Four-eyed Fish

Image: petechar
Anableps sp.

FOUR-EYES!

That's what mean people used to call their bespectacled chums back before geeks and nerds took over the world. I doubt they'd say that if they came across a guy who really, ACTUALLY had four eyes. They'd probably say something like "look at that guy! He has four eyes!" And all his friends would think he meant someone was wearing glasses and they'd wonder what all the fuss was about.

But what about a fish with four eyes?

Image: Bárbol

FOUR-EYES!

And I can say that because the Four-eyed Fish doesn't really, ACTUALLY have four eyes. It just looks like it does. And acts like it does. And kind of... sees the world as if it does. Nice trick if you can pull it off!

What we're looking at here is a genus of three fish found in Central and South America. They spend their time swimming about at the surface of rivers and sometimes even out in coastal sea water. And when I say "the surface", I really mean it. ACTUALLY!

Image: petechar

Half their eyeballs are poking right out of the water. High and dry! The other halves are below the water's surface, where fish eyes usually are. I think you can see some possibilities here! It's great for a fish to see underwater. It's where they live, it's where all their stuff is. But what about "up"? When you're 30 cm (a foot) long like Four-eyes, it's worth looking out for those majestic and deadly eagles. Fish never seem to know what hit 'em...

So, an extra pair of eyes on the top of your head would be great. Probably not possible, but great. And when things are great but not possible, it may be time to compromise. So the Four-eyed Fish doesn't have four eyes, it has two eyes, each one divided in two.

Each eye has one lens. So far, so normal. Each eye also has two pupils and two retinas. No longer normal.

One pupil faces upward, the other downward. The lens is also thicker in the lower half than the upper, so while there is only one lens, it's adapted to provide good sight in both water and air at the same time. They're bifocals!

Now old Four-eyes can cruise the water with confidence, snatching insects that fall on the river, chomping on other small fish and swimming away before the eagles can strike!

Image: alumroot

This amazing ability to see through air like a native seems to have given them ideas. The Four-eyed Fish can not only tolerate fresh, brackish and marine water, it can also survive no water at all! Pretty much. It can take a low tide, but not a complete drying out. So it should be fine so long as you don't use your hair dryer on it.

With all that effort to survive, it's time to lay some eggs. Except that the Four-eyed Fish doesn't lay eggs, they're livebearers. They're even in the order Cyprinodontiformes, which is the one that contains all those aquarium fish like guppies, platies and mollies - the ones where once you have one male and one female, it takes a protracted effort to not have 50. It's a little more complicated for Four-eyes, though...

Image: A. Jaszlics

They have left and right-handed sexual organs. Males have a modified fin called the gonopodium which they use for mating, and it's either right or left-handed. The female's genitalia is also right or left-handed and in this case, opposites very much attract: leftie males mate with rightie females, rightist males with leftist females.

It means that instead of half the population mating with the other half, you have a quarter mating with another quarter. It's not clear how this can be a good thing, but perhaps the Four-eyed Fish just really enjoys dividing things in two.