7. THE DISTRIBUTION AND EVOLUTION OF VISUAL SYSTEMS
337
in fishes from A max 522 to 500 ηΐμ, which is correlated with salinity and
depends upon the change of prosthetic group from retinene 2 to retinenet
(Fig. 7), and a second major transition from A max 500 to 480 m/x, which
is correlated with depth and depends upon the change of opsin (Fig.
13).
It seems probable that the rhodopsins of marine fishes do not greatly
transcend the limits already disclosed (Argyropxelecus affinis, A max 478
m/x (58); Gillichthys mirabilis, a surface form, A max 512 τημ (47). As
Denton and Warren indicate, light from the surface does not penetrate
more deeply than the depths already sampled. These authors note also
that the displacement of A max toward 480 m/χ may have adaptive significance, since it is blue light of about this wavelength that penetrates
most deeply into clear ocean waters. Indeed at still greater depths than
those at which vision is possible with light penetrating from the surface, fishes must see mainly by bioluminescence; recent studies have
shown that bioluminescent radiations also tend to be of maximal intensity at about 475-480 η\μ (60).
Denton and Warren (26), having made their measurements directly
on fresh retinas, have discovered also that deep-sea fishes tend to exhibit
extraordinary densities of visual pigmentation. The change of extinction at about 485 πΐμ on bleaching they found in a few instances to be
as large as 0.98-1.20, corresponding to absorptions of 90-94% of light of
this wavelength reaching the retina. Through this device, more than
through the shift of A max , some of the fishes that frequent great depths
appear to achieve high visual sensitivity.
IX. Eels
We have already described finding in the American freshwater eel,
Anguilla rostrata, mixtures of vitamin A x and A 2 , and corresponding mixtures of rhodopsin and porphyropsin, so that the net A max of the visual
pigment lies at 502-510 τημ. Recently Carlisle and Denton (46), working with the European Anguilla anguilla, have confirmed this result
with so-called yellow eels that have not yet reached sexual maturity, in
the same stage therefore as those used in our experiments. They find
however that so-called silver eels, which are sexually mature and about
to migrate oceanward to spawn, have changed to deep-sea rhodopsin,
with Amax about 485 τημ. This is therefore another and particularly interesting example of a second metamorphosis of visual pigments, involving in this case not a change of their prosthetic group, but a change
of opsin.
My associates Paul and Patricia Brown (61) have recently reexamined this situation at the Stazione Zoologica in Naples. They found
337
in fishes from A max 522 to 500 ηΐμ, which is correlated with salinity and
depends upon the change of prosthetic group from retinene 2 to retinenet
(Fig. 7), and a second major transition from A max 500 to 480 m/x, which
is correlated with depth and depends upon the change of opsin (Fig.
13).
It seems probable that the rhodopsins of marine fishes do not greatly
transcend the limits already disclosed (Argyropxelecus affinis, A max 478
m/x (58); Gillichthys mirabilis, a surface form, A max 512 τημ (47). As
Denton and Warren indicate, light from the surface does not penetrate
more deeply than the depths already sampled. These authors note also
that the displacement of A max toward 480 m/χ may have adaptive significance, since it is blue light of about this wavelength that penetrates
most deeply into clear ocean waters. Indeed at still greater depths than
those at which vision is possible with light penetrating from the surface, fishes must see mainly by bioluminescence; recent studies have
shown that bioluminescent radiations also tend to be of maximal intensity at about 475-480 η\μ (60).
Denton and Warren (26), having made their measurements directly
on fresh retinas, have discovered also that deep-sea fishes tend to exhibit
extraordinary densities of visual pigmentation. The change of extinction at about 485 πΐμ on bleaching they found in a few instances to be
as large as 0.98-1.20, corresponding to absorptions of 90-94% of light of
this wavelength reaching the retina. Through this device, more than
through the shift of A max , some of the fishes that frequent great depths
appear to achieve high visual sensitivity.
IX. Eels
We have already described finding in the American freshwater eel,
Anguilla rostrata, mixtures of vitamin A x and A 2 , and corresponding mixtures of rhodopsin and porphyropsin, so that the net A max of the visual
pigment lies at 502-510 τημ. Recently Carlisle and Denton (46), working with the European Anguilla anguilla, have confirmed this result
with so-called yellow eels that have not yet reached sexual maturity, in
the same stage therefore as those used in our experiments. They find
however that so-called silver eels, which are sexually mature and about
to migrate oceanward to spawn, have changed to deep-sea rhodopsin,
with Amax about 485 τημ. This is therefore another and particularly interesting example of a second metamorphosis of visual pigments, involving in this case not a change of their prosthetic group, but a change
of opsin.
My associates Paul and Patricia Brown (61) have recently reexamined this situation at the Stazione Zoologica in Naples. They found