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The ; ; : tapeta lucida) is a layer of tissue in the
eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...
of many
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
s and some other animals. Lying immediately behind the
retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...
, it is a
retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light or other radiation back to its source with minimum scattering. This works at a wide range of angle of incidence (optics), angle of incidenc ...
. It reflects visible
light Light, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
back through the retina, increasing the light available to the photoreceptors (although slightly blurring the image). The tapetum lucidum contributes to the superior
night vision Night vision is the ability to see in low-light conditions, either naturally with scotopic vision or through a night-vision device. Night vision requires both sufficient spectral range and sufficient intensity range. Humans have poor night v ...
of some animals. Many of these animals are
nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...
, especially
carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...
s, while others are deep-sea animals. Similar adaptations occur in some species of
spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and ran ...
s. Haplorhine primates, including humans, are diurnal and lack a tapetum lucidum.


Function and mechanism

The presence of a tapetum lucidum enables animals to see in dimmer light than would otherwise be possible. The tapetum lucidum, which is
iridescent Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstruc ...
, reflects light roughly on the
interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extra ...
principles of thin-film optics, as seen in other iridescent tissues. However, the tapetum lucidum cells are leucophores, not iridophores. The tapetum functions as a
retroreflector A retroreflector (sometimes called a retroflector or cataphote) is a device or surface that reflects light or other radiation back to its source with minimum scattering. This works at a wide range of angle of incidence (optics), angle of incidenc ...
which 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 In physics, interference is a phenomenon in which two coherence (physics), coherent waves are combined by adding their intensities or displacements with due consideration for their phase (waves), phase difference. The resultant wave may have ...
, thus increasing the quantity of light passing through the retina. In the cat, the tapetum lucidum increases the sensitivity of vision by 44%, allowing the cat to see light that is imperceptible to human eyes. When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head. Apart from its eyeshine, the tapetum lucidum itself has a color. It is often described as iridescent. In
tiger The tiger (''Panthera tigris'') is a large Felidae, cat and a member of the genus ''Panthera'' native to Asia. It has a powerful, muscular body with a large head and paws, a long tail and orange fur with black, mostly vertical stripes. It is ...
s, it is greenish. In
ruminant Ruminants are herbivorous grazing or browsing artiodactyls belonging to the suborder Ruminantia that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microb ...
s, 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 color in
reindeer The reindeer or caribou (''Rangifer tarandus'') is a species of deer with circumpolar distribution, native to Arctic, subarctic, tundra, taiga, boreal, and mountainous regions of Northern Europe, Siberia, and North America. It is the only re ...
changes seasonally, allowing the animals to better avoid predators in low-light winter at the price of blurrier vision.


