In the animal kingdom, eyes have evolved dozens of times. We’re familiar with the camera-style eyes in our own heads, and the weird compound eyes of insects, but there are far weirder ones out there. Scientists are discovering new structures and adaptations all the time. There are eyes with mirrors, eyes with optical fibres, and eyes with bifocal lenses. There are eyes that see in the dark, move around heads, or go into sleep mode. There are even eyes made of rock. This slideshow will take you on a tour of some of these recent eye-opening discoveries.
Eyes don’t even have to be organic. While most animal lenses are made of proteins, the fuzzy chiton – an armoured relative of snails and other molluscs – has lenses made of rock. The lenses are made of aragonite, a type of limestone and the same stuff that the chiton’s shell is made of. These rocky eyes give the chiton a view that’s a thousand times fuzzier than ours, but that’s still good enough to see passing predators. The eyes also erode as the chiton ages, which might explain why it has more than a hundred of them.
Read more: Chitons see with eyes made of rock
Benjamin Franklin is credited with inventing bifocal glasses. These allow wearers to focus on both far and near objects by looking through different parts of the lens. But such lenses have been around for millions of years, on the nightmarish face of the sunburst diving beetle. The beetle’s larva has six pairs of eyes, and the front set is unique in the animal kingdom. Each one has one lens and two retinas, one sitting behind and slightly below the other. The lens manages to focus sharp images onto both of them, so the beetle can see near and far objects at the same time, with equal sharpness. Its bifocal lens gives it two eyes for the price of one.
Read more: The beetle with bifocal eyes
In the deep ocean, the brownsnout spookfish can look up and down at the same time, with some of the animal kingdom’s strangest eyes. Each one is split into two connected parts, so it looks like the spookfish has four eyes. One half points up and the other points down, allowing the fish to look at both sky and abyss simultaneously. The downward eye is unique. Unlike the eyes of all other back-boned animals, which use a lens to focus light, this one uses mirrors. It uses hundreds of tiny crystals, arranged in a curved shape, to collect and focus light.
By reflecting light, rather than refracting it, these outer eyes could produce brighter images with higher contrasts that lens-carrying eyes normally would. That must give the fish a great advantage in the deep sea, where the ability to spot even the dimmest and briefest of lights can mean the difference between eating and being eaten.
Read more: Spookfish eye uses mirrors instead of a lens
The box jellyfish isn’t just a simple blob of goo. It’s an active predator that hunts with 24 eyes. These are clustered into four groups of six. In each cluster, four eyes are simple pits or slits that sense the presence of light. The other two actually see images and they’re remarkably similar to our eyes. They have their own lenses, retinas and corneas, and they’re even made using very similar genes. Even though we’re separated by millions of years of evolution, box jellyfish and back-boned animals have evolved eyes by independently recruiting the same building blocks.
The eye clusters are weighed down by heavy crystals so they're always upright, even if the jellyfish is swimming upside-down. This gives the animal a perpetual view of the sky, which allows it to stay close to the mangrove forests where its prey lives.
(Photos by Anders Garm)
Read more: Jellyfish and human eyes assembled using similar genetic building blocks
Mantis shrimps have the arguably the most incredible eyes of any animal. Each eye has three areas that can independently focus on objects, which means that a single mantis shrimp eye has “trinocular vision”. Our eyes have receptors that are tuned to three colours; those of mantis shrimps are tuned to at least twelve. And they can tune individual light-sensitive cells to local light levels.
Mantis shrimps can even see a special type of light – ‘circularly polarised light’ – that no other animal can. This ability allows them to send secret messages, produced by circularly polarised light reflecting off different parts of their shell. The ability hinges on a structure in their eyes that’s similar to technology found in our CD and DVD players. The mantis shrimp’s biological engineering completely outclasses our man-made efforts; if we could duplicate it, we could have the basis of tomorrow’s multimedia players and hard drives.
Read more: Mantis shrimps have a unique way of seeing; Mantis shrimp eyes outclass DVD players, inspire new technology
When we go to sleep at night, we close our eyes to stop any errant light from disturbing our slumber. But the larvae of zebrafish go one further. They shut down their eyes entirely at night, becoming temporarily blind. Their vision only returns when daylight does. Energy is precious to the baby fish and eyes are gas-guzzling appliances, even when they’re set to standby. It makes sense to just shut them off instead.
Read more: Pocket Science –zebrafish babies go blind at night
Even our own familiar eyes have hidden surprises. In 2009, scientists found that we’re all carrying living optic fibres called Muller cells. These cells help to get round a structural problem in our eyes, where the light-sensing cells of the retina lie behind a tangled mass of nerves and blood vessels. It’s a bit like designing a camera, and sticking the wiring in front of the lens. Light gets through the mess inside the long, cylindrical Muller cells. It reflects down the cell, much like in an optic fibre, to hit the light-sensing cells on the other side. (Image by Elyzhium)
Read more: Living optic fibres bypass the retina’s incompetent design
Many mammals have evolved eyes that can see in the dark. That involves more than just becoming bigger. Their eyes have more light-sensitive rod cells, and these cells have changed at a microscopic level. They have converted the nucleus at the middle of each cell into a light-collecting lens.
In almost all complex cells, DNA is tightly packed around the edge of the nucleus but lightly packed towards its middle. But in the rod cells of nocturnal mammals, it’s the other way round. This inverted arrangement collects light that hits the rod cells and funnels it through to the retina underneath. By moving its DNA around, each cell has become a little optic fibre.
Read more: Nocturnal mammals see in dark by turning displaced DNA into lenses
Like many species that live in perpetual darkness, the blind cavefish has lost its eyes. These fish have evolved from sighted ancestors on several occasions in different Mexican caves. Their eyes have degenerated over a million years of darkness, but their blindness can be easily reversed by a spot of clever breeding. Many genes govern the development of eyes, and different populations of cavefish have lost their vision by disrupting different eye genes. By breeding individuals from different caves, working genes from one parent can compensate for broken ones from another. The result: babies that can see. (Photo by skpy)
Read more: Cross-breeding restores sight to blind cavefish
As babies, flatfishes like plaice and flounders look like every other fish. But as they grow up, one of their eyes moves to the other side of their heads. This allows the adults to lie flat on their sides without getting an eyeful of sand. The evolution of these grotesque fish is beautifully captured by a fossil called Heteronectes. It’s a half-committed flatfish. One of its eyes has begun migrating to the other side of its head but hasn’t made it all the way – it stops at the midline. We couldn’t have wished for a better intermediate form – it’s half-way between the standard fish body plan and the distorted visages of flounders and soles.
Read more: Early flatfish has eye that’s moved halfway across its head
The Hawaiian bobtail squid creates its own light, using special organs filled with glowing bacteria. But these organs don’t just produce light – they sense it too. They are loaded with proteins that can detect light, and they produce nervous signals in bright conditions. They can expand and contract like an iris to control how much light gets through. They’re covered with a thick, transparent tissue that acts like a “lens”. The light organs are effectively an extra set of primitive eyes. They are living, ‘seeing’ flashlights. (Image by William Ormerod)
Read more: Glowing squid use bacterial flashlights that double as an extra pair of “eyes”