Can fish without eyelids also "blink"?

On Monday morning, when you are sitting at your desk, do you feel drowsy, unable to lift your eyelids, and want to use toothpicks to prop up your eyelids to prevent them from closing?

Nothing but hardship

Source: Animation screenshot

Whenever you feel extremely sleepy but have to force yourself to stay awake, do you envy fish without eyelids? If humans also have no eyelids like fish and can sleep with their eyes open, can they also catch up on their sleep without anyone noticing?

Wake up.

Whenever we talk about "sleepy", we will unconsciously associate it with eyelids, as if as long as we hold our eyelids, we can resist the invasion of sleepiness. Many animals, including humans, have eyelids that can close. Today, let's talk about eyelids.

1

What is the use of eyelids?

Eyelids, formally known as eyelids, are important organs that protect our fragile eyeballs. Taking the human eye as an example, the eyelids can be divided into upper eyelids and lower eyelids. The place where the upper and lower eyelids connect is called the canthus (the "canthus" in "a grudge"), which is the corner of the eye. The two corners of an eye are called the inner canthus, which is closer to the nose, and the outer canthus, which is closer to the temple.

Source: Wikipedia

The most important function of eyelids is undoubtedly to protect the eyeball. When the eyes are closed, the eyelids can protect the cornea from trauma. When the eyes are open, the eyelashes on the eyelids can also block dust and foreign matter and reduce strong light.

In the animal kingdom, most vertebrates have eyelids, but this is not always the case. For example, many geckos do not have movable eyelids. There is also a type of vertebrate that is generally considered to have "no eyelids", which is the fish we mentioned earlier.

2

Do fish really have no eyelids?

When searching online for "animals without eyelids", the first answer that pops up is often fish. But fish are so diverse and range in variety, is it true that they all have no eyelids?

Let’s first look at two types of fish that have no eyelids but can “blink”.

First up is the mudskipper. Mudskipper is the general name for mudskippers (tribe Periophthalmini), which belong to the order Goby, family Goby. Most of them live on the mudflats of the sea and stay on the shore for most of the day. Because they need to maintain a good view above the water, their eyes grow on the top of their heads like periscopes. Mudskippers have no eyelids or tear glands, and when they need to keep their eyes moist, they retract their eyes down into their eye sockets, which makes them look like they are blinking.

Mudskippers keep their eyes moist by blinking

Source: discovery

There is another type of fish that has become very popular on the Internet recently and can also "blink", that is, the lantern eye fish (Anomalopidae). The lantern eye fish belongs to the order of the acinus, and each of them has a half-moon-shaped light organ under its eyes, which contains symbiotic luminescent bacteria that can emit cyan or white light.

The lamp eye fish lives in groups, gathering in dark or dark places such as reef caves and shipwrecks during the day, and only comes out to look for food at night. The light organ under their eyes can be opened or closed freely by flipping, which gives people the feeling of "blinking" , but it is not the eyelids of the lamp eye fish that block the light organ, and when it is closed, there is nothing blocking its eyes, only the light organ is blocked.

Light-eyed fish swimming in schools in the dark can be seen turning their light organs on and off.

Source: YouTube: Broken Brain

We have given two examples of fish that can blink but have no eyelids. So are there any fish that have eyelids?

In fact, you have not only seen fish with eyelids, but you may have also eaten them. This kind of eyelid is a transparent membrane structure composed of lipids, and is called adipose eyelid according to its composition.

Fatty eyelids are common in marine fish, including milkfish, herring, mullet, and mackerel, many of which are common edible fish. The thickness and shape of the fatty eyelids vary depending on the type of fish. This transparent membrane may cover the entire eye of the fish or wrap around the eye. The name of the famous milkfish in Taiwan Province is likely derived from the fatty eyelids covering its eyes.

The fish's fat eyelid (outlined in black)

Source: WikiCommons

However, this fat eyelid cannot move. Scientists speculate that its main function may be to help fish enhance their vision, filter out ultraviolet rays, and protect the eyes from impact damage from foreign objects.

3

Are there any fish with movable eyelids?

So far, the fish we have seen either have the ability to "blink" but do not have eyelids, or have eyelids but cannot move. So are there any fish that have a movable "eyelid" that can cover their eyes?

There are sharks . In many documentaries, we can see that when some sharks attack their prey, their eyes are covered with an opaque film, which is their "eyelids".

Nitrate membrane of blue shark

Source: See watermark

The shark's eyelid is called the nictitating membrane, also known as the third eyelid. The nictitating membrane is usually thin and can protect the eyeball while allowing the animal to have a certain degree of vision.

This structure is found in the eyes of frogs, toads, most reptiles, and birds. It can also be found in some mammals, the most common of which is cats. Sometimes when you see your pets rolling their eyes while sleeping, you may have seen their nictitating membranes.

Sleeping until eyes roll back

Source: Lorenz Map

In addition to cats, camels, polar bears and seals all have complete nictitating membranes. Judging from the living environment of these animals, it is clear that their nictitating membranes can play a certain protective role. In most mammals, the nictitating membrane has degenerated into a vestigial organ. For example, the nictitating membrane of humans can only be seen in the inner corner of the eye, called the semilunar fold (bi).

Source: Baidu Encyclopedia

That’s all we have to say about eyelids today. After learning so much about eyelids, I wonder if it can help you slow down the speed at which your eyelids close the next time you feel sleepy? However, for health reasons, it is recommended that you go to bed early and get up early, and maintain a healthy schedule so that you don’t feel sleepy at work.