A Fish with Four Eyes – Anableps

God has equipped living things with some amazing equipment to enable them to survive in all kinds of environments. One example is the fish in the Anableps genus, which have what appears to be four eyes. These “fish with four eyes” live in the coastal waters of Venezuela and Trinidad and along the coastline to the northeast coastal areas of South America.

The Anableps eyes are located above the top of the head, and the fish swim on the water’s surface. They have two eyes out of the water to see what is above the surface and two eyes below the water to see what is there. Actually, they have only two eyeballs, each divided horizontally by a thin band of epithelial tissue. Each eye has two corneas and two pupils. The lens is oval-shaped, while the retina splits into two sections. The lens varies in thickness because of the difference in refraction between the air (index of refraction 1) and the water (1.33). That means light is bent differently for the bottom of the lens than for the top. To correct for that, the lens thickness varies, with the top being thicker than the bottom.

What is the advantage of being able to see above and below the water at the same time? The upper eye can detect insects, while the lower eye detects small fish, diatoms, algae, and small water creatures. That gives these fish with four eyes more food options.

What may not be obvious is how complicated the Anablep’s brain has to be to handle four sets of data. For example, when an upper eye sees a bug on the water’s surface and the lower eye sees a small fish, the brain has to know which part of the eye is sending the signal. Then it has to decide which food to catch and how to do it. Scientists are studying how the Anableps brain performs these tasks.

Anableps also have a very unique reproductive system, but we’ll save that for a later discussion. There is no way to trace an evolutionary process from a typical fish or a single cell organism to a modern Anableps. These fish with four eyes are another example of God’s design that allows life to exist in every environment and situation on Earth.

— John N. Clayton © 2021

Reference: Scientific American, August 2021, page 22