Do Fish Blink? The Full Guide

Have you ever imagined yourself in a pool of water without covering your eyes? In no time you’d feel the urge to blink and that’s a natural response.   

Your eyelid is an important part of your body and it is just as important for almost all living organisms. Is this true for fish too? Have you ever seen a fish blink? Have you seen the eyelid of a fish before? How do fishes swim with their eyes open?

Due to the absence of eyelids, fish do not blink. The only exception is the shark. Sharks have a nictitating membrane that functions just like the eyelid.

Do Fish Blink? No, fishes do not blink due to the absence of eyelids. They lack lachrymal or tear glands. Fishes can do without tear glands as water keeps their eye surface moist. However, fishes have an extension of the skin of the head over the eye surface. Sharks on the other hand possess separate membranes that cover the eye surface when needed and it is called the nictitating membrane. Furthermore, the membranes or skin derivatives are naturally transparent for obvious reasons.

The next time you try to feed your fish in its aquarium, take a close look at their eyes. Now that you know fish do not have eyelids, you may wonder how they sleep or how they manage to protect their eyes from water. This article will explore this and find answers to these questions. We shall also provide interesting facts about fish. 

What is the role of blinking the eyes?

The reason for blinking is the same in all living organisms and not restricted to humans. Blinking is involuntary and happens to protect your eyes.

 The eyelids secrete a mix of oils and mucous whenever you blink which works to keep the eyeballs moisturized. To confirm this, try to keep your eyes open for 10 seconds without blinking. You will undoubtedly feel an itching sensation in your eyes.

The eyelid also performs another important role which is to protect the eyeball from harmful external stimuli. For example, if you stare into bright lights or walk against the wind, your body’s natural response will be to shut the eyelids. This is one of the involuntary actions that your eyelid performs to protect your eyeballs.

This involuntary action is the same for other species as well. How then do fish handle this without an eyelid?

How do fish survive without an eyelid?

As mentioned earlier, the key function of the eyelid for animals that live on land is moisturizing the eyeballs. Because fish live in water, they do not need extra moisture for their eyes. They have evolved over the years to survive without eyelids.

Do fish get water in their eyes?

Yes, water gets into the eyes of fish and they have naturally adapted to it. Fish are aquatic animals and their eyes have adapted to coming in contact with water.

 The contact between water and a fish’s eyeball can be likened to the human eyeball coming in contact with air. 

Although water coming in contact with fish eyeballs does not mean the water gets into their eyeball.

It is worthy of note that the eyeball of a fish is very similar to that of a human. The key difference between the eyes of a human and the eyes of a fish is the lens, that is, the art that lies behind the iris and focuses light on the retina. The lens in human eyeballs is flat and thin due to the immense light the eyes have to handle.

This is not the same as fish. Light does not get into the water so much and the sunlight that reaches underneath a body of water is less than that on land. Thus, the lens of a fish’s eye is thick and shaped just like marble to help them see properly in water, unlike human beings who suffer blurry vision underwater.

Regarding dust particles, you may have observed that the dust in water behaves differently from the dust in the air. This implies that fish do not need eyelids to keep their eyes safe from dust and water.

How do fish sleep without closing their eyes?

The fact that fish don’t have eyelids doesn’t mean they cannot sleep. Fish do sleep! We simply do not catch them sleeping because they sleep differently from what we may imagine. For example, a sleeping fish usually keeps its eyes open (well they don’t have an alternate option) and can be seen swimming slowly. This does not match our definition of sleep, right?

For fish, sleep does not translate to an absolute shut down to the surroundings to relax. Considering the environment fish live in, the possibility of encountering a predator is quite high, and therefore, sleep is a luxury they cannot afford. 

Over the years, fish have evolved to remain alert while asleep. A fish’s sleep can be likened to a state of trance.

If you own an aquarium in your home, you may have encountered your fish hovering in one spot as though in a trance. Well, your fish is asleep. If you try to feed them while in this state, you will notice that it takes a little time for them to come to their senses and swim towards the food.

Most fish still move despite being asleep. This is because, unlike species that live on land, a fish’s respiratory system only works when there is a constant flow of water. To maintain oxygen levels in its body, a fish must ensure there is a consistent flow of water over its gills and this is what seems to us as though the fish is awake and swimming.

