No, fish cannot see in the dark. Similar to humans, fish also don’t have the ability to see in the dark. However, some fish have better vision in low light compared to other aquatic animals. Many of them cannot use their eyes to see things in the dark. Therefore, they use their other senses such as taste, smell, hearing, and some special senses such as magnetoception to experience things around them.
How do fish see in the dark?
You must be wondering if fish cannot see in the dark, then how can fish sense animals nearby them.
Well, you will be surprised to know that all fish don’t use their eyes to sense nearby animals or preys. Some fish identify nearby objects by sensing pressure change in their surrounding while some fish use magnetoception and chemoreception in the dark.
Most of the fish have a lateral line or organ which detects the movement, vibrations, and pressure gradient in the surrounding water. You can easily spot the lateral lines present on both sides of the fish body which runs along the length of the fish. Most of the fish use this lateral line to spot and catch their prey in the dark. Fish such as cookiecutter shark and viperfish which are found living in dark deepwater use their lateral lines to schooling behavior, predation, and orientation.
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There are some fish such as sharks, stingrays, and chimaeras that have an electroreceptive organ that allows perceiving direction, altitude, or location by detecting the magnetic field. This sense is known as magnetoception. They can also detect the small electric field produced by animals around them.
Fish such as catfish use their taste (gustatory receptors), and smell (olfactory receptors) sensing organs to find and catch their prey.
So we now know that in the dark or low light conditions, fish don’t have to depend on their eyes. Fish can use their lateral line, taste, smell, and sound detecting organs to replace the functioning of the vision.
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How the structure of a fish eye looks?
The fish living near the surface of the water is exposed to sufficient light that they can easily locate and hunt their prey using their eye. Fish have better vision underwater as compared to other species due to their eye structure. They also have superior night vision. Let’s understand what makes the vision of fish superior to other species.
We know that most of the vertebrates such as amphibians, reptiles, birds, and mammals are evolved from the fish ancestor. Therefore, fish have almost similar eye structures. Like other vertebrates, the fisheye also consists of the cornea, iris, pupil, lens, retina, and vitreous humour. The light enters the eye through the cornea (the transparent white part which covers the pupil and iris) and reaches to lens by passing through the pupil.
Most fish have fixed pupil size with a varying shape such as circular and slit-like shape. However, some fish such as sharks can adjust their pupil size with the help of their muscular iris. The lens of the fisheye is generally more dense and spherical. This spherical lens refracts the light to focus on the retina, which helps them to have better peripheral vision.
After the lens, it travels through a transparent fluid called vitreous humour to reach the retina. The retina contains photoreceptors such as rod and cone cells. Rod cells provide better vision in low light conditions while cone cells are responsible for color vision and perform well in bright light. More number of rod cells are found in fish who live in low light or dark conditions while more cone cells are found in fish who live near the surface of the water.
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Some fish have a layer of tissue behind the retina called tapetum lucidum which reflects back the light passing through the retina again to the photoreceptors present on the retina. This reflection increases the light available to photoreceptors such as rod and cone cells. Fish with tapetum lucidum have better night vision compared to other fish or aquatic animals.
Most deep fish such as dragonfish have visual pigments that are sensitive to wavelengths around 460-490 nm. They also emit far-red light from photophores (light-producing organ) which is invisible to the other animals.
How fish at various depths see in the dark?
The ocean is divided into four divisions based on the depth: epipelagic zone, mesopelagic zone, bathypelagic zone, and abyssopelagic zone.
The fish which are found up to the depth of 200 m from the top surface of the water are called epipelagic fish. These epipelagic fish have sufficient light to use their eye vision to see and hunt the prey. These fish have more cone cells as compared to rod cells which help them to identify color clearly. The examples of fish found in the epipelagic zone are Atlantic bluefin tuna, basking shark, and whale shark.
The fish living in the next zone that is the mesopelagic zone (200-1000 m) are exposed to low light conditions. They have large eyes with more number of rods cells which provide them better vision in low light. They have superior night vision as compared to epipelagic fish. Mesopelagic fish such as sabertooth fish and lanternfish have adapted to live under low light conditions.
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The zones below the mesopelagic zone are dark with low temperatures and high pressure. The amount of dissolved oxygen is also in these regions. To adapt
They have small eyes which are completely useless due to lack of sunlight. They generally depend on their lateral line and olfactory systems to adapt to conditions of the bathypelagic zone and abyssopelagic zone. Some of the common fishes found in these zones are bristlemouth, anglerfish, fangtooth, viperfish, daggertooth, and barracudina.
Do fish sleep when it’s dark?
Yes, when it is dark in the fish tank, fish do sleep but not similar to humans or other animals. They become motionless and reduce their metabolism and other activities but they remain alert to the attack of predators.
Can aquarium or tropical fish see in the dark?
The answer is still no, the aquarium fish such as angelfish, catfish, and cichlids cannot see in the dark. Similar to humans, they cannot see in the dark and sleep during the nighttime. They can use their olfactory and gustatory sense to move around in the dark.
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