1. VISUAL PIGMENTS
11
tation shortens, but the time required for dark adaptation tends to increase (Ali, 1959). Light adaptation is more rapid and complete when
Salmo is exposed to high intensities; after a preliminary exposure to bright
light, dark adaptation takes longer ( Ali, 1962). Albino Salvelinus, which
lack retinal melanin, undergo normal photomechanical migrations of rod
and cone outer segments ( Ali, 1964b). Three flatfishes ( Microstomus,
Pleuronectes, and Solea) that have immobile pupils have normal photomechanical movement ( Nicol, 1961b, 1965a) ; flatfishes with mobile pupils
have not been investigated. The retina of Carassius shows a persistent
circadian rhythm in its photomechanical changes. The cones of fish kept
in constant darkness for 3 days continued to shift positions in synchrony
with the die1 cycle (John et al., 1967). Goldfish were restrained and
anesthetized by Ali (1964a), and then one eye was exposed to a bright
light. The rods and cones of the dark-adapted eye did not move, but the
retinal melanin expanded partially. Ali suggested that pigment migration
may be influenced by hormones. Walls (1942) commented on the confusion surrounding the mechanisms controlling photomechanical movements; the situation is still far from clear.
A mirror or tapetum lucidum at the back of the eye is a common device to increase the visual sensitivity of nocturnal animals. Although
several morphologically distinct types were described by Walls ( 1942),
only three are common in fishes. A retinal tapetum occurs in the pigment
epithelium of many freshwater fishes ( cyprinids and percids ) . The
epithelial cells contain particles or crystals of the reflective substance
guanine. Melanin is present in the same cells and migrates normally,
occluding the tapetum in bright light. A nonocclusible tapetum of the
same type was said to occur in pelagic deep-sea teleosts (Walls, 1942).
This statement was evidently based on the investigation of Brauer ( 1908),
but Munk (1966) has not found retinal or choroidal tapeta in any deepsea teleost. Some surface-dwelling marine teleosts have a fibrous,
choroidal tapetum which is shiny, like a tendon; this type is not
occlusible.
Almost all elasmobranchs have a choroidal tapetum lucidum just external to the choriocapillary layer; Myliobatis, a pelagic ray, seems to be
the only exception (Denton and Nicol, 1964). The tapetum consists of
flattened, often imbricate cells that contain guanine (Denton and Nicol,
1965; Best and Nicol, 1967). The reflecting plates are typically oriented
perpendicularly to the light incident at each region of the retina. Reflection from these plates is highly directional (specular), which should
minimize blurring of the image (Denton and Nicol, 1964, 1965). In
benthic, neritic species (e.g., Scyliorhinus) the fundus has a black ventral
area; a tapetum occurs elsewhere and is not occlusible (Nicol, 1961a,
11
tation shortens, but the time required for dark adaptation tends to increase (Ali, 1959). Light adaptation is more rapid and complete when
Salmo is exposed to high intensities; after a preliminary exposure to bright
light, dark adaptation takes longer ( Ali, 1962). Albino Salvelinus, which
lack retinal melanin, undergo normal photomechanical migrations of rod
and cone outer segments ( Ali, 1964b). Three flatfishes ( Microstomus,
Pleuronectes, and Solea) that have immobile pupils have normal photomechanical movement ( Nicol, 1961b, 1965a) ; flatfishes with mobile pupils
have not been investigated. The retina of Carassius shows a persistent
circadian rhythm in its photomechanical changes. The cones of fish kept
in constant darkness for 3 days continued to shift positions in synchrony
with the die1 cycle (John et al., 1967). Goldfish were restrained and
anesthetized by Ali (1964a), and then one eye was exposed to a bright
light. The rods and cones of the dark-adapted eye did not move, but the
retinal melanin expanded partially. Ali suggested that pigment migration
may be influenced by hormones. Walls (1942) commented on the confusion surrounding the mechanisms controlling photomechanical movements; the situation is still far from clear.
A mirror or tapetum lucidum at the back of the eye is a common device to increase the visual sensitivity of nocturnal animals. Although
several morphologically distinct types were described by Walls ( 1942),
only three are common in fishes. A retinal tapetum occurs in the pigment
epithelium of many freshwater fishes ( cyprinids and percids ) . The
epithelial cells contain particles or crystals of the reflective substance
guanine. Melanin is present in the same cells and migrates normally,
occluding the tapetum in bright light. A nonocclusible tapetum of the
same type was said to occur in pelagic deep-sea teleosts (Walls, 1942).
This statement was evidently based on the investigation of Brauer ( 1908),
but Munk (1966) has not found retinal or choroidal tapeta in any deepsea teleost. Some surface-dwelling marine teleosts have a fibrous,
choroidal tapetum which is shiny, like a tendon; this type is not
occlusible.
Almost all elasmobranchs have a choroidal tapetum lucidum just external to the choriocapillary layer; Myliobatis, a pelagic ray, seems to be
the only exception (Denton and Nicol, 1964). The tapetum consists of
flattened, often imbricate cells that contain guanine (Denton and Nicol,
1965; Best and Nicol, 1967). The reflecting plates are typically oriented
perpendicularly to the light incident at each region of the retina. Reflection from these plates is highly directional (specular), which should
minimize blurring of the image (Denton and Nicol, 1964, 1965). In
benthic, neritic species (e.g., Scyliorhinus) the fundus has a black ventral
area; a tapetum occurs elsewhere and is not occlusible (Nicol, 1961a,