Research on mudskippers, blinking fish that spend much of their time outside of water, is shedding light on the evolution of blinking in how ancestors of humans, amphibians, birds, reptiles, and other mammals transitioned from life on the water to life on land about 375 million years ago.
The press release via EurekAlert! reports that through studying these amphibious fish, researchers have discovered that they have developed a similar blinking behavior to humans, suggesting that this trait may have evolved to aid in the transition to life on land for tetrapods.
In this photo taken on August 3, 2016 a mudskipper fish walks using its fins on the mud during low tide at Sukadana's beach in Kayong, western Kalimantan island.
Mudskippers Shed Light on the Evolution of Blinking
The study, titled "Mechanical Theory of Nonequilibrium Coexistence and Motility-induced Phase Separation" published in PNAS, was led by Thomas Stewart from Penn State and Brett Aiello from Seton Hill University.
Blinking is an important behavior in animals for various reasons such as keeping the eyes clean and wet and communicating. It is difficult to study how blinking initially evolved as the anatomical changes are mostly present in soft tissues that do not preserve well in the fossil record.
However, mudskippers, fish that evolved their blinking behavior independently, can help scientists test how and why blinking evolved in a living fish that regularly spends time on land. As the press release said, researchers studied mudskippers with high-speed videos and compared their anatomy with that of a closely related water-bound fish that does not blink.
They found that mudskippers blink by momentarily retracting their eyes into sockets covered by a stretchy membrane called a "dermal cup." The mudskipper's blink is about the same length of time as a human blink and appears to have evolved through a rearrangement of existing muscles that changed their line of action and the evolution of a novel tissue, the dermal cup.
Therefore, scientists suggest that mudskippers used what they already had in a different way to conduct a complex behavior. The team also considered why mudskippers blink on land and found that, like humans, they blink more frequently when confronted with dry eyes.
Moreover, they can mix mucus from the skin with water from their environment to produce a tear film, although they have not evolved any tear glands or ducts like humans.
Blinking in Mudskippers Serves the Same Purpose as in Humans
A similar report from Phys.org stated that the research team also conducted experiments to see whether mudskippers blinked to protect their eyes from harm and to clean them of debris. The results showed that mudskippers did indeed blink for these reasons, similar to how humans and other tetrapods blink.
This insight shed light on the potential evolution of blinking in early tetrapods and how it contributed to their adaptation to living on land. Also, it showed that blinking serves the same purpose as in humans and other tetrapods, which is for cleaning, maintaining moisture, and protection.
Blinking is a complex and important behavior that is constantly happening without people realizing it. Through their research on mudskippers, the scientists gained a greater understanding of the origins of blinking in tetrapods and how it may have been one of the many anatomical changes necessary for survival on land.
The team noted that the transition to life on land required a multitude of changes, including feeding, locomotion, and breathing air. The results of this study on mudskippers revealed that blinking was an important adaptation that was likely part of a suite of traits that evolved in early tetrapods as they adapted to their new habitat.
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