Its name is sadly misleading – it doesn`t actually have four eyes, but the two it does have are so specially modified as to be unique within the animal kingdom. Each eye is divided in half horizontally with two separate optical systems, each with its own focal length. The top half is for seeing in the air; the bottom half is for underwater.
This fish swims at the top of the water with its eyes projecting partway into the air. The upper half of each eye can see threatening birds of prey in the air. The lower half, different in structure, can see underwater and enables the fish to find food.
Anableps are mostly found in brackish water where there is heavy algal growth and detritus, and they can withstand wide fluctuations in salinity. In recent years it has been noted that occasionally, Anableps will climb out on land and sit on the mud flats of tidal estuaries where it fulfills many of the same ecological roles as the mudskippers do in the mangrove swamps of Africa and Asia. They eat pretty well anything that moves and quite a lot of stuff that doesn`t, but if you are intending to keep these fascinating creatures as pets you should note that they are primarily surface feeders and will usually ignore food that has fallen to the bottom.
Although most of the emphasis upon anableps is on its peculiar eyes, these are not the only strange things about these singular creatures. Their breeding habits are, if anything, even more extraordinary. Anableps are internally fertilizing and viviparous. In other words, they give birth to live young and provide the embryos with nutrition throughout gestation However the cloaca of a female A. anableps is covered by a single large scale that is unattached on either the right or the left side. The rays of the anal fin of a male Anableps are modified into a tubular gonopodium that can only be bent to the left or the right. Thus, only a “right-handed” male can mate with a “left-handed female” and vice versa. Luckily for the survival of these gloriously peculiar fish the proportion of left-handed males in a population seems to be equal to the proportion of right-handed females . However, surprisingly, it has been reported that right-handed males (60%) are more common than left-handed males (40%)
Anableps are wonderful fish and apparently make charming pets. I have only ever seen them once, when Corinna and I were in Enfield following our near fatal car crash about 18 months ago. I had no way of getting them home so I didn't buy them, although I have been kicking myself ever since. I have often thought that they should be more widely kept and if this article of mine has done something in a small way to further interest in these delightful creatures then I shall have done something very useful for once in my life!