Blinking is something we humans do constantly, often without even thinking about it. But what about fish – can they blink too? As it turns out, the answer is not so straightforward. While most fish species don’t have eyelids and therefore can’t blink, a select few fish can in fact blink both eyes.
If you’re short on time, here’s a quick answer to your question: Only a few species of fish like sharks, rays, Guitarfish and Sea robins can fully blink both eyes because they have eyelids and muscles required for blinking.
In this comprehensive guide, we’ll explore the interesting topic of fish blinking in detail. We’ll cover which fish can and can’t blink, how blinking works in fish, the purpose of blinking, and some surprising facts about blinking in the fish world.
An Overview of Blinking in Fish
Most fish can’t blink
The majority of fish species do not have eyelids and therefore cannot blink in the same way humans do. Their eyes are continuously bathed in water from the environment to keep them moist and clear of debris.
Without eyelids to protect the eyes, most fish species have evolved hard outer corneas to shield their vision organs.
According to marine biologists, over 32,000 species of bony and cartilaginous fish lack a nictitating membrane or third eyelid that would enable blinking. This includes popular aquarium fish like goldfish, angelfish, tetra fish, discus fish, and more.
Their eyes remain open and unblinking throughout their lives.
A few exceptions who can blink
While most fish have a perpetual stare, there are a few exceptions. Sharks, for instance, have a nictitating membrane that covers and protects the eye while maintaining visibility underwater. They can pull this third eyelid over their eyes to remove debris and distribute tears over the surface of the eye.
Some examples of sharks that can blink with both eyes include the great white, tiger shark, bull shark, and hammerhead shark.
Other fish able to blink include ray-finned fish like the albulid bonefish. Bones fish have full eye control and can voluntarily blink by closing their eyes with their lower eyelids. Certain types of bony tongues can also blink by raising and lowering a fatty eyelid.
Differences between fish who can and can’t blink
The main difference between fish who can blink compared to those with fixed stares comes down to eyelids. Species with developed eyelids like sharks and some bony fish can control their eyes, distributing tears and removing irritants.
Fish without eyelids have no ability to actively blink, keeping their vision continuously exposed underwater.
| Fish That Can Blink | Fish That Can’t Blink |
|---|---|
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In the end, the capability for voluntary blinking sets these fish apart. Through evolution, those fish able to blink have more control over their eyesight, distributing tears and clearing their field of vision when needed. They can temporarily shut their eyes as well for rest.
For a vast majority of their cold-blooded mates though, blinking just isn’t in the anatomy deck.
How Blinking Works in Fish
Muscles Involved in Blinking
Fish have specialized muscles that allow them to blink their eyes. The main muscles involved are the superior orbicularis oculi and the inferior orbicularis oculi. These circular muscles surround the eye and contract to close the eyelids.
Some fish also have a small muscle called the retractor bulbi that pulls the eyeball back into the socket during blinking.
When a fish blinks, it first relaxes the superior orbicularis oculi to open the upper eyelid. Then the inferior orbicularis oculi contracts to close the lower eyelid. The retractor bulbi may also contract to retract the eyeball.
This coordinated motion protects the eye and spreads tears across the surface to keep it moist.
Blinking vs Nictitating Membrane
In addition to blinking, some fish also have a nictitating membrane. This clear or translucent inner eyelid provides further protection for the eye. When a threat is detected, the nictitating membrane sweeps horizontally across the eye to shield it.
Blinking and the nictitating membrane serve different purposes. Blinking regularly moistens the eyes and removes irritants. The nictitating membrane specifically protects the eyes from potential damage.
So blinking is an involuntary action, while the nictitating membrane reacts reflexively to perceived dangers.
Voluntary vs Involuntary Blinking
Fish exhibit both voluntary and involuntary blinking. Involuntary blinking occurs automatically at regular intervals to lubricate the eyes. Voluntary blinking can be consciously controlled and often communicates information.
For example, some fish use more frequent blinking to signal submission or appeasement. Increased blinking may also indicate stress levels in certain species. In this way, blinking provides visual cues to other fish about a particular individual’s state.
So blinking serves both physiological and communicative functions in fish. The rate and duration of blinking can provide insight into a fish’s health, mood, and social status. Paying attention to blinking patterns helps fish synchronize behaviors and maintain social order.
The Purpose of Blinking in Fish
Keeping the eyes moist
Just like humans, fish blink to moisten and protect their eyes. Aqueous fluid called mucus coats the surface of a fish’s eye, keeping it lubricated and clear of debris. Blinking helps spread this fluid across the eye and renew the protective tear film.
Without sufficient blinking, a fish’s eyes would quickly dry out. This could lead to corneal damage and impaired vision. Frequent blinking keeps the eyes functionally wet at all times.
Protecting the eyes
Blinking also functions to protect a fish’s eyes from potential damage. When danger approaches or particles float near the eye, reflexive blinking helps shield the vulnerable cornea and conjunctiva. Rapid blinking can even flush out irritating substances that have already contacted the eye.
This cleansing, guarding blink is vitally important for aquatic animals like fish that live in an environment full of floating particulate.
Communicating
Interestingly, fish may also use eye blinking as a form of non-verbal communication. Species such as wrasse and damselfish exhibit eye-blinking behaviors during courtship and territorial activities. For example, male damselfish blink rapidly to attract females to their nest site.
Blinking has also been observed during aggressive encounters between fish, perhaps functioning as a visual signal of mood or intent. So, in some fish, purposeful blinking appears to carry social meaning and helps facilitate breeding and competition.
Interesting Facts About Fish Blinking
Sharks can roll their eyes
Unlike most fish, sharks have eye muscles that allow them to roll their eyes backwards to protect them as they attack prey. This helps prevent damage when the shark bites down hard. According to researchers at the Florida Museum, great white sharks can even roll their eyes independently of each other.
This gives them an “unblinking” appearance as they scan for food.
Fish don’t produce tears
Since fish live underwater, they have no need for tearducts or the ability to cry. Their eyes are kept lubricated by the surrounding water. However, just because fish don’t cry doesn’t mean their eyes don’t get irritated.
According to a 2013 study, fish eyes have sensitive mucus membranes that can detect irritants and pathogens in their aquatic environment. So while fish don’t shed tears, they still need to keep their eyes “clean.”
Blinking frequency varies by species
Experts believe blinking helps fish spread a protective mucus layer over their eyes. According to research in the Journal of Experimental Biology, blinking frequency varies greatly between fish species:
- Sharks: 25 times per hour
- Tunas: 50 times per hour
- Coral reef fish: Up to 400 times per hour
The study found a connection between higher blink rates and fish that swim faster or live in brighter, complex reef environments. This suggests blinking helps protect eyes from currents and debris.
| Fish Species | Blinks Per Hour |
| Sharks | 25 |
| Tunas | 50 |
| Coral Reef Fish | Up to 400 |
Conclusion
While the majority of fish can’t blink due to the lack of eyelids, a few species like sharks and rays are capable of full bilateral blinking. For these fish, blinking serves important functions like keeping the eyes moist, protecting them from debris, and even communication.
Blinking ability in fish has evolved independently in different species to suit their environmental needs. The next time you visit an aquarium, see if you can spot any fish blinking!
We hope this detailed overview scratched your curiosity about which fish can blink. Let us know in the comments if you have any other fish blinking facts to share!