Classification

A
classification Classification is the activity of assigning objects to some pre-existing classes or categories. This is distinct from the task of establishing the classes themselves (for example through cluster analysis). Examples include diagnostic tests, identif ...
of anatomical variants of tapeta lucida defines four types: # ''Retinal tapetum'', as seen in
teleost Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...
s (with a variety of reflecting materials from
lipids Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins Vitamin A, A, Vitamin D, D, Vitamin E, E and Vitamin K, K), monoglycerides, diglycerides, phospholipids, and others. The fu ...
to
phenols In organic chemistry, phenols, sometimes called phenolics, are a class of chemical compounds consisting of one or more hydroxyl groups (− O H) bonded directly to an aromatic hydrocarbon group. The simplest is phenol, . Phenolic compounds ar ...
),
crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include ...
s (with
guanine Guanine () (symbol G or Gua) is one of the four main nucleotide bases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside ...
),
marsupial Marsupials are a diverse group of mammals belonging to the infraclass Marsupialia. They are natively found in Australasia, Wallacea, and the Americas. One of marsupials' unique features is their reproductive strategy: the young are born in a r ...
s (with lipid spheres), and fruit bats (with
phospholipids Phospholipids are a class of lipids whose molecule has a hydrophilic "head" containing a phosphate group and two hydrophobic "tails" derived from fatty acids, joined by an alcohol residue (usually a glycerol molecule). Marine phospholipids typi ...
). The tapetum lucidum is within the
retinal pigment epithelium The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigment A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), ...
; in the other three types the tapetum is within the
choroid The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye. It contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear o ...
behind the retina. Two anatomical classes can be distinguished: occlusible and non-occlusible. #* The brownsnout spookfish has an extraordinary focusing mirror derived from a retinal tapetum. # ''
Choroid The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye. It contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear o ...
al guainine tapetum'', as seen in
cartilaginous fish Chondrichthyes (; ) is a class of jawed fish that contains the cartilaginous fish or chondrichthyans, which all have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fish'', which have skeleto ...
. The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of
guanine Guanine () (symbol G or Gua) is one of the four main nucleotide bases found in the nucleic acids DNA and RNA, the others being adenine, cytosine, and thymine ( uracil in RNA). In DNA, guanine is paired with cytosine. The guanine nucleoside ...
. # ''Choroidal tapetum cellulosum'', as seen in
carnivore A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they ar ...
s,
rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia ( ), which are characterized by a single pair of continuously growing incisors in each of the upper and Mandible, lower jaws. About 40% of all mammal specie ...
s, and
cetacea Cetacea (; , ) is an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively c ...
. The tapetum consists of layers of cells containing organized, highly refractive crystals. These crystals are diverse in shape and makeup: dogs and ferrets use
zinc Zinc is a chemical element; it has symbol Zn and atomic number 30. It is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic tabl ...
, cats use
riboflavin Riboflavin, also known as vitamin B2, is a vitamin found in food and sold as a dietary supplement. It is essential to the formation of two major coenzymes, flavin mononucleotide and flavin adenine dinucleotide. These coenzymes are involved in ...
and zinc, and lemurs use only riboflavin. # ''Choroidal tapetum fibrosum'', as seen in cows,
sheep Sheep (: sheep) or domestic sheep (''Ovis aries'') are a domesticated, ruminant mammal typically kept as livestock. Although the term ''sheep'' can apply to other species in the genus '' Ovis'', in everyday usage it almost always refers to d ...
,
goat The goat or domestic goat (''Capra hircus'') is a species of Caprinae, goat-antelope that is mostly kept as livestock. It was domesticated from the wild goat (''C. aegagrus'') of Southwest Asia and Eastern Europe. The goat is a member of the ...
s, and
horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...
s. The tapetum is an array of extracellular fibers, most commonly
collagen Collagen () is the main structural protein in the extracellular matrix of the connective tissues of many animals. It is the most abundant protein in mammals, making up 25% to 35% of protein content. Amino acids are bound together to form a trip ...
. The functional differences between these four structural classes of tapeta lucida are not known.


Variation across species


Primates

Humans like haplorhine primates lack a tapetum lucidum as they are diurnal. Strepsirrhine primates are mostly nocturnal and, with the exception of several diurnal '' Eulemur'' species, have a tapetum lucidum of riboflavin crystals.


Dogs

In canids, the tapetum lucidum is found in the dorsal half of the eye's fundus. It consists of 9-20 layers of specialized rectangular cells between the choroid and retinal pigment epithelium, thinning towards the periphery. The cells contain zinc-rich rodlets arranged in parallel. The structure appears yellow-green in adults, though blue in puppies until four months of age. Zinc concentration varies among species, with red foxes showing highest levels, followed by Arctic foxes, then domestic dogs. A hereditary zinc-deficiency condition in some beagles results in degenerated tapetal cells with disrupted rodlet arrangement.


Cats

The tapetum lucidum in cats is renowned for its brilliance, even inspiring ancient Egyptians to believe it reflected the sun at night. This reflective layer is composed of 15-20 layers of cells arranged in a central pattern. This structure, denser than that of dogs, results in high reflectance, nearly 130 times that of humans. Its color is heterogeneous, varying with age and species due to factors like rodlet spacing, refractive index, and light interactions. Young cats exhibit a blue appearance, which shifts to yellow with age, with adult coloration ranging from light orange to green. While enhancing night vision, increased light scatter within the tapetum slightly compromises visual acuity.


