The tapetum lucidum // (Latin: "bright tapestry", plural tapeta lucida) is a layer of tissue in the eye of many vertebrates. Lying immediately behind the retina it reflects visible light back through the retina, increasing the light available to the photoreceptors, though blurring the initial image of the light on focus. The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores that hunt their prey at night, while others are deep sea animals.
Similar adaptations occur in some species of spiders, although these are not the result of a tapetum lucidum. Most primates, including humans, lack a tapetum lucidum, and compensate for this by perceptive recognition methods.
Eyeshine is a visible effect of the tapetum lucidum. When light shines into the eye of an animal having a tapetum lucidum, the pupil appears to glow. Eyeshine can be seen in many animals, in nature and in flash photographs. In low light, a hand-held flashlight is sufficient to produce eyeshine that is highly visible to humans (despite our inferior night vision); this technique, spotlighting, is used by naturalists and hunters to search for animals at night. Eyeshine occurs in a wide variety of colors including white, blue, green, yellow, pink and red. However, since eyeshine leads to iridescence, the color varies slightly with the angle at which it is seen and the minerals which make up the reflective tapetum-lucidum crystals.
White eyeshine occurs in many fish, especially walleye; blue eyeshine occurs in many mammals such as horses; green eyeshine occurs in mammals such as cats, dogs, and raccoons; and red eyeshine occurs in coyote, rodents, opossums and birds.
Despite the tapetum lucidum being present in some primates, the human eye lacks this feature, and hence does not produce eyeshine when illuminated. However, in humans and animals, two effects can occur that may resemble eyeshine: leukocoria (white shine, indicative of abnormalities including cataracts, cancers, and other problems) and red-eye effect.
Blue-eyed cats and dogs
Cats and dogs with blue eyes (see eye color) may display both eyeshine and red-eye effect. Both species have a tapetum lucidum, so their pupils may display eyeshine. In flash color photographs, however, individuals with blue eyes may also display a distinctive red eyeshine. Individuals with heterochromia may display red eyeshine in the blue eye and normal yellow/green/blue/white eyeshine in the other eye. These include odd-eyed cats and bi-eyed dogs. The red-eye effect is independent of the eyeshine: in some photographs of individuals with a tapetum lucidum and heterochromia, the eyeshine is dim yet the pupil of the blue eye still appears red. This is most apparent when the individual is not looking into the camera because the tapetum lucidum is far less extensive than the retina.
- Retinal tapetum, as seen in teleosts, crocodiles, marsupials and fruit bats. The tapetum lucidum is within the retinal pigment epithelium; in the other 3 types the tapetum is within the choroid behind the retina.
- Choroidal guanine tapetum, as seen in elasmobranchii (skates, rays, and sharks) and chimaeras. The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine.
- Choroidal tapetum cellulosum, as seen in carnivores, rodents and cetacea. The tapetum consists of layers of cells containing organized, highly refractive crystals. These crystals are diverse in shape and makeup.
- Choroidal tapetum fibrosum, as seen in cows, sheep, goats and horses. The tapetum is an array of extracellular fibers.
The functional differences between these 4 different types of tapeta lucida are not known.
This classification does not include tapeta lucida in birds. Kiwis, stone-curlews, the boat-billed heron, the flightless kakapo and many nightjars, owls, and other night birds such as the swallow-tailed gull also possess a tapetum lucidum. This classification also does not include the extraordinary focusing mirror in the eye of the brownsnout spookfish.
Like humans, some animals lack a tapetum lucidum and they usually are diurnal. These include most primates, squirrels, some birds, red kangaroo, and pig. Among primates only the strepsirrhines, with the exception of several diurnal Eulemur species, have a tapetum lucidum.
When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head, such that in all cases the tapetum lucidum enhances night vision in the center of the animal's field of view.
Apart from its eyeshine, the tapetum lucidum itself has a color. It is often described as iridescent. In tigers it is greenish. In ruminants it may be golden green with a blue periphery, or whitish or pale blue with a lavender periphery. In dogs it may be whitish with a blue periphery.
The tapetum lucidum, which is iridescent, reflects light roughly on the interference principles of thin-film optics, as seen in other iridescent tissues. However, the tapetum lucidum cells are leucophores, not iridophores.
The tapetum lucidum is a retroreflector of the transparent sphere type. Because it is a retroreflector, it reflects light directly back along the light path. This serves to match the original and reflected light, thus maintaining the sharpness and contrast of the image on the retina. The tapetum lucidum reflects with constructive interference, thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum lowers the minimum threshold of vision 6-fold, allowing the cat to see light that is invisible to human eyes.
The tapetum lucidum enables an animal with it to see in dimmer light than the animal would otherwise be able to. This is of use to the animal, but it is of use also to humans. Human uses include scanning for reflected eyeshine to detect and identify the species of animals in the dark, and deploying trained search dogs and search horses at night. Historically, its function was regarded as simply to increase the light intensity of an image on the retina.
Using eyeshine to identify animals in the dark employs not only its color but also several other features. The color corresponds approximately to the type of tapetum lucidum, with some variation between species. Other features include the distance between pupils relative to their size; the height above ground; the manner of blinking (if any); and the movement of the eyeshine (bobbing, weaving, hopping, leaping, climbing, flying).
Manufactured retroreflectors modeled after a tapetum lucidum are described in numerous patents and today have many uses. The earliest patent, first used in "Catseye" brand raised pavement markers, was inspired by the tapetum lucidum of a cat's eye. A more recent use of retroreflectors, helping to provide secure communications between two stations in line of sight, is modeled after the combination of tapetum lucidum and bioluminescent "flashlight" in flashlight fish of the families Anomalopidae and Stomiidae (see Retroreflector). Some of these fish have been shown to use eyeshine both to detect and to communicate with other flashlight fish.
In dogs, certain drugs are known to disturb the precise organization of the crystals of the tapetum lucidum, thus compromising the dog's ability to see in low light. These drugs include ethambutol, macrolide antibiotics, dithizone, antimalarial medications, some receptor H2-antagonists, and cardiovascular agents. The disturbance "is attributed to the chelating action which removes zinc from the tapetal cells."
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