Eye

• 1. Introduction

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  An eye is a sensory organ that lets an organism perceive visual information. It detects light and converts it into electro-chemical impulses inside neurons. That single trick, turning photons into signals a brain can read, sits behind nearly everything we call seeing. Across the animal world, eyes with resolving power have arrived in ten fundamentally different forms. Some are mirrors. Some are clusters of thousands of tiny tubes. Some are crystals of pure mineral. How did a flat patch of light-sensitive protein become a sharp camera focused on the retina? Why does a shrimp carry the most complex colour system on Earth while a snail can only tell light from dark? And what does it mean that 96 percent of animal species, scattered across six of roughly 35 main phyla, all trace their eyes back to one ancient beginning?

• 2. Ten Layouts

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  Ten different eye layouts exist, and they split cleanly into two families. Simple eyes have one concave photoreceptive surface. Compound eyes gather many individual lenses across a convex surface. The word simple misleads here. It does not mean reduced complexity or lower acuity, and almost any eye type can be adapted for almost any behaviour or environment. The pit eye is the most basic resolving form. It is an eye-spot set into a pit, which narrows the angle of light reaching the spot and lets the organism deduce where that light comes from. Found in about 85 percent of phyla, these small structures hold up to about 100 cells across roughly 100 micrometres. A second sorting cuts across the first, dividing eyes by the cellular build of their photoreceptors. Some cells are ciliated, as in vertebrates, while others are rhabdomeric. These two groups are not monophyletic, since the Cnidaria also carry ciliated cells, and some gastropods and annelids carry both. Every technological method humans commonly use to capture an optical image already occurs in nature, with the sole exceptions of zoom and Fresnel lenses.

• 3. Building A Sharper Image

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  Pit eyes leave the image blurred, and the fix is a material with a higher refractive index that forms a lens and shrinks the blur radius. The sharpest version uses material with a high refractive index that decreases toward the edges. This non-homogeneous lens drops the focal length from about 4 times the lens radius to 2.5 radii. No living aquatic organism keeps a homogeneous lens, suggesting the evolutionary pressure to improve was strong enough that this crude stage was quickly outgrown. Heterogeneous eyes have evolved at least nine times: four or more times in gastropods, once in copepods, once in annelids, once in cephalopods, and once in chitons, whose lenses are made of aragonite. Refractive corneas mark the next step, and they only work out of water. There the vitreous fluid carries a higher refractive index than air, so the cornea bends light usefully. Penguins and seals returned to the sea and lost their highly curved corneas, falling back on lens-based vision. A unique feature of most mammal eyes is the eyelid, which wipes the surface and spreads tears across the cornea to prevent dehydration. Eyelashes back them up, multiple rows of highly innervated, sensitive hairs that guard against fine particles and small irritants such as insects.

• 4. Mirrors And Multiple Lenses

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  Some marine organisms carry more than one lens stacked in a single eye. The copepod Pontella has three, its outer lens shaped as a parabolic surface to counter spherical aberration. Another copepod, Copilia, runs two lenses per eye arranged like a telescope. Eagles and jumping spiders also use multiple lenses, with a refractive cornea acting as a negative lens that enlarges the observed image by up to 50 percent over the receptor cells. A stranger design abandons lenses for mirrors. These reflector eyes line the inside with mirrored surfaces that bounce the image to a central focus, and if you peered into such a pupil you would see the same image the organism sees, reflected back out. The scallop Pecten fringes the edge of its shell with up to 100 millimetre-scale reflector eyes, detecting moving objects as they pass successive lenses. The spookfish goes further. Each of its two eyes collects light from both above and below, focusing the upper light with a lens and the lower light with a curved mirror built from many layers of small reflective plates made of guanine crystals.

