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colspan="2" Schematic diagram of the vertebrate eye.>
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colspan="2" Compound eye of Antarctic krillAntarctic krill is a species of krill found in the Antarctic waters of the Southern Ocean. Antarctic krill are shrimp-like invertebrates or crustaceans that live in large schools, called swarms, sometimes reaching densities of 10,000–30,000 individual animals per cubic meter...
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Eyes are
organsIn biology and anatomy, an organ is a collection of tissues joined in structural unit to serve a common function ....
that detect
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
, and send electrical impulses along the
optic nerveThe optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
to the
visualThe visual system is the part of the central nervous system which enables organisms to see.It interprets the information from visible light to build a representation of the world surrounding the body...
and other areas of the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of
animalAnimals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously...
species possess a complex optical system. Image-resolving eyes are present in
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
, molluscs, chordates, annelids and arthropods.
The simplest "eyes", such as those in unicellular organisms, do nothing but detect whether the surroundings are light or
darkDarkness is the absence of light. Scientifically it is only possible to have a reduced amount of light. The emotional response to an absence of light has inspired metaphor in literature, symbolism in art, and emphasis....
, which is sufficient for the
entrainmentIn chronobiology, entrainment of a circadian system is the alignment of its own period and phase to the period and phase of an external rhythm. A common example is the entrainment of endogenous circadian rhythms to the daily light-dark cycle...
of
circadian rhythmA circadian rhythm is a roughly-24-hour cycle in the biochemical, physiological or behavioral processes of living entities, including plants, animals, fungi and cyanobacteria...
s. From more complex eyes, retinal
photosensitive ganglion cellPhotosensitive ganglion cells, also called photosensitive Retinal Ganglion Cells , intrinsically photosensitive Retinal Ganglion Cells or melanopsin-containing ganglion cells, are a recently discovered type of nerve cell in the retina of the mammalian eye which, unlike other retinal ganglion...
s send signals along the
retinohypothalamic tractThe retinohypothalamic tract is a photic input pathway involved in the circadian rhythms of mammals. The origin of the retinohypothalamic tract is the intrinsically photosensitive Retinal Ganglion Cells , which contain the photopigment melanopsin...
to the
suprachiasmatic nucleiThe suprachiasmatic nucleus, or nuclei, , a tiny region on the brain's midline in a shallow impression of the optic chiasm, is responsible for controlling endogenous circadian rhythms...
to effect circadian adjustment.
Overview
Complex eyes can distinguish shapes and
colorColor or colour is the visual perceptual property corresponding in humans to the categories called red, yellow, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...
s. The
visualVisual perception is the ability to interpret information and surroundings from visible light reaching the eye. The resulting perception is also known as eyesight, sight or vision...
fields of many organisms, especially predators, involve large areas of
binocular visionBinocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...
to improve
depth perceptionDepth perception is the visual ability to perceive the world in three dimensions. Although any animal capable of moving around its environment must be able to sense the distance of objects in that environment, the term perception is reserved for humans, who are, as far as is known, the only beings...
; in other organisms, eyes are located so as to maximise the field of view, such as in
rabbitRabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world. There are seven different genera in the family classified as rabbits, including the European rabbit , Cottontail rabbit , and the Amami rabbit...
s and
horseThe horse is a hoofed mammal, a subspecies of one of seven extant species of the family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
s, which have
monocular visionMonocular vision is vision in which each eye is used separately. By using the eyes in this way, as opposed by binocular vision, the field of view is increased, while depth perception is limited. The eyes are usually positioned on opposite sides of the animal's head giving it the ability to see two...
.
The first proto-eyes evolved among animals 540 million years ago, about the time of the
Cambrian explosionThe Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex animals around , as evidenced by the fossil record. This was accompanied by a major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
. The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in 6 of the thirty-plus
[The precise number depends on the author] main phyla. In most
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
s and some molluscs, the eye works by allowing light to enter it and project onto a light-sensitive panel of
cellsThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of an organism that is classified as living, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos...
, known as the
retinaThe vertebrate retina is a light sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
, at the rear of the eye. The
cone cellCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
s (for color) and the
rod cellRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
s (for low-light contrasts) in the retina detect and convert light into neural signals for vision. The visual signals are then transmitted to the
brainThe brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all...
via the
optic nerveThe optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the
vitreous humourThe vitreous humour or vitreous humor is the clear gel that fills the space between the lens and the retina of the eyeball of humans and other vertebrates...
