SOME ASPECTS O F PHOTORECEPTION AND VISION I N FISIIES
191
MacNichol, 1968 ; Wagner et al., 1960 ; Mitarai, 1960 ; Svaetichin,
et al., 1960 ; MacNichol et al., 1961).
VI. TRANSMISSION BY THE LENS
The lenses of fishes are singularly freo from chromatic rtberration
(Pumyhrcy, I!)(; I). If the IcrisoH woro piginonted atid absorbcd certain
components of the incident light, they would alter the composition of
light reaching the retina. It has been discovered that those of diurnal
and of surface oceanic fishes, whose eyes are exposed to some near
ultra-violet light, usually contain pigments absorbing short wavelengths (far blue and near ultra-violet, below 400 mp). In nocturnal
and deep-sea species the lenses are usually transparent to short wavelengths down to about 310 mp. Among certain benthic species of the
neritic zone (Mustelus canis, Tautoga onitis, etc.), the lenses show a
gradual rise in density at wavelengths below 400 mp to 320 mp. Ultraviolet light is absorbed rapidly with increasing depth ; values for
absorption of light of 375 mp range from 5%/m in clear oceanic water
to 90%/m in turbid coastal water. The difference discovered in the
transparencies of lenses are clearly related to the habits of the fishes
and, furthermow, :ire nt1qkcd to tho dvptlis R t whioh they live (Ksnwtly iind M i l k i t i i L i i , I!),Vl; I)rritoii, I05li: Mot,ibiM. 10G7 ; Armstrong and
halch, 196 1 ; Clurko ant1 Denton, 1062).
VII. PHOTOSENSITIVITY AND VISUAL THRESHOLDS
The minimal light which fishes can perceive, the minimal light in
which fishes can see objects, and the minimal light which regulates
their various photoresponuivc activitieu arc frtctoru which have recoivod
some attention.
Reliable values for minimal intensities of light which fishes can
detect, and which govern their activities, are not easy to obtairi, and
recourse has sometimes been made to estimates based on human vision.
The fully dark-adapted human eye can just detect a steady light from
a large field having an intensity of about 3 X
pW/cm2 on the
cornea; this is light of h 510 mp, which is the wavelength of maximal
efficiency for scotopic vision (Pirenne, 19.56 ; Clarke and Dcnton, 1902).
If the eye of a fish is a8 efficient as the human eye in absorbing t h o -
green light, it can just detect daylight having a flux u t the pupil of
3 X 10-8 pW/cm2. IJnfortunately, then, &)OH not mom to tm tiny
rncnmrod valuo nvailrhlu for f i h o l i i t ( t tfrrcnhold in fifih. Hirioo Rome
0thr.r vortotwates niitl at least ono invertd)rato hsvo abtrolutcr throeh l d s as low UH, or lower than, that of mcw, it is highly probablc that
191
MacNichol, 1968 ; Wagner et al., 1960 ; Mitarai, 1960 ; Svaetichin,
et al., 1960 ; MacNichol et al., 1961).
VI. TRANSMISSION BY THE LENS
The lenses of fishes are singularly freo from chromatic rtberration
(Pumyhrcy, I!)(; I). If the IcrisoH woro piginonted atid absorbcd certain
components of the incident light, they would alter the composition of
light reaching the retina. It has been discovered that those of diurnal
and of surface oceanic fishes, whose eyes are exposed to some near
ultra-violet light, usually contain pigments absorbing short wavelengths (far blue and near ultra-violet, below 400 mp). In nocturnal
and deep-sea species the lenses are usually transparent to short wavelengths down to about 310 mp. Among certain benthic species of the
neritic zone (Mustelus canis, Tautoga onitis, etc.), the lenses show a
gradual rise in density at wavelengths below 400 mp to 320 mp. Ultraviolet light is absorbed rapidly with increasing depth ; values for
absorption of light of 375 mp range from 5%/m in clear oceanic water
to 90%/m in turbid coastal water. The difference discovered in the
transparencies of lenses are clearly related to the habits of the fishes
and, furthermow, :ire nt1qkcd to tho dvptlis R t whioh they live (Ksnwtly iind M i l k i t i i L i i , I!),Vl; I)rritoii, I05li: Mot,ibiM. 10G7 ; Armstrong and
halch, 196 1 ; Clurko ant1 Denton, 1062).
VII. PHOTOSENSITIVITY AND VISUAL THRESHOLDS
The minimal light which fishes can perceive, the minimal light in
which fishes can see objects, and the minimal light which regulates
their various photoresponuivc activitieu arc frtctoru which have recoivod
some attention.
Reliable values for minimal intensities of light which fishes can
detect, and which govern their activities, are not easy to obtairi, and
recourse has sometimes been made to estimates based on human vision.
The fully dark-adapted human eye can just detect a steady light from
a large field having an intensity of about 3 X
pW/cm2 on the
cornea; this is light of h 510 mp, which is the wavelength of maximal
efficiency for scotopic vision (Pirenne, 19.56 ; Clarke and Dcnton, 1902).
If the eye of a fish is a8 efficient as the human eye in absorbing t h o -
green light, it can just detect daylight having a flux u t the pupil of
3 X 10-8 pW/cm2. IJnfortunately, then, &)OH not mom to tm tiny
rncnmrod valuo nvailrhlu for f i h o l i i t ( t tfrrcnhold in fifih. Hirioo Rome
0thr.r vortotwates niitl at least ono invertd)rato hsvo abtrolutcr throeh l d s as low UH, or lower than, that of mcw, it is highly probablc that