Birds

Kiwis, stone-curlews, the boat-billed heron, the flightless
kākāpō The kākāpō (; : ; ''Strigops habroptilus''), sometimes known as the owl parrot or owl-faced parrot, is a species of large, nocturnal, ground-dwelling parrot of the superfamily Strigopoidea. It is endemic to New Zealand. Kākāpō can be u ...
, and many
nightjar Nightjars are medium-sized nocturnal or crepuscular birds in the family Caprimulgidae and order Caprimulgiformes, characterised by long wings, short legs, and very short bills. They are sometimes called bugeaters, their primary source of food ...
s, owls, and other night birds such as the swallow-tailed gull possess a tapetum lucidum. Nightjars use a retinal tapetum lucidum composed of lipids.


In spiders

Most species of spider also have a tapetum, which is located only in their smaller, lateral eyes; the larger central eyes have no such structure. This consists of reflective crystalline deposits and is thought to have a similar function to the structure of the same name in vertebrates. Four general patterns can be distinguished in spiders: # Primitive type (e.g.
Mesothelae The Mesothelae are a suborder of spiders (order Araneae). , two extant families were accepted by the World Spider Catalog, Liphistiidae and Heptathelidae. Alternatively, the Heptathelidae can be treated as a subfamily of a more broadly circum ...
, Orthognatha)a simple sheet behind the retina # Canoe-shape type (e.g. Araneidae,
Theridiidae Theridiidae, also known as the tangle-web spiders, cobweb spiders and comb-footed spiders, is a large family of Araneomorphae, araneomorph spiders first described by Carl Jakob Sundevall in 1833. This diverse, globally distributed family includes ...
)two lateral walls separated by a gap for the nerve fibres # Grated type (e.g.
Lycosidae Wolf spiders are members of the Family (biology), family Lycosidae (), named for their robust and agile hunting skills and excellent eyesight. They live mostly in solitude, hunt alone, and usually do not spin spider web, webs. Some are opportunis ...
, Pisauridae)a relatively complex, grill-shaped structure # No tapetum (e.g.
Salticidae Jumping spiders are a group of spiders that constitute the family (biology), family Salticidae. , this family contained over 600 species description, described genus, genera and over 6,000 described species, making it the largest family of spide ...
)


Animals lacking a tapetum lucidum

Animals without tapetum lucidum include haplorhine primates, squirrels, some birds, red kangaroo, and pigs.Orlando Charnock Bradley, 1896, ''Outlines of Veterinary Anatomy. Part I. The Anterior and Posterior Limbs,'' Baillière, Tindall & Cox, p. 224
Free full text on Google Books
/ref>


Eyeshine

Eyeshine is a visible effect of the tapetum lucidum. When light shines into the eye of an animal having a tapetum lucidum, the
pupil The pupil is a hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black becau ...
appears to glow. Eyeshine can be seen in many animals, in nature, and in flash
photograph A photograph (also known as a photo, or more generically referred to as an ''image'' or ''picture'') is an image created by light falling on a photosensitivity, photosensitive surface, usually photographic film or an electronic image sensor. Th ...
s. In low light, a hand-held flashlight is sufficient to produce eyeshine that is visible to humans (despite their inferior
night vision Night vision is the ability to see in low-light conditions, either naturally with scotopic vision or through a night-vision device. Night vision requires both sufficient spectral range and sufficient intensity range. Humans have poor night v ...
). Eyeshine occurs in a wide variety of
color Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...
s including
white White is the lightest color and is achromatic (having no chroma). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully (or almost fully) reflect and scatter all the visible wa ...
,
blue Blue is one of the three primary colours in the RYB color model, RYB colour model (traditional colour theory), as well as in the RGB color model, RGB (additive) colour model. It lies between Violet (color), violet and cyan on the optical spe ...
,
green Green is the color between cyan and yellow on the visible spectrum. It is evoked by light which has a dominant wavelength of roughly 495570 nm. In subtractive color systems, used in painting and color printing, it is created by a com ...
,
yellow Yellow is the color between green and orange on the spectrum of light. It is evoked by light with a dominant wavelength of roughly 575585 nm. It is a primary color in subtractive color systems, used in painting or color printing. In t ...
,
pink Pink is a pale tint of red, the color of the Dianthus plumarius, pink flower. It was first used as a color name in the late 17th century. According to surveys in Europe and the United States, pink is the color most often associated with charm, p ...
, and red. However, since eyeshine is a type of
iridescence Iridescence (also known as goniochromism) is the phenomenon of certain surfaces that appear gradually to change colour as the angle of view or the angle of illumination changes. Iridescence is caused by wave interference of light in microstru ...
, the color varies with the angle at which it is seen and the minerals which make up the reflective tapetum lucidum crystals. Individuals with heterochromia may display red eyeshine in the blue eye and other-colored eyeshine in the other eye. These include odd-eyed cats and bi-eyed dogs. Although human eyes lack a tapetum lucidum, they still exhibit a weak reflection from the
choroid The choroid, also known as the choroidea or choroid coat, is a part of the uvea, the vascular layer of the eye. It contains connective tissues, and lies between the retina and the sclera. The human choroid is thickest at the far extreme rear o ...
, as can be seen in photography with the
red-eye effect The red-eye effect in photography is the common appearance of red pupils in color photographs of eyes. It occurs when using a photographic flash at low lighting or at night. When a flash passes through the eyes and rebounds at the back of the ...
and with
near-infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
eyeshine. Another effect in humans and other animals that may resemble eyeshine is leukocoria, which is a white shine indicative of abnormalities such as
cataract A cataract is a cloudy area in the lens (anatomy), lens of the eye that leads to a visual impairment, decrease in vision of the eye. Cataracts often develop slowly and can affect one or both eyes. Symptoms may include faded colours, blurry or ...
s and
cancer Cancer is a group of diseases involving Cell growth#Disorders, abnormal cell growth with the potential to Invasion (cancer), invade or Metastasis, spread to other parts of the body. These contrast with benign tumors, which do not spread. Po ...
s.


