A school of happy Anableps anableps

Our love affair with Anableps goes back to March of 1981 when we were lucky enough to obtain a group of six of these charming rascals from our favourite local pet store. They were about 3 inches (8cm), (TL). These fish had been, like almost all Anableps to this day, captured in the wild and transported from somewhere on the coast of South America or the east coast of Central America to a wholesaler and then to our pet store. They don't travel very well and were abnormally dark in colour, obvioiusly stressed, but alive. These live-bearers are called "Four-eyed fish" for rather obvious reasons.

The "Four-eyed" fish

We knew nothing about our Anableps beyond what we had gleaned from a couple of excellent articles by Edward Taylor but, fortunately, that was enough to get us started. We've been learning ever since. The common name, "Four-eyes", is a slight exaggeration. There are not four distinct eyes, but rather two divided eyes. Still, the separate functions of the two divisions and the beauty of the eye's adaptation to the necessities of the Anablep's life is quite remarkable.

A strange fish indeed

Bear in mind, as you read and examine the drawings and photographs that Anableps are surface dwellers. It is true that they can leap quite remarkable distances above the water and, if required plunge to the bottom for a while, but they much prefer cruising along the surface. This makes them fairly easy prey for birds to spot. It is also likely that predatory fish could, in spite of the Anablep's whitish bellies, spot them and sneak up from underneath. For best survival chances the Anableps must be able to spot these attackers in time to take evasive action. Their very special eyes provide the means. A band of pigment divides the eye horizontally just at the waterline. At least Anableps ordinarily swim such that the waterline coincides with that band. Just behind and avove that band is a golden iris flap to shield the upper pupil from the glare at the water's surface. The lens behind this upper pupil is flattened to provide an undistorted image of what's going on above water, (in air), for the lower retina. See the simple human eye diagram below for comparison.) Anableps really see their keepers very clearly!

Anablep eye at top-left, human eye top-right

The part of the lens that is behind the lower pupil is rounded as in an ordinary fish eye. This provides the best image of the underwater scene to the upper retina. As with human eyes, all images are inverted. It's up to the brain to turn them around and right side up.

As you can see the Anableps is abel to keep track of what is above and what is below at pretty much the same time. It's likely that they can only attend to one image at a time but I'll gladly leave it to some scientist to determine how quickly they can flip back and forth. By the way, these eyes aren't quite as delicate as they appear. They are well protected by a stron bony socket.

The below water lens

Read Part Two in this series to learn of even stranger stuff!