusted as per the requirements. The pupil can take many shapes, like a circle or a small slit-like.
It has been noted that the fish lens has a texture that is highly dense and spherical based on the needs of the body. This allows them to see the front as well as the other sides. The fish with big eyes on top of its body is able to see better. When a school of fish is together, they are better equipped to escape from the predator first and then catch the prey.
At times, koi fish eyes seem to be protruding out and are called 'popping eyes'. They can still see across the water content even in a tank. Halibut fish have normal eyes when they are born. As they grow, they convert into so-called one-sided eye fish. The blood and the two eyes' content move to one side of the body.
Monocular Vs. Binocular Vision In Fish
Fish have different positions for their two eyes. The position of the fish eye depends on the environment they live in.
Fish have monocular as well as binocular vision. Monocular vision is not efficient at perceiving depth. It is a 2-D image. Binocular vision uses both eyes to track things.
How far can fish see?
Fish can see as far as 49 ft (15 m) even in clear water. Water is a dense medium, and light does not penetrate much.
Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for fish eyes, then why not take a look at fish with legs, or River Chub facts.
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