Eyes (comparative anatomy)

There are many kinds of eyes in the animal kingdom, each type with its own set of benefits for a particular species. The most basic function of the eye is to act as a light-sensitive organ, to detect the presence or absence of light. In addition to this, elaborations are often made, such as the ability to form an actual image on a retina, or the ability to control the amount of light entering the eye; each elaboration is made to provide the visual needs of individual species. The animal kingdom comprises eyes from the very rudimentary to exceptionally complex. The two basic types of eyes are the simple eye and the compound eye. The simple eye is made up of a single light-sensitive region, whereas the compound eye comprises several such elements.

Simple Eyes

The most elementary simple eye consists of a photosensitive membrane, or eye spot. Light should enter the membrane from only one direction, and so eyes have a pigmented backing to stop entry of light from the wrong side. To give the light entering a sense of directionality, the photosensitive membrane is often shaped like a cup; this basic eye type is called a pigmented cup eye. An example of an animal which possesses a simple eye of this nature is the cephalopod, Nautilus pompilius, commonly called chambered or pearly nautilus, is known as a living fossil as it is thought to exemplify the behavior and physiology of ancient organisms. N. pompilius has a pinhole eye with a pigmented backing; the pinhole aperture is a primitive method to restrict the amount of light entering the eye. This eye has no other formal optics. An elaboration of this type of eye would be to add a spherical lens; this would allow the light entering to be focused and to form an image on the photosensitive membrane. In many animals, the lens can change shape (become thinner or fatter) using surrounding musculature, to properly focus the image on the retina; this ability is called accommodation. The lens changes the angle at which light bends, and hence the focal length of the eye. The focal length of the eye is the distance from the eye to the part of the retina on which the image is focused. Other animals move the entire lens forward or backward to accommodate. One drawback that comes along with spherical lenses is that the entire image is rarely totally focused on the retina because, as a result of the spherical shape of the lens, the light rays become focused over several focal lengths (this is called spherical aberration). To correct this, many lenses have a refractive index gradient across their length; basically, the density of the lens is different across its length, which causes the light rays to bend differently and corrects for spherical aberration, allowing proper focusing of the entire image on the retina. In spiders, a different kind of optics is found; instead of a lens, these creatures focus light rays with their corneas. The structure, unlike the lens, is fixed and does not change shape to accommodate.

Compound Eyes

Compound eyes are the most abundant type of eye in the animal kingdom and can be thought of as being derived from the simple pigmented-cup type of eyes. The simplest compound eye consists of several pigmented-cup-type units, each of which samples a different angle of visual space. Found only in invertebrates, such as certain types of worms, this kind of eye gives very poor-quality images, but does give the animal a sense of the direction from which light is coming.

A more complicated version of this eye is the apposition compound eye, found in many insects and crabs, where each cup has its own optics. Each individual unit is called an ommatidium, comprising the rhabdom (containing the light-sensitive cells), directly contacting a light-focusing apparatus (either a lens or a cornea). To fit as many ommatidia into the eye as possible, the facets are hexagonal. Another elaboration is the superposition compound eye; this eye has a space between the cornea (or lens) and the rhabdom of each ommatidium. As such, this allows light from many corneal facets to converge onto each rhabdom. This increases the sensitivity as compared to the apposition eye. Several mechanisms are used to bend the light from each ommatidium; the simplest is the reflecting superposition eye, found in shrimp, which uses a series of mirrors along the edge of each facet to reflect the light onto the rhabdoms.

Photoreceptors

The photoreceptive element common to all eyes differs from species to species, from the cup-shaped retinas of simple eyes to the complex rhabdom structures of compound eyes. In simple eyes, the light-sensitive element is platelike, with projections containing flat layers or discs of membranes. In vertebrates, the two photoreceptor types are the cones (cone-shaped projections), which have layers of photosensitive membrane, and the rods (rod-shaped projections), which contain free-floating discs. Compound eyes have rhabdomeric microvillar photoreceptors; the microvilli are finger-like structures that project from the rhabdomeres and are light-sensitive. Within each of these structures lies the actual light-sensitive pigment, a membrane-bound protein known as opsin. In the rhabdoms, the orientation of the opsin molecule is parallel to the axis of the microvilli; this fact aids in the perception of polarized light. There are many types of opsin protein, and they can be categorized according to which wavelength of the light spectrum they preferentially absorb. The opsin protein interacts with a Vitamin A-derived molecule called the chromophore. Its chemical name is 11-cis retinal. The absorption of a photon of light changes the chemical interaction between the opsin and the chromophore, and it is this chemical change which initiates the cascade of events that leads to a nerve signal, known as phototransduction.

The actual biochemistry of phototransduction is very complex but involves the amplification of the signal of reception of light, and its transformation into a nerve impulse. The passage of the information through the nervous system is also very complicated and details are different in different species, but in general, the receptors converge onto axons and the nerve impulse travels through the optic nerve for higher processing. Of the two types of photoreceptors existing in vertebrates, rods are very light-sensitive and are used in situations where light is limited (scotopic conditions). Cones are less sensitive and are used in situations where light is abundant (photopic conditions). The vertebrate retina almost always possesses both rods and cones, although some nocturnal animals or animals living in an environment where light is scarce, for example deep-dwelling fishes, have all-rod retinas. Mammals have rod-dominated retinas, which are probably remnants of the time millions of years ago when mammals were nocturnal. There are several kinds of cones, which are characterized according to which opsins they carry within the membrane layers; this dictates which wavelengths of light the cone preferentially absorbs. Absorption of light by combinations of at least two different cone types will allow a species color vision, given the cognitive ability to process such information.

Another important feature of eyes is their position on the head relative to each other. Most vertebrates and some invertebrates have two eyes, allowing them a certain amount of binocular vision depending on the angle between them. In fact, both eyes project a slightly different image onto each retina, but the brain can compute this as a single image. This results in a larger field of view and a sense of depth. These are definite advantages, for example for a hunting animal, which needs to know how far it is from the prey target.

Research concerning the eyes of the kingdom continues to emerge. For example, scientists at the University of Arkansas investigated the range of colors that animals can see. They concluded that, in general, animals that live on the land have a greater color range than those that live in water. Additionally, those living in open plains have the best color-range vision. To help individuals understand how animals see the world, researchers at the University of Sussex and George Mason University created a software and hardware camera system that records in four color channels, including blue, green, red, and UV. The device is over 90 percent accurate and allows ecologists, filmmakers, and other interested individuals the opportunity to understand animals' vision.

Principal Terms

Binocular Vision: The ability to utilize image information from both eyes to form a single image with depth information

Chromophore: The molecule which interacts with opsin; absorption of light changes the interaction and starts the phototransduction cascade

Ommatidium: Individual unit of the multifaceted compound eye

Opsin: A membrane-bound protein, or pigment, which absorbs light

Optic Nerve: The main nerve taking information from the eyes to higher processing areas

Photoreceptor: Cell containing membranes which house light-sensitive pigments

Retina: The light-sensitive membrane at the back of the eye

Bibliography

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