, with a focusing
lensThe lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a sharp real...
and often an
irisThe iris is a membrane in the eye, responsible for controlling the diameter and size of the pupil and the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel...
; the relaxing or tightening of the muscles around the iris change the size of the
pupilThe pupil is an opening located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils...
, thereby regulating the amount of light that enters the eye, and reducing aberrations when there is enough light.
The eyes of
cephalopod{Taxobox| name = Cephalopods| fossil_range = | image = Tafel 054 300.jpg| image_caption = A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur| regnum = Animalia| image_width = 220px| phylum = Mollusca| classis = Cephalopoda...
s,
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
,
amphibianAmphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic animals that metamorphose from a juvenile water-breathing form, to an adult air-breathing form. Though amphibians typically have four limbs, the Caecilians are notable for being limbless. Unlike other land...
s and
snakeSnakes are elongate legless carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic amniote vertebrates covered in overlapping scales...
s usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens — similar to how a
camerathumb |right|Cameras from Large to Small, Film to Digital A camera is a device that records images, either as a still photograph or as moving images known as videos or movies...
focuses.
Compound eyes are found among the
arthropodAn arthropod is an invertebrate that has an exoskeleton , a segmented body, and jointed attachments called appendages. Arthropods are animals belonging to the Phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360-degree field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many
StrepsipteraThe Strepsiptera are an order of insects with nine families making up about 600 species...
, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images.
Possessing detailed hyperspectral color vision, the
Mantis shrimpMantis shrimp or stomatopods are marine crustaceans, the members of the order Stomatopoda. They are neither shrimp nor mantids, but receive their name purely from the physical resemblance to both the terrestrial praying mantis and the shrimp. They may reach 30 cm in length, although...
has been reported to have the world's most complex color vision system.
TrilobiteTrilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. Trilobites first appear in the fossil record during the Early Cambrian period and flourished throughout the lower Paleozoic era before beginning a drawn-out decline to extinction when, during the...
s, which are now extinct, had unique compound eyes. They used clear
calciteCalcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 470°C, and vaterite is even less stable....
crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.
In contrast to compound eyes, simple eyes are those that have a single lens. For example,
jumping spiderThe jumping spider family contains more than 500 described genera and over 5,000 species, making it the largest family of spiders with about 13% of all species. Jumping spiders have good vision and use it for hunting and navigating. They are capable of jumping from place to place, secured by a...
s have a large pair of simple eyes with a narrow
field of viewThe field of view is the angular extent of the observable world that is seen at any given moment.Different animals have different fields of view, depending on the placement of the eyes. Humans have an almost 180-degree forward-facing field of view, while some birds have a complete or...
, supported by an array of other, smaller eyes for
peripheral vision Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of peripheral vision...
. Some insect
larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e, like
caterpillarCaterpillars are the larval form of a member of the order Lepidoptera . They are mostly phytophagous in food habit, with some species being entomophagous. Caterpillars are voracious feeders and many of them are considered pests in agriculture...
s, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called
ocelli- Overview :A 'simple eye' refers to a type of eye design or optical arrangement that contains a single lens. ‘Simple’ in this case does not refer to the number of cells present in the eye or the visual acuity of the eye. Indeed, the optical properties and anatomy of simple eyes is often incredibly...
, can be found in animals like some of the
snailThe word snail is a common name for almost all members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word snail is used in a general sense, it includes sea snails, land snails and freshwater snails. Snails lacking a shell or having only a very small one are...
s, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out of direct
sunlightSunlight, in the broad sense, is the total spectrum of the electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the atmosphere, and the solar radiation is obvious as daylight when the Sun is above the horizon. Near the poles in summer, the days are longer and the...
.
In organisms dwelling near
deep-sea ventA hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots....
s, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents - in this way the bearers can spot hot springs and avoid being boiled alive.
Evolution
The common origin (
monophylyIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it consists of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly...
) of all animal eyes is now widely accepted as fact based on shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, as the first predator to gain true imaging would have touched off an "arms race". Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.
