🦈 Do Fish Blink?

No, most of the fish cannot blink because they lack eyelids. The main role of eyelids in humans and other land animals is to keep the eyes (cornea) moist. However, fish actually don’t need eyelids because their surrounding environment is water. As fish don’t need and have eyelids, they can’t blink their eyes. It is interesting to note that there are some sharks that have a transparent nictitating membrane which they can blink during feeding time to protect their eyes.

Image Credit: Christel SAGNIEZ

Can fish blink?

Have you ever observed the eyes of the fish? If yes, then you already know the answer to the question “Can fish blink their eyes?”

Fish cannot blink their eyes because fish don’t have eyelids to blink. Yes, you heard that correct almost all species of fish lack eyelids. However, the shark is the only species of fish that have a nictitating membrane which acts as an eyelid and can blink.

Before we try to understand why fish don’t need to blink, let’s learn why do we need to blink?

Why do we need to blink?

The main functioning of eyelids is to protect our eyes. An eyelid made up of thin skin that covers our eyes. These eyelids can have opacity from opaque to transparent depending on their functioning. These eyelids have meibomian glands (also known as tarsal glands) which secretes an oily substance called meibum. It is important for the cornea (present on the surface of the eye) to remain moist all the time. This lipid present in the meibum helps to keep our eyes moist by preventing the evaporation of the eye’s tear film.

Human Eye, Image credit: PIXNIO

Do You Know?

Human eyelids have about 50 glands on the upper eyelid and 25 glands on the lower eyelid.

Eyelids help to block light so that we can sleeping comfortably. They also help to maintain the moisture level of the cornea while sleeping. These eyelids can respond quickly (blink reflex) to cover our eyes to protect them from foreign substances such as dust.

Read: 🐟 Can Fish See In The Dark?

Now we know why blinking of an eye is important.

Why do fish not blink?

Fish don’t have eyelids similar to humans. Therefore, fish cannot blink their eyes. Actually, they don’t even need to blink. The primary function of the eyelids is to keep the eyes moist. However, fish don’t need to maintain the moisture in their eyes because they are surrounded by water all the time. Thus, blinking is not required in the case of fish.

How they protect their eyes if they don’t blink?

Apart from keeping eyes moist, eyelids also help to protect eyes from dust and other debris particles with blink reflex. The eyes of the fish have adapted to their surrounding water conditions. It is similar to the human eye which is adapted to air. Whenever our human eye is exposed to undesired substances such as dirt, chemicals, and smoke, the tears along with blinking lids remove these dust particles. However, in the case of fish, the surrounding water takes care of it. The surrounding water naturally removes the dust particles. The fish eyes have evolved their eyes such that they can survive in the saltwater. Therefore they don’t need eyelids and don’t have to blink.

According to Okinawa Churashima Research Center, Some fish such as whale shark have teeth on their eyeballs. These tiny denticles present in large numbers provide protection to their eye.

Read: 🐠 Can Fish Drown?

Eyelid replacement in Fish

Now, we know that fish don’t have eyelids and cannot blink. However, it is interesting to note that some species of fish have developed a replacement for typical eyelids. For example, sharks have a nictitating membrane cover to protect their eyes. It is a thin, tough, and transparent or translucent membrane over the eyeball which protects it while hunting their prey.

Some fish such as bonefish have a fatty layer around their eye which they use as eyelids to block light from entering the eye. Some fish have lots of chromatophores (light-reflecting cells) with pigments in their eye that helps to cut off the bright light from entering into the eye.

Butterflyfish, Image Credit: LASZLO ILYES

Fun Fact

Butterflyfish have a fake eye like spots near their tail to confuse the predator about which end is the head of the fish.

Read: How Do Fish Taste?

What fish can blink with both eyes?

The only species of fish that can blink is a shark. The shark eye has a nictitating membrane that acts as a protective layer over the eye. This membrane covers the shark eye during the feeding time in which the prey might cause damage while protecting itself. This a transparent membrane through which shark can easily see and hunt their prey. Some species of shark such as the great white shark and the whale shark lack this transparent eyelid membrane.

Shark Eye, Image credit: Jean-Lou Justine

Read: Do Fish Have Tongues?

How fish sleep without eyelids?

Now you must thinking about how do fish sleep without eyelids? or do fish sleep without closing their eyes?

Yes, fish do sleep, and that also with their eyes open. Fish do not sleep as humans or other land animals do. Fish generally take rest in which they reduce their metabolism and other activities by almost half. Fish living in oceans are always prone to the risk of attack by predators. Therefore, they have to be always alert in order to avoid danger. Fish have adapted to this environment such that they can stay relax and alert simultaneously. Because fish don’t have eyelids, they have to sleep or rest with their eyes open. Similar to humans, most of the fish also sleep during the night time. Since it is dark in the night and fish cannot see in the dark. Therefore, they don’t get bothered by keeping their eyes open while resting.

Thus, fish do not blink due to lack of eyelids.

Read: 🦈Do fish have teeth?

Read: 🐟 Can a fish survive in milk?

Read: ⭐ How Do Starfish Eat?

Professor Atom

View posts by Professor Atom
Professor Atom is a science enthusiast and alumni of IIT Bombay. According to him, every question can be solved with curiosity and mind mapping. ( Curiosity = Asking Questions = Learning )

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