SOME ASPECTS OF PHOTORECEPTION AND VISION IN FISHES
193
moving bars (subtending an angle of 11' at the eye) in a minimal
illumination of 1 x 10-6 mL. It has been estimated that a t this
brightness the equivalent of about 1 x
pW/cm2 would be falling
upon the eye (Clarke, 1936). Lepomis is a shallow-water fish with a
duplex retina ; fishes living habitually in dimly-:it waters may be able
to distinguish objects at much lower intrnsities. Thus, the mackerel
continues to school at very weak light-intensities, which approach visud
threshold for humans, and which are well below those at which aggregations of sunfish break up (Schlaifer, 1942).
Some observations on young salmon (Oncorhynchw) reveal that they
cease feeding at light-intensities below lo-' to
ft c (roughly IO-'to
10-6 pW/cm2) (Ali, 1959), and that they rise off the bottom and begin
migrating at intensities below 0.01 ft c (approx.
pW/cm2)
(McDonald, 1960). The down-stream movement of young rainbow trout
is influenced by much lower light-intensities (<0.0001 ft c) (Northcote,
1962). Both these responses involve visual discrimination (of food
objects or objects on the bottom). Kinetic movements of trout and
salmon larvae (Salmo trutta, S. irideus and S. salar) have a light threshold of 0.005 m c (=O-00047 ftc) (Woodhead, 1967). The results
demonstrate the importance of ambient light-levels on behaviour
patterns, but they do not necessarily reveal absolute threshold values.
Activities at or near absolute thresholds may be important, however,
only in some special environments.
There are enormow populations of luminescent animals in the
oceans, hardly any in fresh waters (see, for example, Harvey, 1952.).
Since the light-organs of these animals arc often tiny-indeed many of
thc light-producirig animal8 themr;lc!lvos arc! quito mall- it iR pri~otical
t,o coiiHidor l , l i ( ~ i i i IIH I ) o i i i t , H ~ U ~ W H ,
i~ii(l ho wfirnrd,o Irow f l u thoy onn tw
porwivctl. 'I'lio t i u i t i ~ ~ t i
vycr ( m i j u H t p(m:vivo II ~ r n a l l ,
Htcady ligtit-source
hwviiig IL fliiu of I.(! x I 0 l o pW/oniJ at tho surfttco of the cornoa
(l'irotitto, I!)%; : Nic:ol, I!)T,H). Oliirko (in Clnrku nnct Donton, 1962) states
that IL miall light could just bo detectrtl in clear oceanic water (transmission '37%/m) by a human observer at a depth of 66 m, when the
intensity was 2.8 x 10-0 pW/cm2 at the eye. Pirenne's value used by
Nicol obtained as it was under ideal conditions by a co-operative
human observer, is probably too high for natural conditions in the tima.
Clarke's figure, on the other hand, is likely to be too low because of the
difficult conditions under which it was obtained (foveal vision, partial
dark-adaptation, etc.). As previously considered, many fish may have
thresholds as low as or lower than that of man. Moreover, in deep
oceanic waters, luminescent lights are umally blue and Rhould have
maximal eBciency for stimulating scotopic visual pigments of hathy-
193
moving bars (subtending an angle of 11' at the eye) in a minimal
illumination of 1 x 10-6 mL. It has been estimated that a t this
brightness the equivalent of about 1 x
pW/cm2 would be falling
upon the eye (Clarke, 1936). Lepomis is a shallow-water fish with a
duplex retina ; fishes living habitually in dimly-:it waters may be able
to distinguish objects at much lower intrnsities. Thus, the mackerel
continues to school at very weak light-intensities, which approach visud
threshold for humans, and which are well below those at which aggregations of sunfish break up (Schlaifer, 1942).
Some observations on young salmon (Oncorhynchw) reveal that they
cease feeding at light-intensities below lo-' to
ft c (roughly IO-'to
10-6 pW/cm2) (Ali, 1959), and that they rise off the bottom and begin
migrating at intensities below 0.01 ft c (approx.
pW/cm2)
(McDonald, 1960). The down-stream movement of young rainbow trout
is influenced by much lower light-intensities (<0.0001 ft c) (Northcote,
1962). Both these responses involve visual discrimination (of food
objects or objects on the bottom). Kinetic movements of trout and
salmon larvae (Salmo trutta, S. irideus and S. salar) have a light threshold of 0.005 m c (=O-00047 ftc) (Woodhead, 1967). The results
demonstrate the importance of ambient light-levels on behaviour
patterns, but they do not necessarily reveal absolute threshold values.
Activities at or near absolute thresholds may be important, however,
only in some special environments.
There are enormow populations of luminescent animals in the
oceans, hardly any in fresh waters (see, for example, Harvey, 1952.).
Since the light-organs of these animals arc often tiny-indeed many of
thc light-producirig animal8 themr;lc!lvos arc! quito mall- it iR pri~otical
t,o coiiHidor l , l i ( ~ i i i IIH I ) o i i i t , H ~ U ~ W H ,
i~ii(l ho wfirnrd,o Irow f l u thoy onn tw
porwivctl. 'I'lio t i u i t i ~ ~ t i
vycr ( m i j u H t p(m:vivo II ~ r n a l l ,
Htcady ligtit-source
hwviiig IL fliiu of I.(! x I 0 l o pW/oniJ at tho surfttco of the cornoa
(l'irotitto, I!)%; : Nic:ol, I!)T,H). Oliirko (in Clnrku nnct Donton, 1962) states
that IL miall light could just bo detectrtl in clear oceanic water (transmission '37%/m) by a human observer at a depth of 66 m, when the
intensity was 2.8 x 10-0 pW/cm2 at the eye. Pirenne's value used by
Nicol obtained as it was under ideal conditions by a co-operative
human observer, is probably too high for natural conditions in the tima.
Clarke's figure, on the other hand, is likely to be too low because of the
difficult conditions under which it was obtained (foveal vision, partial
dark-adaptation, etc.). As previously considered, many fish may have
thresholds as low as or lower than that of man. Moreover, in deep
oceanic waters, luminescent lights are umally blue and Rhould have
maximal eBciency for stimulating scotopic visual pigments of hathy-