A blinking fish reveals clues to how our ancestors evolved from water to land

The mudskipper, a peculiar fish that blinks, spends a large portion of the day outside of the water and is offering hints as to how and why blinking may have developed during the transition of our own ancestors to life on land. According to recent studies, these amphibious fish have developed a blinking habit that has many of the same functions as our own. The findings imply that a number of features, including blinking, may have developed in tetrapods—the group of creatures that includes mammals, birds, reptiles, and amphibians—some 375 million years ago to enable the shift to living on land.

Thomas Stewart, an associate professor of biology at Penn State, and Brett Aiello, an assistant professor of biology at Seton Hill University, conducted the study, which was published on April 24 in the Proceedings of the National Academy of Sciences.

Stewart added, "Animals blink for a variety of reasons. We utilize blinking for communication as well as to keep our eyes moist and clean and protect them from harm. The anatomical alterations that allow blinking are primarily in soft tissues, which don't survive well in the fossil record, making it difficult to study how this activity initially developed. We can examine how and why blinking may have evolved in a living fish that frequently leaves the water to spend time on land thanks to the mudskipper, which independently acquired its blinking behavior.

The researchers studied the activity using high-speed movies and compared the anatomy of mudskippers with that of a closely related water-bound fish that doesn't blink in order to comprehend how mudskippers gained the capacity to blink. The eyes of the mudskipper protrude like frog eyes from the top of its heads. The fish's eyes are temporarily retracted into their sockets during a blink, where they are protected by a flexible membrane known as a "dermal cup." The duration of the mudskipper's blink is comparable to that of a human blink.

"Blinking in mudskippers appears to have evolved through a rearrangement of existing muscles that changed their line of action and also by the evolution of a novel tissue, the dermal cup," claimed Aiello. This is an extremely intriguing conclusion since it demonstrates how a very simple system may be utilized to carry out a complicated action. The evolution of this new habit didn't need the development of many novel traits; instead, mudskippers simply began making use of their existing resources in new ways.

The research team thought about the functions that blinking serves in humans and other tetrapods to understand why mudskippers blink on land. The scientists looked at whether mudskippers blink to keep their eyes moist, as tears are essential for humans to maintain healthy and oxygenated eye cells.

Aiello stated, "We discovered that mudskippers, much like humans, blink more frequently when faced with dry eyes. It's amazing that they can moisten the eyes with their blinks despite the fact that these fish lack tear ducts or glands. Unlike humans, who make their tears from glands on our eyelids and surrounding our eyes, mudskippers appear to create their tears by combining skin-derived mucus with water from their environment.

The study team also investigated if mudskippers' blinking might be stimulated to protect the eye from potential harm and to clear the eyes of debris or dust. The response was "yes" in both instances. Therefore, blinking in mudskippers appears to carry out three of the primary purposes of blinking in humans and other tetrapods—protection, cleaning, and preserving moisture.

"Our study, which considered the behavior and anatomy of a living fish that underwent a transition to life on land, similar to the earliest tetrapods, helps us to reimagine how and why these early tetrapods might have been blinking," claimed Aiello. "The chance to investigate how and why this behavior first emerged offers an incredible opportunity to learn more about how humans came to be as they are and gives us insight into changes associated with significant transitions in the history of animals—like inhabiting land," says Dr. Peters.

Humans and other tetrapods continually blink during the course of the day, frequently without even realizing it, according to Aiello. He noted that although it appears to be a minor motion, blinking is actually highly sophisticated and intriguing since it is a single behavior that may carry out a variety of tasks, all of which are essential for the safety and health of the vertebrate eye.

"The transition to life on land required many anatomical changes, including changes for feeding, locomotion, and breathing air," said Stewart. "We think that it was likely part of the suite of traits that evolved when tetrapods were adapting to live on land," the authors write. "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."