Seeing is believing!

The eyesight of fish varies greatly, depending on the environment in which they live.  Amongst those with the most highly-developed sense of vision is the so-called four-eyed fish (Anableps anableps) which occurs in parts of Central and South America. These fish inhabit quite shallow, clear water and spend much of their time near the surface. Their eyes are positioned high on their head, rather like those of crocodiles, so the fish can remain largely submerged but see above the water surface at the same time.

Each eye is divided into two parts, with the upper portion allowing them to see well in air, while the lower is adapted to underwater viewing. These two segments of the eye operate independently, so they can scour the water looking for possible food and also watch for any danger from above at the same time. This often results in the pupil resembling an hourglass in shape, rather than being rounded. Four-eyed fish make fascinating aquarium occupants if you can provide a relatively spacious tank, since they grow to about 25cm (10in) long.

Fish that lose their sight

By way of contrast, the blind cave fish (Astyanax fasciatus mexicanus) has no vision at all, as its name suggests. Although these fish actually hatch with functional eyes, skin soon grows over them, so they lose their sight at an early age. This is because vision is of no value in the dark Mexican caves which they inhabit. Like other cave-dwelling species, they have also lost their body pigmentation and so are white in colour.

The ancestors of today’s blind cave fish become isolated about 1 million years ago, and have since adapted to their subterranean environment, whereas their normally-sighted and pigmented relatives are still to be found in nearby lakes and rivers. They are popular aquarium fish, often being kept as part of a community aquarium, and are certainly at no disadvantage when feeding alongside other species.

The significance of the lateral line

All fish also have a secondary means of finding their way around their environment. This takes the form of a jelly-filled tube present beneath the skin, running down each side of the body. These is known as the lateral line, and operates rather like a radar.

As the fish swims, so it detects pressure waves in the water being reflected back off obstacles in its path or the approach of a likely predator, enabling it to take appropriate action. It is often possible to see the lateral line as a paler area if you look closely along the fish’s body. With this highly efficient system to guide them, so even blind cave fish will therefore be able to swim safely around their aquarium.