Almost all animals have eyes to sense light in their environment – even in dark habitats such as the deep ocean, where the only source might come from the odd burst of bioluminescence.

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But although species across the animal kingdom have evolved various structures for sight, they all need special cells called photoreceptors.

What are photoreceptors?

Photoreceptors are cells that transform light energy into electrical signals that can then be interpreted by the brain. There are two types based on shape: rods and cones.

Rod cells are built for contrast (black versus white) and contain the light-sensitive pigment rhodopsin.

A cone cell will carry one of many pigments, each tuned to a specific range of wavelengths – matching a colour – on the visible spectrum.

The number and type of photoreceptors an animal has dictates which colours it can perceive. A human retina has roughly 120 million rods and six million cones. The cones are split into three kinds or ‘spectral classes’ for trichromatic vision (around red, green and blue wavelengths).

In comparison, butterflies and mantis shrimp have a dozen classes to cover a broader range of the spectrum (from deep ultraviolet to far-red light), which gives them hyperspectral vision.

But there’s more to sight than sensing light, right?

Very poetic! Yes, vision also requires neural circuits. The electrical signal generated by a photoreceptor is relayed via branching neurones that connect to other nerve cells, which amplify or dampen signals before they reach the brain.

The brain then processes complex patterns in that visual information to detect the edges of objects and form an image of the outside world.

Speaking of circuitry, how’s this for ‘intelligent design’? In vertebrates like humans, light only hits photoreceptors after passing through the neural wiring. But in cephalopods, eyes are wired sensibly: behind light-sensitive cells.

Why do animals have pairs of eyes?

Well, one reason they come in pairs is so animals can calculate distance. But like most features, sight is an adaptation to an ecological niche, leading to a trade-off.

Fallow deer doe looking towards camera.
Fallow deer doe in Ashdown Forest, Sussex, England. © James Warwick/Getty

Prey (such as deer) typically have monocular vision – an eye on each side of their head so two fields of view can be combined into one large visual field, helping them watch out for hunters.

A female lioness running
A female lioness running full speed, in the Kalahari Desert, Botswana. © Jami Tarris/Getty

But predators (such as lions) have binocular vision, with forward-facing eyes so fields can overlap for depth perception, helping them spot food.

Eyes have a layer of photoreceptors and neurones – the retina – spread across a curved surface so that the brain can compare light and shade on separate cells to deduce direction.

Evolution has since elaborated on that basic plan to create a variety of eyes – 10 distinct forms, in fact, with two main kinds: simple and compound.

What is a simple eye?

The organ is simple because it’s made up of a single chamber with a concave retina at the back of the eye.

Some worms, larvae and molluscs have open ‘pit eyes’, but more complex creatures have a closed eyeball with a window (cornea) to help direct light beams to numerous photoreceptors on the retina.

Many animals have a diaphragm (iris) that adjusts the amount of light entering the eye through its aperture (pupil).

To produce sharp images, scallops and certain crustaceans use mirrors, but the majority of species focus light using lenses. Fish and other aquatic organisms have a spherical lens, whereas land animals have a disc lens and the eye is filled with a gel or ‘vitreous fluid’ that refracts light as it travels from air into the fluid medium (like a ‘bent’ straw in a glass of water).

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Illustration of the anatomy of a human eye
Illustration of the anatomy of a human eye. © Mark Garlick/Science Photo Library/Getty

What is a compound eye?

It’s composed of multiple chambers, optical units called ommatidia, arranged in a convex structure. While each ommatidium only captures a blurry image, because it directs light at narrow angles to relatively few photoreceptors, the signals from each facet are pieced-together by the brain to from one pixelated image.

Close-up of a common blue damselfly.
Close-up of a common blue damselfly. © Mik Roman/Getty

The multifaceted eyes of insects aren’t inferior, merely different. If a simple eye is like an HD television, a compound eye resembles the wall of screens in a CCTV control room: each individual screen doesn’t show much detail, but a change to one is noticeable and reveals sudden motion.

This helps explain why a house fly can easily avoid being swatted by, say, a rolled-up copy of BBC Wildlife.


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Main image: Eye of cuttlefish, in Ambon, Moluccas, Indonesia. © Reinhard Dirscherl/ullstein bild/Getty

Authors

JV ChamaryScience communicator

JV Chamary is an award-winning journalist with a PhD in evolutionary biology. He writes 'The Big Question' column for BBC Wildlife, and spent several years as the features editor on BBC Science Focus.

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