Four-Eyed Fish
Tuesday, 19 September 2023
The four-eyed fish (Anableps anableps) is back in the Mangrove. These fish have a wide distribution in the wild in the northern part of South America. There, they live in both fresh and brackish water in small streams and mangrove areas near river mouths. The four-eyed fish can be seen in the section with the fiddler crabs. In this article, we would like to introduce you to this fish.
Two eyeballs, four pupils
The name “four-eyed fish” is factually inaccurate. Like other fish, the four-eyed fish has two eyes, but the pupils in the four-eyed fish are split horizontally. The eyes are ingeniously adapted for swimming at the water’s surface. In the early larval stage, the eyes look the same as those of all other fish, with a round lens, a pupil, and an iris in each eye. During development, the pupil and the cornea become progressively separated by a pigment line until they are completely distinct. The upper pupil receives incoming images from above the waterline, while the lower pupil receives incoming images from below. Both the cornea (on the outside of the eye) and the retina (a layer in the eye with light-sensitive cells) differ in thickness at the top and bottom of the eye. Both are adapted to differences in the refractive index of light and light intensity in air and water. But that’s not all. A flexible iris valve develops behind the cornea where the pigment line is located, exactly at the waterline. This valve protects the pupil from light reflection on the water’s surface. The lens is asymmetric in shape (flattened at the top and round at the bottom), causing light entering from above and below to be refracted differently, allowing them to see clearly above and below water. A bony eye socket for protection largely encloses the protruding eyes.
Commuters
Four-eyed fish commute daily with the tide to submerged parts of the mangrove forest. There, they find fallen insects in the water, their favourite prey. But due to their split eyes, they can also effectively hunt small fish and water insects. If they are stranded in a drying puddle at low tide, it’s not a disaster; they can survive for a while without water. Sometimes, they sort of “crawl” out of the water, which has also been observed at the Mangrove.
Few but large offspring
Four-eyed fish are livebearers. The genders can be easily distinguished by the relatively large gonopodium of the male. The gonopodium is an organ found in various types of live-bearing bony fish. It consists of fused rays of the anal fin and is used during mating. After mating, the female carries the fertilised eggs in her belly. The vulnerable fish larvae have a better chance of survival than those that live in open water. When the fish are born, they are already five centimetres long!
