Today, advanced technology has made it possible to make bifocals from just one piece of glass, with curves of different strength in the top and bottom parts. And now we have multifocals-or varifocals-to aid our sight. But, long before humans invented bifocals, a fish which lives in brackish waters-the Four-Eyed Fish Anableps-has had bifocal sight since its beginnings.
The Anableps lives in the waters stretching from southern Mexico down through South America. It is found in the mouths of rivers, in water which is neither fresh nor saltwater, but is somewhere in between. This type of water naturally occurs at the mouths of rivers and swamps near the sea.
Anableps are a schooling fish, and if kept in an aquarium should be kept in groups. They are livebearers, and can grow to around one foot long, with a life span of six years.
The name ‘The Four-Eyed Fish’ isn’t completely accurate of course. At first glance these fish appear to have four eyes, one pair looking up and another pair looking down. But that’s an optical illusion. They have two big round eyes, but each eye is divided horizontally into two parts by a band of skin. Anableps are surface dwellers. To catch insects they can leap high above the water and, if necessary plunge to the bottom for a while. But for most of the time, they cruise along the surface. It’s because of this that the upper halves of their eyes protrude above the water and scan the sky, while the lower parts remain submerged and look underwater. In this way the four-eyed fish searches for food below and at the same time can see any predators such as birds above the water.
The thicker lens in its eyes below the band of skin enables the four-eyed fish Anableps to see clearly below the surface. The flatter, upper part surveys the sky. In other words it has its very own bifocals!
To keep their lenses clean all they have to do is duck their heads underwater. That stops their eyes drying out, keeping them clear and sparkling.
So, although man has come on leaps and bounds with scientific advancements, much of what he’s done is what nature has already thought of.