We don't tend to think about what it means to blink. This simple act, which is so common among terrestrial animals, occurs in many diverse ways. Ducks, for instance, blink by raising their lower eyelids, while mongooses often utilize a thin membrane, sometimes referred to as a third eyelid, which passes across the eye from the head towards the tail. Aquatic animals, however, largely lack the ability to blink. This led a multidisciplinary team from the USA and Australia, headed by Brett Aiello, to hypothesize that blinking may have arisen when animals transitioned from water to land. To investigate why blinking first evolved, the team examined mudskippers, amphibious fish that spend most of their time on land. Perhaps not so coincidentally, they also belong to the minority of aquatic animals that blink. Aiello and colleagues began by analysing how mudskippers blink their eyes before delving into why.
From watching movies of Indian mudskippers (Periophthalmus barbarous) and African mudskippers (Periophthalmodon septemradiatus) blinking on land, the team discovered that these land-frequenting fish blink by lowering most of their eye into a cavity in their heads and raising a membrane to cover the rest of it, which is notably different from blinking in ducks and mongooses. How long mudskippers blink for, however, was comparable to spontaneous blinks in humans. When the researchers compared the eye muscles of mudskippers with those of a non-blinking fully aquatic fish, they found no notable differences. This suggested that the mudskippers must be using an existing set of muscles to blink. Likewise, Aiello and colleagues didn't find any tear glands in mudskippers, but they did discover cells on the head that produce mucus that would spread around, wetting the mudskipper's eyes when blinking. So, blinking may serve to wet the eye in mudskippers as it does in land-dwelling animals.
Next, the group turned their attention to discovering why mudskippers blink. To test whether blinking indeed serves to wet the eye, they filmed mudskippers under simulated windy conditions using fans. The windy conditions caused 30 times more evaporation than normal, which made the mudskippers blink more. Moreover, the researchers noticed that dehydrated mudskippers more often rolled their bodies on the damp tank floor, a behavior which captures water to wet the eye. Together, these observations indicate that mudskippers blink and roll to moisten the eye while on land.
Preventing the eye from drying out isn't the only reason that animals blink their eyes. To consider whether blinking also cleans debris from the surface of the mudskipper's eyes, the researchers dusted the eyes with dried brine shrimp eggs. The mudskippers removed ∼97% of the dusty substance from their eyes after just one blink. To test whether blinking provides protection from bumps, the researchers lightly tapped the eye surface with a cotton swab, causing the fish to blink. This reflexive blink usually happened within 30 ms and lasted about twice as long as a spontaneous blink. Based on this, Aiello and colleagues suggest that blinking serves an additional function in these fish – to protect the eye from impact injury by encroaching objects. Thus, mudskippers blink for at least three reasons and these shared experiences might explain why this complex behavior occurs in a fish that is so distantly related to animals that live in the air. Although we remain largely unaware of our eye blinking in everyday situations, the behavior serves several functions, each tied to the challenges of leaving life in the water behind.