Usage

Humans can scan for eyeshine to detect and identify the species of animals in the dark and deploy search dogs and search horses at night. The color corresponds approximately to the type of tapetum lucidum, with some variation between species. It has been speculated that some flashlight fish may use eyeshine both to detect and to communicate with other flashlight fish. American scientist Nathan H. Lents has proposed that the tapetum lucidum evolved in vertebrates, but not in
cephalopods A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...
, which have a very similar eye because of the backwards-facing nature of vertebrate photoreceptors. The tapetum boosts photosensitivity under conditions of low illumination, thus compensating for the suboptimal design of the vertebrate retina.


In photography

Traditionally, it has been difficult to take retinal images of animals with a tapetum lucidum because ophthalmoscopy devices designed for humans rely on a high level of on-axis illumination. This kind of illumination causes backscatter when it interacts with the tapetum. New devices with variable illumination can make this possible, however.


Pathology

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 Ethambutol (EMB, E) is a medication primarily used to treat tuberculosis. It is usually given in combination with other tuberculosis medications, such as isoniazid, rifampicin and pyrazinamide. It may also be used to treat ''Mycobacterium avi ...
,
macrolide Macrolides are a class of mostly natural products with a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. Macrolides belong to the polyketide class of natural products. ...
antibiotics, dithizone,
antimalarial medication Antimalarial medications or simply antimalarials are a type of antiparasitic chemical agent, often natural product, naturally derived, that can be used to treat or to prevent malaria, in the latter case, most often aiming at two susceptible target ...
s, some receptor H2-antagonists, and cardiovascular agents. The disturbance "is attributed to the chelating action which removes zinc from the ''tapetal'' cells."


See also

*
Defense mechanism (biology) Anti-predator adaptations are mechanisms developed through evolution that assist Predation, prey organisms in their constant struggle against predators. Throughout the animal kingdom, adaptations have evolved for every stage of this struggle, na ...
*
Emission theory (vision) Emission theory or extramission theory (variants: extromission) or extromissionism is the proposal that visual perception is accomplished by eye beams emitted by the eyes. This theory has been replaced by '' intromission theory'' (or ''intromis ...
*
Nocturnal bottleneck The nocturnal bottleneck hypothesis is an evolutionary biology hypothesis to explain the origin of several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively ...
*
Walleye The walleye (''Sander vitreus'', Synonym (taxonomy), synonym ''Stizostedion vitreum''), also called the walleyed pike, yellow pike, yellow pikeperch or yellow pickerel, is a freshwater perciform fish native to most of Canada and to the Northern ...


Notes


References


External links

* {{Vision in animals Vertebrate anatomy Animal communication Science of photography Eye anatomy