1. VISUAL PIGMENTS
23
freshwater fishes, but the arguments and uncertainties brought out by
Lythgoe ( 1966) could also be applied here (see Section 11, D ) . Schwanzara (1967) found a tendency of surface-feeding freshwater fishes to have
retinene, pigments, while bottom dwellers have retinenez pigments. This
is consistent with the filtering undergone by sunlight as it penetrates to
greater depths. She found that retinene, pigments are more common in
tropical fishes than in Temperate Zone species, and the converse; this
was also evident within a single primary freshwater family, the
Cyprinidae. These trends may be related to some general difference in
spectral quality of light in tropical and temperate freshwaters, but comparative data are lacking. At the least, speculation about the possible
visual significance of retinenez should provoke experiments that may give
insight into this problem in biochemical evolution.
D. A Multiplicity of Opsins
The visual pigments of fishes are more conspicuously diverse than
those of all other vertebrates combined (Dartnall and Lythgoe, 1965).
These differences are referable to a series of species-specific opsins, as well
as to the occurrence of retinenee in some fishes. A histogram summarizes
the published results (Fig. 9 ) . Only the most abundant visual pigment
of each species is presented, except in cases where retinenel-retinene2
pairs have been described. Presumably, these are visual pigments of the
rods. The source of less abundant pigments may be either rods or cones
but is unknown in most cases. The visual pigments of more than 180
species have been subjected to adequate spectrometric analysis, and the
list is growing so rapidly that any table would be obsolete before its
publication. The figure should be regarded as a progress report, therefore,
and not as the conclusion of a completed survey. Availability affects the
choice of species in any survey; as far as possible, however, efforts have
been made to sample species from a variety of taxa and of habitats (see
Lythgoe, 1966; Schwanzara, 1967).
Marine fishes can have any of a series of rhodopsins. The A, , , , values
are not normally distributed about some wavelength such as 500 nm, but
there seem to be clusters at several wavelengths as described by Dartnall
and Lythgoe (1965). These authors suggested that such a distribution
implies a limited number of possible variations in opsin structure. Study
of hybrid salmonids, Salvelinus, indicated that a single-factor difference
distinguishes the opsins of two species in which the visual pigments have
A,,,,, 9 nm apart ( McFarland and Munz, 1965). This first genetic information is at least consistent with the view of Dartnall and Lythgoe. A
23
freshwater fishes, but the arguments and uncertainties brought out by
Lythgoe ( 1966) could also be applied here (see Section 11, D ) . Schwanzara (1967) found a tendency of surface-feeding freshwater fishes to have
retinene, pigments, while bottom dwellers have retinenez pigments. This
is consistent with the filtering undergone by sunlight as it penetrates to
greater depths. She found that retinene, pigments are more common in
tropical fishes than in Temperate Zone species, and the converse; this
was also evident within a single primary freshwater family, the
Cyprinidae. These trends may be related to some general difference in
spectral quality of light in tropical and temperate freshwaters, but comparative data are lacking. At the least, speculation about the possible
visual significance of retinenez should provoke experiments that may give
insight into this problem in biochemical evolution.
D. A Multiplicity of Opsins
The visual pigments of fishes are more conspicuously diverse than
those of all other vertebrates combined (Dartnall and Lythgoe, 1965).
These differences are referable to a series of species-specific opsins, as well
as to the occurrence of retinenee in some fishes. A histogram summarizes
the published results (Fig. 9 ) . Only the most abundant visual pigment
of each species is presented, except in cases where retinenel-retinene2
pairs have been described. Presumably, these are visual pigments of the
rods. The source of less abundant pigments may be either rods or cones
but is unknown in most cases. The visual pigments of more than 180
species have been subjected to adequate spectrometric analysis, and the
list is growing so rapidly that any table would be obsolete before its
publication. The figure should be regarded as a progress report, therefore,
and not as the conclusion of a completed survey. Availability affects the
choice of species in any survey; as far as possible, however, efforts have
been made to sample species from a variety of taxa and of habitats (see
Lythgoe, 1966; Schwanzara, 1967).
Marine fishes can have any of a series of rhodopsins. The A, , , , values
are not normally distributed about some wavelength such as 500 nm, but
there seem to be clusters at several wavelengths as described by Dartnall
and Lythgoe (1965). These authors suggested that such a distribution
implies a limited number of possible variations in opsin structure. Study
of hybrid salmonids, Salvelinus, indicated that a single-factor difference
distinguishes the opsins of two species in which the visual pigments have
A,,,,, 9 nm apart ( McFarland and Munz, 1965). This first genetic information is at least consistent with the view of Dartnall and Lythgoe. A