Blink Fish Game _VERIFIED

Scientists think that blinking evolved in land animals when they made the transition from the oceans roughly 375 million years ago. Therefore, studying this example of convergent evolution has offered clues as to how our primordial ancestors first took to Earth's shores. The researchers published their findings April 24 in the journal Proceedings of the National Academy of Sciences.

"Animals blink for many reasons," co-author Thomas Stewart, an assistant professor of biology at Penn State, said in a statement. "It helps us keep our eyes wet and clean, it helps us protect our eyes from injury, and we even use blinking for communication.

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"Studying how this behavior first evolved has been challenging because the anatomical changes that allow blinking are mostly in soft tissues, which don't preserve well in the fossil record," he added. "The mudskipper, which evolved its blinking behavior independently, gives us the opportunity to test how and why blinking might have evolved in a living fish that regularly leaves the water to spend time on land."

The researchers tracked the locations in which the fish blinked. They found that, while submerged, the mudskippers barely blinked at all but when in the air, they blinked often. When the researchers increased the airflow and the subsequent rate of evaporation in the tank, the mudskippers began blinking more frequently. The fish also blinked to remove debris from their eyes.

"We found that, just like humans, mudskippers blink more frequently when confronted with dry eyes," lead author Brett Aiello, an assistant professor of biology at Seton Hill University in Pennsylvania, said in the statement. "What's incredible is that they can use their blinks to wet their eyes, even though these fish haven't evolved any tear glands or ducts. Whereas our tears are made by glands around our eyes and on our eyelids, mudskippers seem to be mixing mucus from the skin with water from their environment to produce a tear film."

To understand how the freakish fish evolved the ability to blink, as well as retrieve clues on how our fishy ancestors did the same, the researchers compared the mudskipper's anatomy with those of close relatives that don't blink. They found that the creatures' eyes had evolved to rest above a socket covered in a stretchy membrane called a dermal cup and that they were sucked downward into this cup to conduct the blink, which lasts about as long as a human's blink.

The muscles that performed this blinking action are not new but rather a simple rearrangement of existing ones. Therefore, the researchers think ancient fish likely didn't need complex adaptations to begin blinking. Instead, the fish probably achieved the feat with a shuffling of their rudimentary biology.

"The transition to life on land required many anatomical changes, including changes for feeding, locomotion and breathing air," Stewart said. "Based on the fact that mudskipper blinking, which evolved completely independently from our own fishy ancestors, serves many of the same functions as blinking in our own lineage, we think that it was likely part of the suite of traits that evolved when tetrapods were adapting to live on land."

Fish have unique eye structures that help them see clearly and effectively in their aquatic habitats. While humans and other land animals blink to keep the surface of our eyes moist and free of debris, fish are immersed in water, which necessitates a different type of blinking.

Fish eye protection and moisture come primarily from their surrounding water, which keeps their eyes constantly wet. This eliminates their need for the same blinking mechanisms found in terrestrial animals. However, fish do have other methods of dealing with irritants and debris that may affect their vision.

Various adaptations and behaviours in fish help keep their eyes clean and functioning well, such as specialized eye movements or secretions. The wide array of eye characteristics among fish species can offer insights into their habitats and how they have adapted to maintain their vision and eye health.

The cornea is an essential part of fish eyes, as it serves as the outer protective layer that allows light to enter the eye. Fish corneas are usually thicker than those of other animals, as they need to withstand underwater pressure and protect the sensitive tissues within the eye.

The fish cornea is a transparent layer, enabling the light to pass through and reach the other components of the eye, such as the lens and retina. The refractive power of the corneal surface is significantly reduced in freshwater due to the small difference between the refractive indices of the corneal surface and the water. Because seawater has a refractive index similar to that of the cornea, the cornea contributes even less to the total refractive power of the eye in marine environments.

Unlike many terrestrial animals, fish do not have eyelids. Their eyes are protected by a thin mucous layer, known as the conjunctiva, which helps to protect their eyes from debris and bacteria in the water.

