Why do Sharks have eyelids? How do sharks use their eyelids?
Why do Sharks have eyelids?
Yes, it’s true that sharks have eyelids. They don’t have eyelids like us, in fact, they have eyelids made up of nictitating membranes and skin layers.
Sharks have eyelids because it helps them protect their eyes from abrasion, moisten it, and also to cover the eyes while hunting and also when the shark is being attacked.
They have skin eyelids that can cover their eyes. The color of the upper eyelids is the same as the color of the snout.
Sharks also have a full nictitating membrane in the form of the third eyelid that is also referred to as the plica semilunaris, membrana nictitans, or palpebra tertia in scientific terms.
The third eyelids of sharks are transparent and translucent nictitating membrane that can be drawn across the eye for protection and to moisten the eyes while maintaining proper vision.
It is to be noted that, Sharks do have eyelids, but they do not blink because the surrounding water is just enough to clean their eyes.
Actually, the most unique things about sharks’ eyes are its eyelids, as most of the fishes don’t have eyelids.
How do sharks use their eyelids?
First of all when the shark is about to attack its prey, it will close its third eyelid with its nictitating membrane. This closing protects its eyes from the prey while attacking.
Next, if the attacked prey tries to fight back in order to protect itself from being killed, then this may lead to a deadly fight between the shark and the prey. During such a case, the shark will close its other eyelids to protect its eyes from any damage or scars.
The action of eyelids may be voluntary or involuntary and it totally depends on the action and situation the shark is in.
It is often seen that the sharks can also try to cover its eyes with all of its eyelids, when fighting with other sharks and also during mating to protect its eyes from being damaged.
Also, without adequate moisture, shark’s eyes can become dry and uncomfortable. Dry eyes can lead to vision loss and damage in the eyes.
So, the third eyelid made up of the nictitating membrane can keep the eyes of the shark moist by interlocking the moisture providing the eyes better health and vision.
The third eyelids’ key function is to regularly spread the moisture of glandular secretions and other secretions on the eye surface to keep it moist since the cornea must be continuously moist. The eyelids keep the eyes from drying out.
It is also to be noted that the eyelids (transparent third eyelid) may act as outer lens, increasing the shark’s ability to focus on specific objects and interpreting its surroundings better.
The third eyelid can also provide the shark with a better capability of seeing polarized light along with low light. However, tapetum lucidum tissue is the prominent reason how sharks are able to see in the low light created by murky or deep waters.
Do all sharks have eyelids? Do sharks have two eyelids?
All sharks have at least two eyelids viz. an upper eyelid and the lower eyelid. The upper and lower eyelids can close and open based on the voluntary or involuntary wishes of the shark.
There’s another eyelid, and this is the third eyelid made up of nictitating membrane. This is transparent and help to see clearly, focused, and keep the eye moist.
Uniqueness amongst the fishes is the shark’s eyelids. They use all of their eyelids primarily as a protective measure.
It has been seen that the upper eyelid and the lower eyelid don’t close all the way covering the whole eye of the shark.
However, the third eyelid which is known as the nictitating membrane is the transparent and translucent membrane that will fully protect the eye and cover it.
The nictitating membrane can slide down to fully protect the eye in unpredictable and potentially dangerous situations.
And it has been clearly seen that when it’s feeding time, or when the shark has an encounter with another shark, it will close the eyelids to protect the eyes from abrasion.
And, it is also to be noted that all of the sharks don’t have eyelids. Just, for instance, the great white and the whale shark don’t have eyelids and so they roll their entire eyes into the back of their head just in order to protect it. This gives their eyes a white-eyed look.
Just like the Whale sharks’ eyes have no eyelids and the eyes poke out on either side of their heads, which makes them vulnerable to exposure.
And so, the eye surface is less protected from mechanical damage than other regions of the body.
Do sharks close their eyes when they bite?
Yes, due to their voluntary or involuntary responses, sharks are known to close their eyes when they attack and bite their prey.
This closing of the eyes is done in order to protect the shark eyes from getting damaged by the counter-attack of the prey in order to save itself from getting killed.
The upper eyelid and the lower eye partially closes covering the eyes. Meaning that the eyelids don’t close all the way, and leaves a small opening for the eyes to see.
On the other hand, the third inner eyelid that is the nictitating membrane being transparent closes the eyes completely. And there seems to be no problem for the shark to see as they can easily see through this transparent membrane.
Some of the species of sharks like the Great white shark and the Whale Shark lacks all of the eyelids including the nictitating membrane. And so, they, therefore, roll their eyes back in their heads for protection when bitting or fighting.
And, so not all of the sharks close their eyes when they bite because a few of the shark species can actually roll their eyes, as they do lack eyelids for protection.
Rolling of their eyes back when they bite into their prey, make their eyes turn creamy-white. And, that’s awesome!
Not only from their prey, but they can also retract their eyes if needed to protect the eyes from other creatures in the sea, as they do not have eyelids.
Why majority fishes do not have eyelids but sharks do have?
Not all fishes have eyelids, but some fishes like some sharks, milkfish, jacks, mullets, and mackerel have eyelids.
The eyelids of some of the fishes are the adipose eyelids that are transparent like the nictitating membrane of sharks and fully covers the eyes. For example in milkfish, mullets, etc.
While in the majority of the fishes, they do not need any kind of eyelid as they can keep the surface of their eyes moist and perfect for vision without eyelids.
And also, there’s no need for many of the fishes to keep their eyes safe from predators as they rarely indulge in one to one fight with their predators.
But, sharks being the apex predators needs to keep themselves safe and working and so need to protect their eyes from getting damaged when attacking a prey. So, many of the shark species have developed mechanisms to safe their eyes.
So, that’s why the majority of the fishes do not have eyelids but sharks do have eyelids.
Sharks are all cartilagenous fishes, meaning that their body is made out of cartilage instead of bones like other fish. They are included in Class Cyclostomata of the Animal Kingdom.
And yes, sharks are fish. They do live in water and use their gills to filter oxygen from the water.
In fact, it will be much better to say that the sharks are special types of fish that are known because their body is made out of cartilage.
