Video: How a Mudskipper blinks. © Brett Aiello
Targeting the mudskipper’s eyes: A study of the blinking of these amphibious fish links the evolution of blinking to the transition to life on land. For their partly terrestrial life, the mudskippers developed this concept independently of the terrestrial vertebrates. It also serves to moisturize, protect and cleanse the eyes, the results show. This proves that blinking was one of the adaptations that allowed our distant ancestors to go ashore, the scientists say.
When you read these lines, it happens many times unconsciously: Closing the eyelids regularly is important for the health of our eyes. This also applies to other land vertebrates. Different membrane systems are briefly closed when they blink. In fish, on the other hand, this concept is not common. So it stands to reason that blinking is linked to life on land. But how and why did it evolve in the ancestors of land vertebrates? For this question, it is difficult to get clues from anatomical adaptations in fossils. Because traces of the corresponding soft tissue have usually not been preserved.
Mudskipper wink analyzed
That’s why the team of researchers, led by Pennsylvania State University (Penn State) at State College, has now turned to creatures that can serve as models for the vertebrates that first emerged from the water about 375 million years ago: mudskippers (periophthalmos ). These are fish from the goby family (Gobiiformes) that spend most of their lives out of water. Much later than the pioneers of terrestrial vertebrates, they set out again to go ashore. With their powerful pectoral fins, they are now hopping on the muddy subsoil of their habitats in Africa and Asia.
In addition, another unusual feature of the mudskipper is already known: In contrast to their purely aquatic relatives, these fish blink their eyes. “The mudskippers, who appear to have independently generated their blinking behavior, allowed us to explore how and why the concept might have evolved,” said senior author Thomas Stewart of Penn State. As part of their study, the scientists have now researched the corresponding structures and behaviors in detail for the first time. To do this, they compared the eye structures of the mudskippers with those of their aquatic relatives, the black-mouthed gobies. They also conducted experiments to shed light on the purposes of fishy blinking.
Presumably similar development as before primeval times
The team reports that when the fish blink, they briefly retract their protruding eyes into their sockets. This process takes about as long as human blinking. The anatomical investigations have now shown that the eyes are covered by an elastic membrane – a skin shell. “The concept appears to have evolved through a rearrangement of existing muscles, altering how they function, and through the development of a new tissue — the skin shell,” says lead author Brett Aiello of Seton Hill University in Greensburg. “This is a very interesting result because it shows that very basic things can be used to create a complex system. The mudskippers simply made good use of what they already had,” says the researcher.
As for the purpose of the concept, it became clear: “Like humans, mudskippers blink more often when confronted with dry conditions,” reports Aiello. However, no glandular fluid is used for moistening: “Mudskippers mix mucus from the skin with water from their environment to create a tear film,” says Aiello. The research team was also able to demonstrate through experiments that the fish blink to protect their eyes from potential threats or to clear them of dust or debris. The researchers conclude that the concept also appears to fulfill the three main functions of blinking in terrestrial vertebrates in mudskippers.
According to them, the study now also sheds light on developments around 375 million years ago: “The transition to life on land required many anatomical changes, including for feeding, locomotion and breathing,” says Stewart. “Based on our results, we now assume that blinking was also one of the traits that evolved as the early tetrapods adapted to life on land,” says the scientist.
Source: Pennsylvania State University, professional article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2220404120