• 5. The Compound Crowd

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  A compound eye can hold thousands of individual photoreceptor units called ommatidia, each pointing in a slightly different direction across a convex surface. Some eyes carry up to 28,000 such sensors arranged hexagonally, giving a full 360 degree field of vision and exquisite sensitivity to motion. The cost is resolution, since the tiny lenses run into the limits of diffraction. To see as sharply as a human simple eye, a person would need compound eyes around 11 metres in radius. These eyes fall into two groups. Apposition eyes form multiple inverted images and are the most common and presumably ancestral form, gathering one point of information per eye and combining them in the brain. Superposition eyes form a single erect image and split into three kinds: refracting, reflecting, and parabolic. The refracting superposition eye, found in nocturnal insects, can create images up to 1,000 times brighter than equivalent apposition eyes, trading away resolution for that light. Long-bodied decapod crustaceans, including shrimp, prawns, crayfish, and lobsters, stand alone in using reflecting superposition eyes, which swap lenses for corner mirrors. Trilobites, now extinct, built their compound lenses from clear calcite crystals, setting them apart from the soft eyes of most arthropods. Some carried a single lens while others packed thousands per eye.

• 6. Eyes Without Edges

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  The brittle star Ophiocoma wendtii has no discrete eyes at all. Its whole body is covered with ommatidia, turning its entire skin into a compound eye, and the same is true of many chitons. The tube feet of sea urchins carry photoreceptor proteins that together act as a compound eye, lacking screening pigments yet still detecting the direction of light by the shadow the urchin's opaque body casts. Other animals run hybrid designs that refuse easy labels. The mysid shrimp Dioptromysis paucispinosa has a refracting superposition eye, but behind it sits a single large facet three times the diameter of the others, fronting an enlarged crystalline cone that throws an upright image onto a specialised retina. The result is a simple eye nested inside a compound one. Males of the order Strepsiptera assemble their compound eye from a series of simple eyelets, each a complete image-forming retina, much like the schizochroal eyes of some trilobites. The pit organs of pit vipers add another twist, sensing thermal infra-red radiation alongside ordinary optical eyes. Those organs run on a transient receptor potential channel called TRPV1, an ion channel, rather than the G-protein coupled receptors of true photoreceptors.

• 7. Tuned To A Life

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  Predators on the African plains, scanning a flat horizon, carry a horizontal line of high-density ganglia matched to where their acuity is needed most. Tree-dwelling creatures that need good all-round vision instead spread their ganglia symmetrically, with acuity falling off from the centre outward. Hyperiid amphipods feed on prey drifting above them in deep water, so their eyes are nearly split in two, the upper region tuned to catch silhouettes against the faint sky. Deeper-living hyperiids, facing dimmer light, grow larger upper-eyes and may lose the lower portion entirely. In the giant Antarctic isopod Glyptonotus, a small ventral compound eye sits completely separated from a much larger dorsal one. Lifestyle even shapes colour and acuity. A bird of prey holds far greater visual acuity than a human and can in some cases detect ultraviolet radiation. A human eye with excellent acuity tops out near a theoretical 50 cycles per degree, while a rat resolves only about 1 to 2. The mantis shrimp carries the world's most complex colour vision system, with detailed hyperspectral colour vision. Most organisms are confined to wavelengths between 400 and 700 nanometres, a narrow window that likely reflects the submarine origins of the organ, since water blocks all but two small slivers of the electromagnetic spectrum.

• 8. One Ancient Origin

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  The PAX6 gene anchors one of the most striking claims in biology: that every animal eye shares a single origin. The common origin of all animal eyes is now widely accepted, built on the genetic features they share, and traced to a proto-eye that evolved some 650 to 600 million years ago. The first proto-eyes appeared around the time of the Cambrian explosion, and most early advances likely took only a few million years. Once the first predator gained true imaging, an arms race began, leaving prey and rival predators that could not see at a steep disadvantage in surviving and reproducing. The earliest eyes were patches of photoreceptor protein in single-celled animals, able only to register ambient brightness. Eye-spots then depressed into cups, the openings narrowed, the photoreceptor cells multiplied, and an effective pinhole camera emerged that could dimly make out shapes. The ancestors of modern hagfish, thought to be the protovertebrate, were pushed into deep dark water, where a convex eye-spot gathering more light proved advantageous and steered the vertebrate eye onto its own path. Convergence can deceive here. Cephalopod and vertebrate eyes look so alike in geometry that one seems descended from the other, yet their reversed ciliary and rhabdomeric opsin roles and different lens crystallins prove this is parallel evolution. The story closes with an unexpected human chapter. Animal eyes, especially non-compound spherical ones, are eaten by people in multiple culinary cultures.