It is not surprising that eyes are one of the most intricate and complex parts of the body, given that they developed over 540 million years ago.

What might come as a surprise is that, despite all land animals having evolved in unique ways from fish millions of years ago, it would seem that it is fish’s eyes that are the most diverse and highly adapted on the planet.

There are many things which distinguish the eyes of a fish from our own. One of the most notable differences is the size of the eye itself. Fish eyes are, in general, proportionally much larger than other species. This is because fish need to be able to see in low light conditions.

Another unique quality of fish is that they don’t have eyelids. Eyelids serve the purpose of keeping out debris and keeping our eyes moist. However, the nature of a fish’s habitat means that their eyes are kept moist without the need of eyelids.

Unsurprisingly, the eyes of fish differ significantly between species. Fish use their vision in different ways, depending on what they eat and where they live. Those that live near the sea surface in clear water have very different eyes to those that live in caves, or in the deep dark depths of the ocean where there is very little light.

About the only thing all fish eyes have in common, is the substance of the lens.

Here are two examples of evolution at its very best, two completely unique eyes that are perfectly adapted to each fish’s living and feeding habits:

The Four-Eyed Fish (Anableps anableps)

This little fish has a slightly misleading name. It doesn’t actually have four eyes, it only has two. But they are so highly developed that they provide the same benefits of having four eyes.

The Four-Eyed fish lives at the surface of the water and has large protruding eyes.

It’s eyes point upwards which allows it to see the insects that it feeds on, and keep an eye out for aerial predators.

However, surely this would leave it vulnerable to marine predators that could attack it from below? And this is where Four-Eyes gets its name from…

Incredibly, each eye is split vertically by a membrane to form two pupils, and the lens changes in thickness to allow visibility in both air through one half and water through the other. This allows the fish to see both above and below the water at the same time!

Barreleye (Macropinna microstoma)

Another example of extreme evolution of the eyes is unquestionably the Barreleye fish. This species typically lives at the depth of the ocean where sunlight is replaced by complete darkness.

This fish gets its name from its barrel shaped eyes. Looking at the image, the nostrils are easily confused for the eyes. The eyes are luminous tubular shapes within the fish’s head. These help to collect light and enable the fish to see in dark conditions.

One of the most remarkable features about the Barreleye fish is that it’s head looks transparent. This is because of a fluid-filled shield that covers the head – allowing the eyes to absorb light and have visibility.

For many years it was thought that the Barreleye fish could only see a very narrow tunnel vision directly above it’s head. But more recent research has shown that the fish can rotate it’s eyes within it’s transparent shield to be able to look upwards to spot prey, and then forward to see what’s ahead of them. The ability to look upwards without moving it’s body allows it to surprise its prey.

It is only within the last ten years that there has been any significant research on the Barreleyed fish. In 2004 scientists from Monterey Bay Aquarium Research Institute used remotely operated vehicles to record it in its natural habitat. This was a really groundbreaking piece of research as there had been no previous knowledge of that it’s head was transparent. As any records of having caught the fish showed that by the time it reached the surface, the fluid-filled shield had shattered.

It does make you wonder, have we only just begun to uncover what’s really out there? There must still be so much to learn and many more weird and wonderful examples of evolutionary genius to be discovered?

This article is contributed by Steph McLean. She works for Lenstore.co.uk, an ecommerce retailer which sells contact lenses online. She has a strong interest in science, in particular biology and the nature of evolution.