Eyes in various animals show adaption to their requirements. For example,
birds of preyBirds of prey are birds that hunt for food primarily on the wing, using their keen senses, especially vision. They are defined as any bird that kills its prey with its talons. Their talons and beaks tend to be relatively large, powerful and adapted for tearing and/or piercing flesh. In most cases,...
have much greater visual acuity than humans, and some can see
ultravioletUltraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than x-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of
parallel evolutionParallel evolution is the development of a similar trait in different not closely related species , but descending from the same ancestor.-Parallel vs...
, despite their distant common ancestry.
The earliest eyes, called "eyespots", were simple patches of
photoreceptorA photoreceptor, or photoreceptor cell, is a specialized type of neuron found in the eye's retina that is capable of phototransduction. The great biological importance of photoreceptors is that as cells they convert light into the beginning of a chain of biological processes...
cells, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the lightsource. This gradually changed as the eyespot depressed into a shallow "cup" shape, granting the ability to slightly discriminate directional brightness by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective
pinhole cameraA pinhole camera is a very simple camera with no lens and a single very small aperture. Simply explained, it is a light-proof box with a small hole in one side. Light from a scene passes through this single point and projects an inverted image on the opposite side of the box...
that was capable of slightly distinguishing dim shapes.
The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized colour filtering, blocked harmful radiation, improved the eye's
refractive indexThe refractive index of a medium is a measure of how much the speed of light is reduced inside the medium. For example, typical soda-lime glass has a refractive index close to 1.5, which means that in glass, light travels at 1 / 1.5 = 2/3 the speed of light in a vacuum...
, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent
crystallinIn biology, a crystallin is a water-soluble structural protein found in the lens of the eye, accounting for the transparency of the structure and also in cornea, where it can play a similar role...
protein.
The gap between tissue layers naturally formed a bioconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the
corneaThe cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is approximately 43...
and
irisThe iris is a membrane in the eye, responsible for controlling the diameter and size of the pupil and the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel...
. Separation of the forward layer again forms a humour, the
aqueous humourThe aqueous humour is a thick watery substance filling the space between the lens and the cornea.-Locations:The anterior segment is the front third of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens...
. This increases refractive power and again eases circulatory problems. Formation of a nontransparent ring allows more blood vessels, more circulation, and larger eye sizes.
Types of eye
Nature has produced ten different eye layouts — indeed every way of capturing an image has evolved at least once in nature, with the exceptions of
zoomA zoom lens is a mechanical assembly of lens elements with the ability to vary its focal length , as opposed to a fixed focal length lens...
and
Fresnel lensA Fresnel lens is a type of lens developed by French physicist Augustin-Jean Fresnel for lighthouses; a similar design had previously been proposed by Buffon and Condorcet as a way to make large burning lenses....
es. Eye types can be categorized into "simple eyes", with one concave chamber, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution — the physics of compound eyes prevents them from achieving a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures. Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or
rhabdomRhabdoms are transparent rods, found in the center of each ommatidium in the compound eye of arthropods. These rods are constructed from the eight photoreceptor cells in the ommatidium. Each photocell is long and thin. They pack their area of the ommatidium completely filling the space...
ic. These two groups are not monophyletic; the cnidaira also possess cilliated cells,
and some annelids possess both.
Simple eyes
Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times; in vertebrates, cephalopods, annelids, crustacea and cubozoa.
Pit eyes
Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eye". They are small, comprising up to about 100 cells covering about 100 µm. The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material.
Pinhole eye
The pinhole eye is an "advanced" form of pit eye incorporating several improvements, most notably a small
apertureIn optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane. The aperture determines how collimated the admitted rays are,...
(which may be adjustable) and deep pit. It is only found in the
nautiloidNautiloids are a large and diverse group of marine cephalopods belonging to the subclass Nautiloidea that began in the Late Cambrian and are represented today by the living Nautilus. Nautiloids flourished during the early Paleozoic era, where they constituted the main predatory animals, and...
s. Without a lens to focus the image, it produces a blurry image, and will blur out a point to the size of the aperture. Consequently, nautiloids can't discriminate between objects with an
angular separationIn mathematics and all natural sciences , the angular distance between two point objects, as observed from a location different from either of these objects, is the size of the angle between the two directions originating from the...
of less than 11°. Shrinking the aperture would produce a sharper image, but let in less light.
Spherical lensed eye
The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered — hence increasing the resolution obtainable. The most basic form, still seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges — this decreases the focal length and thus allows a sharp image to form on the retina. This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing
spherical aberrationthumb|right|Spherical aberration. A perfect lens focuses all incoming rays to a point on the optic axis. A real lens with spherical surfaces suffers from spherical aberration: it focuses rays more tightly if they enter it far from the optic axis than if they enter closer to the axis. It therefore...
. Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.
Heterogeneous eyes have evolved at least eight times — four or more times in gastropods, once in the
copepodCopepods are a group of small crustaceans found in the sea and nearly every freshwater habitat. Many species are planktonic , but more are benthic , and some continental species may live in limno-terrestrial habitats and other wet terrestrial places, such as swamps, under leaf fall in wet forests,...
s, once in the annelids and once in the cephalopods. No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".
This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles.
The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field — this fast response is further accelerated by the large nerve bundles which rush the information to the brain. Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity.
This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above).
Weaknesses
One weakness of this eye construction is that
chromatic aberrationIn optics, chromatic aberration is the failure of a lens to focus all colors to the same point. It occurs because lenses have a different refractive index for different wavelengths of light...
is still quite high — although for organisms without color vision, this is a very minor concern.
A weakness of the vertebrate eye is the
blind spotA blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
at the
optic discThe optic disc or optic nerve head is the location where ganglion cell axons exit the eye to form the optic nerve. There are no light sensitive rods or cones to respond to a light stimulus at this point. This causes a break in the visual field called "the blind spot" or the "physiological blind spot"...
where the optic nerve is formed at the back of the eye; there are no light sensitive rods or cones to respond to a light stimulus at this point. By contrast, the
cephalopod{Taxobox| name = Cephalopods| fossil_range = | image = Tafel 054 300.jpg| image_caption = A variety of cephalopod forms from Ernst Haeckel's 1904 Kunstformen der Natur| regnum = Animalia| image_width = 220px| phylum = Mollusca| classis = Cephalopoda...
eye has no blind spot as the retina is in the opposite orientation.
Multiple lenses
Some marine organisms bear more than one lens; for instance the copeopod
Pontella has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. Copillas eyes have two lenses, which move in and out like a telescope. Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution.
Refractive cornea
In the eyes of most terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air, relieving the lens of the function of reducing the focal length. This has freed it up for fine adjustments of focus, allowing a very high resolution to be obtained. As with spherical lenses, the problem of spherical aberration caused by the lens can be countered either by using an inhomogeneous lens material, or by flattening the lens. Flattening the lens has a disadvantage: the quality of vision is diminished away from the main line of focus, meaning that animals requiring all-round vision are detrimented. Such animals often display an inhomogeneous lens instead.
As mentioned above, a refractive cornea is only useful out of water; in water, there is no difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures which have returned to the water — penguins and seals, for example — lose their refractive cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strong cornea.
Reflector eyes
An alternative to a lens is to line the inside of the eye with "
mirrors", and reflect the image to focus at a central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out.
Many small organisms such as
rotiferThe rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...
s, copeopods and platyhelminths use such organs, but these are too small to produce usable images. Some larger organisms, such as scallops, also use reflector eyes. The scallop
PectenPecten is Latin for comb, and can refer to:*Pecten oculi - a structure in the bird retina which contains most of the vasculature*Various pectineal lines in anatomy*Pecten , a respiratory structure...
has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses.
There is at least one vertebrate, the spookfish, whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from the above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of
guanineGuanine is one of the five main nucleobases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, thymine, and uracil. In DNA, guanine is paired with cytosine. With the formula C
5H
5N
5O, guanine is a derivative of purine, consisting of a fused...
crystalA crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is crystallography...
s.
Compound eyes
A compound eye may consist of thousands of individual photoreception units. The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the
polarizationPolarization is a property of waves that describes the orientation of their oscillations. This article primarily covers the polarization of electromagnetic waves such as light, although other types of wave also exhibit polarization....
of light. Because the individual lenses are so small, the effects of
diffractionDiffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings...
impose a limit on the possible resolution that can be obtained. This can only be countered by increasing lens size and number — to see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their head.
Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image. Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.
Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.
Apposition eyes
Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum. Some annelids and bivalves also have apposition eyes. They are also possessed by Limulus, the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point. (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.)
Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information.
The typical apposition eye has a lens focusing light from one direction on the
rhabdomRhabdoms are transparent rods, found in the center of each ommatidium in the compound eye of arthropods. These rods are constructed from the eight photoreceptor cells in the ommatidium. Each photocell is long and thin. They pack their area of the ommatidium completely filling the space...
