1. VISUAL PIGMENTS
25
by passage through a long column of clear oceanic water (Denton and
Warren, 1957; Wald et al., 1957; Munz, 1958a). The generally blue
luminescence of deep-sea animals should also favor selection of bluesensitive visual pigments ( Munz, 1958a). There is no obvious correlation
between the phylogenetic relationships of fishes and the maxima of their
visual pigments. It was tempting, therefore, to generalize from the deepsea fishes and seek ecological correlations instead (Munz, 1958b, 1964,
1965). The predominant colors of sunlight are certainly not the same, as
filtered by different seawater types such as oceanic, coastal, and inshore.
But the attempt to associate visual pigment maxima with these photic
environments has broken down as additional species have been sampled
( Dartnall and Lythgoe, 1965).
His own experience as a diver led Lythgoe (1966, 1968) to emphasize
that visual contrast, rather than sensitivity, may be the main selective
agent. Water acts as a color filter, progressively narrowing the spectrum
of transmitted light as well as decreasing its intensity. Light reflected
from an object, such as the silvery side of another fish, travels a shorter
distance underwater than light scattered back from the water behind it.
Background illumination is therefore more monochromatic than light
reflected from the object. Its maximum is further from the spectral region
dominant in sunlight at the water surface and closer to the wavelength
of maximum transmission by the water. This means that the greatest
visual contrast between such an object and the background occurs if the
visual pigment has its A, ,
at a wavelength removed from the transmission maximum; the object is therefore seen to be brighter than the background. Should the difference be too great, of course, visual sensitivity
would be drastically reduced. Lythgoe has shown that the visual pigments of marine fishes from several different photic environments appear
to fit the requirements for a balance of visual contrast and relatively high
sensitivity. He also pointed out that understanding of these problems is
hampered by our lack of definite knowledge of the origin of the various
visual pigments in rods or cones and whether the pigments occur in separate receptors or are mixed indiscriminately. In summary, current thinking suggests that the many different visual pigments of fishes probably
have been selected for their adaptive advantage in different photic
environments.
E. Pigments of Color Vision
Of great interest are the mechanisms responsible for color vision, both
in ourselves and in other animals, such as teleosts, that can discriminate
25
by passage through a long column of clear oceanic water (Denton and
Warren, 1957; Wald et al., 1957; Munz, 1958a). The generally blue
luminescence of deep-sea animals should also favor selection of bluesensitive visual pigments ( Munz, 1958a). There is no obvious correlation
between the phylogenetic relationships of fishes and the maxima of their
visual pigments. It was tempting, therefore, to generalize from the deepsea fishes and seek ecological correlations instead (Munz, 1958b, 1964,
1965). The predominant colors of sunlight are certainly not the same, as
filtered by different seawater types such as oceanic, coastal, and inshore.
But the attempt to associate visual pigment maxima with these photic
environments has broken down as additional species have been sampled
( Dartnall and Lythgoe, 1965).
His own experience as a diver led Lythgoe (1966, 1968) to emphasize
that visual contrast, rather than sensitivity, may be the main selective
agent. Water acts as a color filter, progressively narrowing the spectrum
of transmitted light as well as decreasing its intensity. Light reflected
from an object, such as the silvery side of another fish, travels a shorter
distance underwater than light scattered back from the water behind it.
Background illumination is therefore more monochromatic than light
reflected from the object. Its maximum is further from the spectral region
dominant in sunlight at the water surface and closer to the wavelength
of maximum transmission by the water. This means that the greatest
visual contrast between such an object and the background occurs if the
visual pigment has its A, ,
at a wavelength removed from the transmission maximum; the object is therefore seen to be brighter than the background. Should the difference be too great, of course, visual sensitivity
would be drastically reduced. Lythgoe has shown that the visual pigments of marine fishes from several different photic environments appear
to fit the requirements for a balance of visual contrast and relatively high
sensitivity. He also pointed out that understanding of these problems is
hampered by our lack of definite knowledge of the origin of the various
visual pigments in rods or cones and whether the pigments occur in separate receptors or are mixed indiscriminately. In summary, current thinking suggests that the many different visual pigments of fishes probably
have been selected for their adaptive advantage in different photic
environments.
E. Pigments of Color Vision
Of great interest are the mechanisms responsible for color vision, both
in ourselves and in other animals, such as teleosts, that can discriminate