7. THE DISTRIBUTION AND EVOLUTION OF VISUAL SYSTEMS
327
In all known cases rhodopsin and porphyropsin are present in the same
proportions as the retinal vitamins A from which they are derived. In
consequence of these relationships one can observe a regular displacement of A max in the total visual pigment of the retina, depending on the
proportions of rhodopsin and porphyropsin that are present, and correlated to a surprising degree with ecological habit. This is illustrated
in Fig. 7. Beginning with the wholly marine spiny dogfish, which posi.o
0.8
c
o
| 0.6
LL)
04
0.2
450
500
550
600
Wavelength
FIG. 7. Spectra of the visual pigments of various fishes, illustrating the transition from an exclusively rhodopsin to an exclusively porphyropsin system, correlated
with salinity relations. The permanently marine dogfish (1) possesses rhodopsin alone
(Xniax approx. 497 m/j); the catadromous eel (2) and brackish-water killifish (3),
predominantly rhodopsin; the anadromous brook trout (4), predominantly porphyropsin; and the anadromous white perch (5), wholly porphyropsin (X m& x 522
ιημ). (From Wald, 43.)
sesses typical rhodopsin (A max 498 m/x), one goes to the catadromous eel
and brackish water killifish (A max 498-505 m/x), marine fishes which can
invade fresh water; to the anadromous brook trout (A max 510-515 m/x),
a freshwater fish that can enter the sea; and finally to the white perch,
another anadromous fish that possesses exclusively porphyropsin (A max
522 m/x) (43).
Is this pattern of visual systems determined genetically or by the
environment? A first answer to this question is provided by those labrid
and corid fishes which, though wholly marine, possess porphyropsin.
But it is the euryhaline fishes, since they can be observed in both environments, which demonstrate most clearly that to a first approximation
the pattern of visual systems is independent of the immediate environment. The salmonids which we found to have both systems mixed in
their retinas had never been out of fresh water. Alewives just coming
into fresh water from the sea on their spawning migration have vitamin
A 2 and porphyropsin alone. Eels taken from a landlocked freshwater
327
In all known cases rhodopsin and porphyropsin are present in the same
proportions as the retinal vitamins A from which they are derived. In
consequence of these relationships one can observe a regular displacement of A max in the total visual pigment of the retina, depending on the
proportions of rhodopsin and porphyropsin that are present, and correlated to a surprising degree with ecological habit. This is illustrated
in Fig. 7. Beginning with the wholly marine spiny dogfish, which posi.o
0.8
c
o
| 0.6
LL)
04
0.2
450
500
550
600
Wavelength
FIG. 7. Spectra of the visual pigments of various fishes, illustrating the transition from an exclusively rhodopsin to an exclusively porphyropsin system, correlated
with salinity relations. The permanently marine dogfish (1) possesses rhodopsin alone
(Xniax approx. 497 m/j); the catadromous eel (2) and brackish-water killifish (3),
predominantly rhodopsin; the anadromous brook trout (4), predominantly porphyropsin; and the anadromous white perch (5), wholly porphyropsin (X m& x 522
ιημ). (From Wald, 43.)
sesses typical rhodopsin (A max 498 m/x), one goes to the catadromous eel
and brackish water killifish (A max 498-505 m/x), marine fishes which can
invade fresh water; to the anadromous brook trout (A max 510-515 m/x),
a freshwater fish that can enter the sea; and finally to the white perch,
another anadromous fish that possesses exclusively porphyropsin (A max
522 m/x) (43).
Is this pattern of visual systems determined genetically or by the
environment? A first answer to this question is provided by those labrid
and corid fishes which, though wholly marine, possess porphyropsin.
But it is the euryhaline fishes, since they can be observed in both environments, which demonstrate most clearly that to a first approximation
the pattern of visual systems is independent of the immediate environment. The salmonids which we found to have both systems mixed in
their retinas had never been out of fresh water. Alewives just coming
into fresh water from the sea on their spawning migration have vitamin
A 2 and porphyropsin alone. Eels taken from a landlocked freshwater