, while light from other directions is absorbed by the dark wall of the
ommatidiumThe compound eye of insects is composed of units called ommatidia. An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea. Each ommatidium is innervated by one axon and thus...
. In the other kind of apposition eye, found in the
StrepsipteraThe Strepsiptera are an order of insects with nine families making up about 600 species...
, lenses are not fused to one another, and each forms an entire image; these images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye. Because images are combined additively, this arrangement allows vision under lower light levels.
Superposition eyes
The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used mostly by nocturnal insects. In the parabolic superposition compound eye type, seen in arthropods such as
mayfliesMayflies are insects which belong to the Order Ephemeroptera . They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies...
, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as
shrimpShrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
,
prawnPrawns are Decapods, belonging to the sub-order Dendrobranchiata . They are similar in appearance to shrimp, but can be distinguished by the gill structure which is branching in prawns , but is lamellar in shrimp...
s,
crayfishCrayfish, crawfish, or crawdads — members of the superfamilies Astacoidea and Parastacoidea — are freshwater crustaceans resembling small lobsters, to which they are related...
and
lobsterClawed lobsters compose a family of large marine crustaceans. Lobsters are economically important as seafood, forming the basis of a global industry that nets US$31.8 billion in trade annually....
s are alone in having reflecting superposition eyes, which also has a transparent gap but uses corner
mirrorA mirror is an object with at least one polished and therefore specularly reflective surface. The most familiar type of mirror is the plane mirror, which has a flat surface...
s instead of lenses.
Parabolic superposition
This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes.
Other
Good fliers like flies or honey bees, or prey-catching insects like praying mantis or
dragonfliesA dragonfly is a type of insect belonging to the order Odonata, the suborder Epiprocta or, in the strict sense, the infraorder Anisoptera. It is characterized by large multifaceted eyes, two pairs of strong transparent wings, and an elongated body...
, have specialized zones of
ommatidiaThe compound eye of insects is composed of units called ommatidia. An ommatidium contains a cluster of photoreceptor cells surrounded by support cells and pigment cells. The outer part of the ommatidium is overlaid with a transparent cornea. Each ommatidium is innervated by one axon and thus...
organized into a
foveaThe term fovea comes from the Latin, meaning pit or pitfall. As an anatomical term, there are several foveae around the body, including in the head of the femur.Fovea of the eye=...
area which gives acute vision. In the acute zone the eyes are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution.
There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp Dioptromysis paucispinosa. The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialized retina. The resulting eye is a mixture of a simple eye within a compound eye.
Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of numerous ocelli on each side of the head, organized in a way that resembles a true compound eye.
The body of
Ophiocoma wendtiiThe brittle star Ophiocoma wendtii inhabits coral reefs from Bermuda to Brazil. It is known for its advanced compound eyes.Brittle stars have long, thin arms emanating from a small, disk-shaped body and are about the size of an outstretched human hand...
, a type of
brittle starBrittle stars, or ophiuroids, are echinoderms, closely related to sea stars. They crawl across the seafloor using their flexible arms for locomotion. The ophiuroids generally have five long slender, whip-like arms which may reach up to in length on the largest specimens...
, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many
chitonChitons are small to large, primitive marine molluscs in the class Polyplacophora.There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura....
s.
Relationship to lifestyle
Eyes are generally adapted to the environment and lifestyle of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the African plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.
Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered. In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition. Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia. Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example if the animal is on a slope.
An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey. In deep water organisms, it may not be the centre of the eye that is enlarged. The hyperiid amphipods are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey — or predators — against the faint light of the sky above. Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether. Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy.
Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop. On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.
It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function.
Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head.
Acuity
Visual acuityVisual acuity is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye and the sensitivity of the interpretative faculty of the brain....
is often measured in cycles per
degreeA degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
(CPD), which measures an
angular resolutionAngular resolution or 'spatial resolution' describes the resolving power of any image-forming device such as an optical or radio telescope, a microscope, a camera, or an eye.- Definition of terms :...
, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white — black stripe cycles. For example, if each pattern is 1.75 cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white — black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye.
For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central
foveaThe term fovea comes from the Latin, meaning pit or pitfall. As an anatomical term, there are several foveae around the body, including in the head of the femur.Fovea of the eye=...
region.
Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair. A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an
optotypeAn optotype is a standardized symbol for testing vision. Optotypes can be specially shaped letters, numbers, or geometric symbols. For instance, to determine visual acuity, optotypes of different sizes are presented to a person and the smallest size is determined at which the person can reliably...
, corresponds to 20/20 (normal vision) in humans.
Color
All organisms are restricted to a small range of the electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700 nm.
This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.
The most sensitive pigment,
rhodopsinRhodopsin, also known as visual purple, is a pigment of the retina that is responsible for both the formation of the photoreceptor cells and the first events in the perception of light. Rhodopsins belong to the G-protein coupled receptor family and are extremely sensitive to light, enabling vision...
, has a peak response at 500 nm. Small changes to the genes coding for this protein can tweak the peak response by a few nm; pigments in the lens can also "filter" incoming light, changing the peak response. Many organisms are unable to discriminate between colors, seeing instead in shades of "grey"; colour vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of
cone cellCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
s, from which the more sensitive
rod cellRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
s evolved. Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioral tests can this be deduced.
Most organisms with colour vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light — this precludes the possibility of any UV light being detected, as it does not even reach the retina.
Rods and cones
The retina contains two major types of light-sensitive photoreceptor cells used for vision: the
rodsRod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of photoreceptor, cone cells. Because they are more light sensitive, rods are responsible for night vision. Named for their cylindrical shape, rods are concentrated...
and the
conesCone cells, or cones, are photoreceptor cells in the retina of the eye that function best in relatively bright light. The cone cells gradually become sparser towards the periphery of the retina....
.
Rods cannot distinguish colors, but are responsible for low-light (scotopic) monochrome (
black-and-whiteBlack-and-white is a number of monochrome forms in visual arts. Most forms of visual technology start out in black and white, then slowly evolve into color as technology progresses....
) vision; they work well in dim light as they contain a pigment, rhodopsin (visual purple), which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are none at the
foveaThe term fovea comes from the Latin, meaning pit or pitfall. As an anatomical term, there are several foveae around the body, including in the head of the femur.Fovea of the eye=...
and none at the
blind spotA blind spot, also known as a scotoma, is an obscuration of the visual field. A particular blind spot known as the blindspot, or physiological blind spot, or punctum caecum in medical literature is the place in the visual field that corresponds to the lack of light-detecting photoreceptor cells on...
. Rod density is greater in the peripheral retina than in the central retina.
Cones are responsible for
color visionColor vision is the capacity of an organism or machine to distinguish objects based on the wavelengths of the light they reflect or emit. The nervous system derives color by comparing the responses to light from the several types of cone photoreceptors in the eye. These cone photoreceptors are...
. They require brighter light to function than rods require. There are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colors). The color seen is the combined effect of
stimuliIn physiology, a stimulus is a detectable change in the internal or external environment. The ability of an organism or organ to respond to external stimuli is called sensitivity. When a stimulus is applied to a sensory receptor, it elicits or influences a reflex via stimulus transduction...
to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on this spot, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibers of the
optic nerveThe optic nerve, also called cranial nerve II, transmits visual information from the retina to the brain.-Anatomy:The optic nerve is the second of twelve paired cranial nerves but is considered to be part of the central nervous system as it is derived from an outpouching of the diencephalon during...
. When rods and cones are stimulated by light, the nerves send off impulses through these fibers to the brain.
Pigment
The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related – although problems of convergence do exist.
Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides.
The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.
There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells. The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. However, some ganglion
cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes. Likewise, c-opsins have been found to be expressed in the brain of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form. C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins.
Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group. Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.
See also
- Arthropod eye
The arthropods ancestrally possessed compound eyes, but the type and origin of this eye varies between groups, and some taxa have secondarily developed simple eyes...
- Human eye
The human eye is an organ which reacts to light for several purposes.As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
- Mammalian eye
-Dimensions:Dimensions vary only 1–2 mm among humans. The vertical diameter is 24 mm; the transverse being larger. At birth it is generally 16–17 mm, enlarging to 22.5–23 mm by three years of age. Between then and age 13 the eye attains its mature size...
- Naked eye
The naked eye is a figure of speech referring to human visual perception that is unaided by enhancing equipment, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
- Sanpaku
Sanpaku gan or Sanpaku is a Japanese term that means “three whites” and is generally referred to in English as "Sanpaku eyes".-Use in Physiognomy:...
eyes
External links