Fish lenses are generally larger and more rounded compared to those of land-dwelling animals, helping them to focus light more effectively underwater. These lenses are also quite flexible, and fish can adjust the shape of their lenses by contracting or relaxing the nearby muscles, allowing them to easily focus on various distances.

The retina, the light-sensitive tissue lining the back of the eyeball, is crucial for processing visual information in fish eyes. Like other vertebrates, fish retinas have both rod and cone cells that detect light and colour, respectively.

Some fish have a retroreflector behind the retina, which enhances vision in low light. Many fish species adapt to their specific environment, with some possessing more rod cells for better low-light vision or more cone cells for distinguishing fine details and colours.

The thin, opaque nature of the nictitating membrane enables sharks to have an unhindered view of their surroundings even while blinking. This is particularly important for aquatic creatures as it allows them to effectively navigate their underwater environment and swiftly detect possible predators or prey.

Some species, like mudskippers, exhibit a blinking motion when they are exposed to air. This blinking action helps keep their eyes moist, which is essential for maintaining their vision when they are out of water.

Mudskippers blink by sucking down their eye into their eye socket, where they are covered by a stretchy membrane called a dermal cup. The evolution of this behaviour did not necessitate the evolution of many new parts, such as new muscles or special glands.

By comparing the anatomy and behaviour of mudskippers to the fossil record of early tetrapods, researchers from Georgia Institute of Technology, Seton Hill University, and Pennsylvania State University concluded in a 2023 paper that blinking emerged in both groups as an adaptation to transition life on land. These findings contribute to the understanding of our own biology and raise a host of new questions about the variety of blinking behaviours observed in living species.

Fish, like many other living organisms, require periods of rest. However, their sleep patterns and behaviours can be quite different from those of mammals or birds. Even when they are asleep, fish continue to swim in order for their gills to absorb oxygen for the body continuously. Fish sleep can be classified into a few distinct types: suspended animation, reduced activity, and rapid eye movement (REM) sleep.

Protopterus annectens, an African lungfish, can survive in suspended animation (aestivation) for three to five years without food or water. When water becomes available, they awaken. Animals in suspended animation are inactive for an extended period of time, consuming no food or water and producing no urine or waste.

Reduced activity sleep, on the other hand, is more common among fish. During this phase, fish experience a noticeable decrease in their movement and responsiveness to external stimuli. They may seek out shelter or hover in one place while still maintaining a level of alertness to potential threats or opportunities.

An example of this can be seen in zebrafish, which have been studied for their sleep regulation. Zebrafish have also been shown to experience REM sleep, which is characterized by rapid eye movement and resembles mammalian sleep in certain aspects.

Instead, these marine mammals have well-adapted eyes to cope with the underwater environment, and they can produce a thick mucus to protect their eyes from the saltwater without needing to blink as often.

But there's one real oddball of a fish that doesn't just crawl around on land; it also blinks its bulbous eyes. We're talking, of course, about the amphibious mudskippers, and scientists are studying them to understand how blinking emerged from the water with our ancestors.

There is a dearth of preserved eyelid and lacrimal tissue in the fossil record, which makes it difficult to ascertain exactly when and how blinking began. But there are some animals today that live sort of in between air and water, much like those early ancestors might have. Scientists turned to these to see if, and why, they blink.

Mudskippers, unlike most fish, have protruding eyes, a bit like a frog's. The high-speed videos showed that mudskippers do blink and how they do it. The team also compared the anatomy of the mudskippers' eyes to those of similar fish that don't blink.

There are many different kinds of blinking in the animal kingdom, with variations of which membranes and how many of them close over the eye. Mudskippers blink by retracting their eye entirely down into the socket. When the eyeball is fully retracted, they are momentarily covered by a membrane called a dermal cup before the blink ends, and the eyes re-emerge.

Next was to figure out why the mudskippers were blinking. Interestingly, they found that the mudskippers rarely blinked while underwater; they only did so when they bumped into objects in the tank, such as water filters or other mudskippers. Predominantly, they blinked while crawling about in the air. 006ab0faaa