Tapetum lucidum

The tapetum lucidum (

The tapetum lucidum contributes to the superior night vision of some animals. Many of these animals are nocturnal, especially carnivores, while others are deep-sea animals. Similar adaptations occur in some species of spiders.^[2] Haplorhine primates, including humans, are diurnal and lack a tapetum lucidum.^{[Note 1]}

The presence of a tapetum lucidum enables animals to see in dimmer light than would otherwise be possible. The tapetum lucidum, which is

1. Retinal tapetum, as seen in
   fruit bats (with phospholipids).^[3]: 16 The tapetum lucidum is within the retinal pigment epithelium; in the other three types the tapetum is within the choroid
   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.^[7]
2. Choroidal guainine tapetum, as seen in cartilaginous fish.^[8] The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine.^[4]
3. 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: dogs and ferrets use zinc, cats use riboflavin and zinc, and lemurs use only riboflavin.^[3]: 17
4. Choroidal tapetum fibrosum, as seen in
   cows, sheep, goats, and horses. The tapetum is an array of extracellular fibers, most commonly collagen.^[3]
   ^: 17

The functional differences between these four structural classes of tapeta lucida are not known.[3]

This section is missing information about bird anatomy: are they all retinal? If so, they should be moved up to the 4-type list. Please expand the section to include this information. Further details may exist on the talk page. (August 2023)

This classification does not include tapeta lucida in birds. Kiwis, stone-curlews, the boat-billed heron, the flightless kākāpō, and many nightjars, owls, and other night birds such as the swallow-tailed gull also possess a tapetum lucidum.^[9] Nightjars use a retinal tapetum lucidum composed of lipids.^[10]

Like humans, some animals lack a tapetum lucidum and they usually are

Apart from its eyeshine, the tapetum lucidum itself has a color. It is often described as iridescent. In tigers, it is greenish.^[13] In ruminants, it may be golden green with a blue periphery,^[11] or whitish or pale blue with a lavender periphery. In dogs, it may be whitish with a blue periphery.^[11] The color in reindeer changes seasonally, allowing the animals to better avoid predators in low-light winter at the price of blurrier vision.^[14]

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 visible to humans (despite their inferior night vision). Eyeshine occurs in a wide variety of colors including white, blue, green, yellow, pink, and red. However, since eyeshine is a type of iridescence, the color varies with the angle at which it is seen and the minerals which make up the reflective tapetum lucidum crystals.

Although human eyes lack a tapetum lucidum, they still exhibit a weak reflection from the choroid, as can be seen in photography with the red-eye effect and with near-infrared eyeshine.^[15][16] Another effect in humans and other animals that may resemble eyeshine is leukocoria, which is a white shine indicative of abnormalities such as cataracts and cancers.

Cats and dogs with a blue

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:[17]

1. Primitive type (e.g.
   Orthognatha
   ) – a simple sheet behind the retina
2. Canoe-shape type (e.g.
   Araneidae, Theridiidae
   ) – two lateral walls separated by a gap for the nerve fibres
3. Grated type (e.g.
   Pisauridae
   ) – a relatively complex, grill-shaped structure
4. No tapetum (e.g.
   Salticidae
   )

It has been speculated that some flashlight fish may use eyeshine both to detect and to communicate with other flashlight fish.^[18] American scientist Nathan H. Lents has proposed that the tapetum lucidum evolved in vertebrates, but not in cephalopods, 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.^[19]

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.^[20] This kind of illumination causes backscatter when it interacts with the tapetum. New devices with variable illumination can make this possible, however.

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."^[21]

• Defense mechanism (biology)
• Emission theory (vision)
• Nocturnal bottleneck
• Walleye