Anableps anableps - Part 2 - Mating Explained

By Tom and Pat Bridges
First published in "The Scat" - St. Catherine's Aquarium Society, Canada. October1999
Aquarticles

Anableps anableps
Photo: E.Naus

Anableps' mating seen from above - often called "s"ing                                This young male exhibits amazing reach

Back in the early 1700's when the first Anableps were sent back to Europe, Artedi, the most famous icthyologist of his day, made much of the unusual eyes, but did not mention that these fish were livebearers. I'm very certain that no one could have missed thta bit of information if any males had been captured. It came to light about half a century later. This oversight was likely due to the tendency for female Anableps to school together in the wild and, along with the fact that both males and females look alike when they are young, gave rise to the myth that male Anableps are very rare. When we first confided to a fellow hobbyist that we had six Anableps of which three were males, his response was, "Are you sure you have males!"

Statistically, I expect that about half of all Anableps born in the wild are males; in our tanks we have encountered the problem of having too many males and too few females at times. The myth of a scarcity of males needs to die, and yes we were sure! If you look at the pictures and drawings on this page you will easily see why.

Male Anableps - note what Artedi never saw

Close up of the Anableps' fleshy gonopodium

Note that gonopodium can only move in one direction

The maleness of Anableps is not subtle. What starts out as a normal-looking anal fin forms into a substantial gonopodium. No fragile-looking bony appendage such as you see on a guppy or swordtail.

Males can move their gonopodiums quite vigorously but, only in one direction. Some are right-gonopodiumed and some are left. There are no "switch hitters".

If the female's genital pore could be easily approached from either side, there would be no problem and no mystery.

Don't make the embarrassing mistake that I originally did, and think that the anus is the genital opening. The female Anableps' genital opening is covered by a "scale flap" called a foricula and, you guessed it, some have a foricula that hinges on the right and some on the left. As the picture and drawing on the lower right of a post-mortem mature female illustrates, the foricula, as pried up with the tip of my scalpel, is quite large and secure.

Female Anableps - note the normal anal fin

A strong light can help you guess the direction of the foricula

A post-mortem examination of a foricula

What happens is this. When a right-gonopodiumed male approaches a left-foricula(ed) female along her left side and moves his member into position, she may, if she's ready, lower the flap and permit him to inject sperm. That compatible situation is the one pictured here. If the male approaches a female with an incompatible genital arrangement, presumably he fails to impregnate her. By the way, females grow larger than males and may just turn, and drive them away if they're not interested for any reason.

The mystery is why the process of natural selection over millions of years has not eliminated what appears to be an impediment to Anableps' reproduction. It's assumed that there is some overriding advantage to this arrangement that science has yet to discover. The problem for hobbyists when trying for compatible pairings is that while the orientation of males can be observed, that of females can only be guessed at.

All we can offer is our observation that the foriculas of young females are smaller and seem less secure. We think an incompatible male might sometimes succeed with a yougn female, but with an older one, it's doubtful.