SOME ASPECTS OF PIIOTOREC~RPTION AND VISION IN FISIIES
181
extended at 0.001 f t c (Mi, 1959, 1961a, b). The threshold for the
retinomotor shift to the “ d a r k ” position drops during tho night in
Lateolabrax and Cyprinus: from 0.004 to 0.001 ft c before and after
midnight in Lateolabrax, and from 0-00005 to 0-000006 ft c in Cyprinw,
(Tamura, 1957). There are obviously considerable differences in lightlevels controlling the photomechanical changes of different fishes, and
Rome degree of diurnal rhythmicity may exist (Welsh, 1938). In general
it appears that pigment, cones and rods begin to adopt a position
characteristic of darkness when the illumination falls below 1 ft c and
positions of maximal retraction (pigment, rods) or extension (cones)
are attained at levels below 0.01 f t c. This level of illumination is
encountered during twilight, and in a landscape lit by the moon at
phases between full and quarter. Full retreat of pigment, elongation
of cones and shortening of rods are attained in dim light conditions
that correspond to those associated with the switch to scotopic and
rod vision in man.
Since these retinomotor changes in duplex retinae are almoet
certainly conccrncd wit,h enhancing vision, it would be interesting to
know at what light-intensity the change from phototopic to scotopio
vision occurs in fish. The data for retinomotor changes oould then be
interpreted more adequately in terms of vision. Since retinomotor
changes are so extensive in fishes, one wonders if the change from cone
to rod vision and vice versa depends primarily on the ability of cones
and rods themselves to respond ; or if it depends rather on screening or
exposure of the receptors.
Most observers have concerned themselves with only one level of
illumination (and darkness), but Ali (1962) has considered the effect of
different light-intensities on the extent and temporal cmrw of ratimmotor changes. He exposed yearliiigs of Atlantic salmon to two different
intensities, viz. 900 and 0.1 ft c and determined the extent of movement of the retinal pigment and cones. In the stronger light the pigment
became fully extended in 40 min and the cones fully retracted in 46 min.
In the weaker light the pigment expanded, reaching a steady state in
36 min, and the corm shortened to a steady state in 66 min. The
pigment was expanded to a greater degree and the cones mora retracted
in the stronger light. Placed in darknem, thct pigmcnt and mnen h)ok
longer to return to the I ‘ dark ” position uflm sxpowuro to the, ntrotlg
light than after expomrc to the weak light.
Possibly a fish does not often oncounter abrupt ohmgon from light
to total darkness, arul it i H worthwhilo dimovering the s p d at whioh
movomerit of pigrncnt and recoptom procoob undor oonditionH of
changing light-intonaitios cncounturod in nature, Hay at twilight 1)olow
181
extended at 0.001 f t c (Mi, 1959, 1961a, b). The threshold for the
retinomotor shift to the “ d a r k ” position drops during tho night in
Lateolabrax and Cyprinus: from 0.004 to 0.001 ft c before and after
midnight in Lateolabrax, and from 0-00005 to 0-000006 ft c in Cyprinw,
(Tamura, 1957). There are obviously considerable differences in lightlevels controlling the photomechanical changes of different fishes, and
Rome degree of diurnal rhythmicity may exist (Welsh, 1938). In general
it appears that pigment, cones and rods begin to adopt a position
characteristic of darkness when the illumination falls below 1 ft c and
positions of maximal retraction (pigment, rods) or extension (cones)
are attained at levels below 0.01 f t c. This level of illumination is
encountered during twilight, and in a landscape lit by the moon at
phases between full and quarter. Full retreat of pigment, elongation
of cones and shortening of rods are attained in dim light conditions
that correspond to those associated with the switch to scotopic and
rod vision in man.
Since these retinomotor changes in duplex retinae are almoet
certainly conccrncd wit,h enhancing vision, it would be interesting to
know at what light-intensity the change from phototopic to scotopio
vision occurs in fish. The data for retinomotor changes oould then be
interpreted more adequately in terms of vision. Since retinomotor
changes are so extensive in fishes, one wonders if the change from cone
to rod vision and vice versa depends primarily on the ability of cones
and rods themselves to respond ; or if it depends rather on screening or
exposure of the receptors.
Most observers have concerned themselves with only one level of
illumination (and darkness), but Ali (1962) has considered the effect of
different light-intensities on the extent and temporal cmrw of ratimmotor changes. He exposed yearliiigs of Atlantic salmon to two different
intensities, viz. 900 and 0.1 ft c and determined the extent of movement of the retinal pigment and cones. In the stronger light the pigment
became fully extended in 40 min and the cones fully retracted in 46 min.
In the weaker light the pigment expanded, reaching a steady state in
36 min, and the corm shortened to a steady state in 66 min. The
pigment was expanded to a greater degree and the cones mora retracted
in the stronger light. Placed in darknem, thct pigmcnt and mnen h)ok
longer to return to the I ‘ dark ” position uflm sxpowuro to the, ntrotlg
light than after expomrc to the weak light.
Possibly a fish does not often oncounter abrupt ohmgon from light
to total darkness, arul it i H worthwhilo dimovering the s p d at whioh
movomerit of pigrncnt and recoptom procoob undor oonditionH of
changing light-intonaitios cncounturod in nature, Hay at twilight 1)olow