The Strangest Eyes in the Animal Kingdom

When you see the world in a particular way, it can be easy to overlook the fact that not everybody sees it alike.

We mean that literally. Leaving aside philosophical questions about the subjective experience of colour, organisms have evolved very different ways of viewing their surroundings, with eye structures and arrangements suited to their particular ways of life.

Some examples are familiar. The horizontal pupils of herbivores provide a panoramic view, helping them spot approaching predators and avoid obstacles while fleeing. Nocturnal hunters, by contrast, have vertical pupils that enhance their vision at night.

Other eyes across the vast and wonderfully diverse animal world work in ways we can scarcely imagine. Here are some of the animal kingdom's strangest eyes – jeepers creepers, indeed.

Cuttlefish

No other creature has pupils quite like those of the cuttlefish. Their W-shaped form appears to help these animals compensate for the vertically uneven light conditions common in the underwater depths where they live. That is only the beginning.

The eye of a cuttlefish. (A. Martin UW Photography/Moment/Getty Images)

Cuttlefish possess just one kind of photoreceptor, which ought to limit them to monochrome vision. However, the odd, broad pupils of cuttlefish and other cephalopods may enable a wholly different method of perceiving colour: exploiting the way light passing through a prism separates into a rainbow.

This effect, called chromatic aberration, can cause difficulties when the lenses of our own eyes do not focus every colour at the same point, changing sharp tonal contrasts into a gentler blend of hues. Cuttlefish may instead have made this apparent flaw useful.

A narrower pupil reduces the effect, making cephalopods' wide pupils especially susceptible to it. Although it may blur the image, that blurring depends on colour – potentially allowing these supposedly colour-blind animals to distinguish colours. They may even detect colours we do not know exist. It might also account for their ability to match their colouring to their surroundings for camouflage.

Unlike other cephalopods, cuttlefish can rotate their eyes, which lets them view the world in 3D. Scientists recently discovered that these mobile eyes produce stereoscopic vision, giving cuttlefish one more environmental advantage.

Birds

Despite their tiny, bead-like eyes, birds may be able to see far more than we can.

As noted, cephalopods have only one type of photoreceptor. Humans have four: three cones and a rod. This gives us colour sensitivity across three peak wavelengths, known as trichromatic vision. The rod serves low-light vision.

Birds have six: four cones that provide tetrachromatic vision, one rod, and an unusual double cone used for perceiving movement without colour.

A migratory Himalayan bluetail (Tarsiger rufilatus). (Nitat Termmee/Moment/Getty Images)

They also have a protein in their eyes that may enable them to perceive magnetic fields. Migratory birds navigate remarkably well, although precisely how they do so was unclear for a long time. Scientists recently narrowed the explanation to a group of blue-light-sensitive proteins known as cryptochromes.

Birds' magnetoreception – their capacity to sense magnetic fields – appears to depend on blue light, which indicates that this sense could be based on vision. There is a distinct possibility that the magnetic filter associated with blue may result from a quantum peculiarity. More recent laboratory work has demonstrated how a magnetic field influences a quantum property of cryptochromes that controls their electrons.

Anableps anableps

Meet the largescale four-eyes (Anableps anableps), a member of the four-eyed fish genus.

The unusual eyes of the four-eyed fish. (Charles Peterson/Flickr/CC BY-NC 2.0)

This remarkable creature does not truly have four eyes. Instead, its two eyes have evolved an extraordinary adaptation. It occupies the water's surface, spending most of its time there hunting insects that hover over aquatic habitats.

Its eyes sit on top of its head, ideally placed for spotting airborne insects. Yet part of each optical organ remains beneath the water, which is where the arrangement becomes especially intriguing. Each pupil is split in two: one half lies above the waterline (dorsal), while the other is underwater (ventral), facing down into the cloudy depths.

This allows the fish to see both above and below the water simultaneously – two environments in which light travels differently – so it can monitor predators as well as prey. Its lens thickness also varies to accommodate the different refractive indices of air and water, as does the thickness of its corneal epithelium.

The proteins within the retina's photoreceptor cells differ slightly too. Those in the dorsal retina are more sensitive to green light, while those in the ventral retina respond more strongly to yellow light. Since these fish often inhabit muddy places such as mangroves, this is believed to enhance sight in murky water.

Mantis shrimps

The most complicated eyes known in the animal kingdom belong to bottom-dwelling marine crustaceans that spend their lives in burrows among rocks and on the seafloor.

Humans have four photoreceptors. Birds have six – impressive enough. Yet mantis shrimps of the order Stomatopoda, those exceptionally industrious little creatures, have 16 in their compound eyes. What do they do with all those photoreceptors? They see. They see everything. Do not play hide-and-seek with a mantis shrimp.

A peacock mantis shrimp (Odontodactylus scyllarus). (prilfish/Flickr, CC BY 2.0)

In truth, we do not know why mantis shrimps require such elaborate visual organs, largely because it is so difficult for us to conceive of what they see. Alongside conventional colour photoreceptors, they have receptors sensitive to ultraviolet light. That alone is not exceptional: certain insects, birds and even reindeer can perceive ultraviolet. Mantis shrimps, however, can see five distinct ultraviolet frequency bands.

They can also detect polarised light: the orientation of the oscillations in a travelling light wave. Many animals, including cuttlefish, can perceive linearly polarised light. As far as we know, mantis shrimps alone can see circularly polarised light.

Each eye sits on a stalk and can move independently. Every eye can also judge depth. Humans depend on binocular vision for depth perception; a mantis shrimp needs only one eye. It can even see cancer before symptoms appear.

If that is not an ocular superpower, we do not know what is.

Chitons

What are eyes made from? Usually tissue: a cellular structure. Unless, that is, you are a marine mollusc called a chiton, belonging to the class Polyplacophora.

West Indian fuzzy chitons (Acanthopleura granulata). (Hans Hillewaert/Wikimedia Commons/CC BY-SA 4.0)

These fairly small animals spend their lives shielded by thick plates of interlocking armour while creeping over rocks and grazing on whatever they encounter. You might expect such an animal to have soft eyes capable of looking beyond the shell's edges, watching for predators and detecting the day-night cycle.

You would be mistaken. Chitons do have eyes, but they are set into their armour and made from mineral – specifically, a form of calcium carbonate called aragonite.

The simple eyes of chitons are scattered over their shells among hundreds of sensory structures called aesthetes. They comprise an aragonite lens beneath a cornea and some kind of retina. To scientists' surprise, these tiny primitive structures can genuinely form images.

What remains unknown is how their brains process this visual information – chitons do not have much going on in that area.

Chiton eyes. The dark spots are eyes; the smaller bumps are aesthetes. (Wyss Institute at Harvard University)

Nevertheless, they may help us understand some of evolution's more unusual historical routes. Trilobites, for instance, also possessed mineral eyes, with lenses formed from calcite.

These extinct animals had the first genuinely complex eyes that we know about, so studying them may reveal much about the evolution of vision on Earth in all its dazzling complexity.

An earlier version of this article was published in January 2022.