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."
