Classification

Classification

A classification of anatomical variants of tapeta lucida^[2] defines 4 types:

• Retinal tapetum, as seen in teleosts, crocodiles, marsupials and fruit bats. The tapetum lucidum is within the retina; 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). The tapetum is a palisade of cells containing stacks of flat hexagonal crystals of guanine.^[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.
• 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.^[2]

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^[4] This classification also does not include the extraordinary focusing mirror in the eye of the brownsnout spookfish.^[5]

Aye-aye

Sportive lemur

Like humans, some animals lack a tapetum lucidum and they usually are diurnal.^[2] These include most primates, squirrels, some birds, red kangaroo, and pig.^[6] Primates that have a tapetum lucidum include the aye aye and sportive lemur.

When a tapetum lucidum is present, its location on the eyeball varies with the placement of the eyeball in the head,^[7] 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 luminum itself has a color. It is often described as iridescent. In tigers it is greenish.^[8] In ruminants it may be golden green with a blue periphery,^[6] or whitish or pale blue with a lavender periphery. In dogs it may be whitish with a blue periphery.^[6]