F. W. MUNZ
22
of seasonal succession of retinenel and retinene, pigments ( Bridges,
1965b; Beatty, 1969~). Pacific salmon, Oncorhynclzus, undergo somewhat similar changes in their visual pigments during the life cycle
(Beatty, 1966). The retinene balance of juvenile salmon is partly controlled by light but is not altered as easily as in the cyprinids. Salinity
has no effect on the visual pigments at this stage. Young salmon, even in
winter, always have a considerable proportion of rhodopsin. Salmon
caught on the high seas (in winter) had rhodopsin alone. Associated
with the spawning migration is a nearly complete conversion to the
retinene, system. This can start in the ocean, but may be hastened by
entry into freshwater. Beatty suggested that sexual maturation may
accelerate this process. The sockeye salmon, Oncorhynchus nerka, is best
adapted to freshwater for it can complete its life cycle without entering
the sea ( landlocked form called “kokanee”). Contrary to expectations,
Beatty found that this species, and especially the landlocked form, never
has much porphyropsin. The predicted effects of salinity have not actually
been demonstrated on the visual pigments of any fishes but probably
could be if the right species were used. Further experiments are needed
to unravel the nature of environmental, dietary, and hormonal factors
that control the proportions of retinene, and retinene, pigments. A clue
in this direction was provided by the demonstration that a centrarchid,
Lepomis, converts vitamin A, to retinene, in the eye and that thyroxine
inhibits the conversion (Naito and Wilt, 1962). Thyroxine has the opposite effect of increasing the proportion of retinene, pigment in the eyes
of salmonids; its mechanism of action is not known (Munz and Swanson, 1965; Beatty, 1969b). In a frog, Rana, the immediate precursor of
retinene? appears to be retinene,, rather than vitamin A, (Ohtsu et al.,
1964). Whether vitamin A, and A, can ever be interconverted directly
is not known, but there is sometimes little relation between the forms of
vitamin A in the liver and retinene in the eye (e.g., Munz, 1965; Beatty,
1966).
The biological significance of retinene, pigments may possibly be related to their frequent occurrence in mixtures with retinene, pigments
(Munz, 1965). Proportions of the two pigments can be altered within
individual fish ( e.g., Scardinius) in response to environmental light levels.
An outstanding feature of freshwater photic environments is their instability, both seasonally and on a geological time scale. An adaptable
visual system, therefore, may have selective advantage for some freshwater fishes. Although it is difficult to describe the light in so variable
an environment, it is probably richer in long wavelengths (redder) than
light in the sea. An increased sensitivity to red light (which would result
from a visual system based on retinene,) may be advantageous to many
22
of seasonal succession of retinenel and retinene, pigments ( Bridges,
1965b; Beatty, 1969~). Pacific salmon, Oncorhynclzus, undergo somewhat similar changes in their visual pigments during the life cycle
(Beatty, 1966). The retinene balance of juvenile salmon is partly controlled by light but is not altered as easily as in the cyprinids. Salinity
has no effect on the visual pigments at this stage. Young salmon, even in
winter, always have a considerable proportion of rhodopsin. Salmon
caught on the high seas (in winter) had rhodopsin alone. Associated
with the spawning migration is a nearly complete conversion to the
retinene, system. This can start in the ocean, but may be hastened by
entry into freshwater. Beatty suggested that sexual maturation may
accelerate this process. The sockeye salmon, Oncorhynchus nerka, is best
adapted to freshwater for it can complete its life cycle without entering
the sea ( landlocked form called “kokanee”). Contrary to expectations,
Beatty found that this species, and especially the landlocked form, never
has much porphyropsin. The predicted effects of salinity have not actually
been demonstrated on the visual pigments of any fishes but probably
could be if the right species were used. Further experiments are needed
to unravel the nature of environmental, dietary, and hormonal factors
that control the proportions of retinene, and retinene, pigments. A clue
in this direction was provided by the demonstration that a centrarchid,
Lepomis, converts vitamin A, to retinene, in the eye and that thyroxine
inhibits the conversion (Naito and Wilt, 1962). Thyroxine has the opposite effect of increasing the proportion of retinene, pigment in the eyes
of salmonids; its mechanism of action is not known (Munz and Swanson, 1965; Beatty, 1969b). In a frog, Rana, the immediate precursor of
retinene? appears to be retinene,, rather than vitamin A, (Ohtsu et al.,
1964). Whether vitamin A, and A, can ever be interconverted directly
is not known, but there is sometimes little relation between the forms of
vitamin A in the liver and retinene in the eye (e.g., Munz, 1965; Beatty,
1966).
The biological significance of retinene, pigments may possibly be related to their frequent occurrence in mixtures with retinene, pigments
(Munz, 1965). Proportions of the two pigments can be altered within
individual fish ( e.g., Scardinius) in response to environmental light levels.
An outstanding feature of freshwater photic environments is their instability, both seasonally and on a geological time scale. An adaptable
visual system, therefore, may have selective advantage for some freshwater fishes. Although it is difficult to describe the light in so variable
an environment, it is probably richer in long wavelengths (redder) than
light in the sea. An increased sensitivity to red light (which would result
from a visual system based on retinene,) may be advantageous to many