Can a catfish blink?

Do fish blink? Do catfish blink?
Catfish eyes

Catfish cannot blink their eyes. This is a misconception that is common among most catfish owners. A lot of people believe that the catfish blinks its eyes when it recognizes its owners.

 In reality, this is untrue. Just as was mentioned earlier, fish lack eyelids, and this includes the catfish.

In reality, the catfish only rotates its eyes quickly. When this occurs, the catfish rotates its eyes downward which results in the pupil & eyelid disappearing for a few seconds from the human view. 

While this is happening, the reflective tissues surrounding the eye become momentarily visible and when the eye rotates back to its original position, an illusion of a wink is created.

How fascinating is that?

Do sharks blink?

Do sharks blink?
Shark eyes

Sharks are known to blink their eyes but it is quite different from how you may have imagined it. 

The covering over the eyes of a shark is called a nictitating membrane but this doesn’t mean they blink in the same way human blinks. These nictitating membranes are found on the upper & lower parts of the shark’s eye. When sharks hunt for prey, these membranes are usually closed as protection.

The nictitating membranes of a shark are transparent and the reason for this is quite obvious. Not every shark, however, has these membranes to keep their eyes safe.

Among 453 identified species of sharks, up to 15 do not have nictitating membranes and thus, use other methods to protect their eyes. For example, the great white shark does not have the nictitating membrane and so it rolls its eyes into its head for protection while attacking prey.

Do whales and dolphins blink?

There is a wide variety of particulates suspended in the sea, while mammals such as dolphins and whales do not possess eyelashes to keep these particulates out of the eyes just as we humans do, they are equipped with hardened glands which continuously cleans the eyes with an oil-based protein mucous, which has the effect of flushing any irritants which may lodge in the eye. Fun fact whales do not blink as much as humans do.

Do whales and dolphins have eyelids?

Yes, All mammals do. Dolphins and whales have fatty eyelids with special tear glands. Dolphins do not blink as much as humans do, since they live underwater, but if the need arises they blink and spread thick, jelly-like tears that protect their eyes over time. Dolphins also close their eyes to sleep, and they only close one eye at a time as half of their brain needs to stay awake.

Why do we blink?

Blinking is a reflex action, some sort of automatic response by the mammal body. Blinking serves the purpose of lubricating the eyes by closing it to keep out dust and irritants, including bright light and foreign objects.

Related Question

Why do fish not blink?

Fishes do not blink due to the absence of eyelids. The purpose of blinking is to keep the eyes moist. Humans blink because they do not live in water, so it makes no sense for fishes to blink since they spend most of their time in the water.

Which is the only fish that can blink?

Sharks. They are the only fish capable of blinking

Which animals do not blink their eyes?

When it comes to blinking, no animal with compound eyes can blink. Flies do not blink, spiders, shrimps, and a host of others also cannot blink. This is also true for animals without eyes, and the list is endless. Most aquatic animals also do not blink, because blinking is just a mechanism used to maintain water film over the pupil – which has no use in water.

Which fish can blink with both eyes?

Sharks are the only fish that can blink with both eyes!

Do Pufferfish blink?

Yes, Pufferfish can blink but not in the way and manner humans do. These amazing creatures are one of the only bony fishes capable of closing their eyes, and the mechanism responsible has been demystified.

They are able to blink by sinking their eyeballs deep into their sockets, just like the aperture of a camera, then pucker the skin surrounding the eye together.

When researchers direct gentle streams of water towards their eyes, they respond in a blink-like manner. However, the Pufferfish do not rely on eyelids to blink, instead, they close their eyes in a circular way, thus the blink effect.

Do Betta Fish Blink?

No, Betta fish do not blink because they do not possess eyelids as humans do. These fishes are near-sighted -meaning their acute vision is only effective for up to 12-14 inches away. Their eyesight plays a crucial role with respect to their combative tendencies, which is why they’ll thrust or chase your finger if you dip it in the water surface.

Final Thoughts: Do Fish Blink?

Nearly 85% of global fish stocks face exploitation, in recovery from exploitation or grossly exploited. A recent report indicates that there may be fewer than 100 cod over the age of 13 years residing in the North Sea between the United Kingdom and Scandinavia.


See Also

Sharks 101 | Shark Parts, Habits, and Biology