192
J. A. C. NIOOL
many fishes are equally sensitive. Densities of retinal pigment in the
human eye are only about 0.15 and some 30% of incident blue-green
light is absorbed ; densities in fish retinae are usually much greater than
this, 0.6 in the skipper (Scontberesox t?auru.s), and 0.58 in the conger eel
(Conger conger). The retinae of these fish absorb about 75% of the light
reaching their surfaces, a gain of over twice that of the human. Indeed,
in deep-sea fish such as Gonostoma and Xenodermichthys, densities reach
1.0 or more, permitting absorptions of 90% or greater (Denton, 1959).
It is known that the owl, nocturnal, has an absolute threshold one
tenth lower than that of man, and fishes habitually living in dim
light may have thresholds as low or lower (Pirenne, 1956). Denton
and Warren (1957), following their investigation of visual pigments
of deep-sea fish, have discussed this problem at length. Tho scotopic
pigments or visual golds of such fish absorb maximally in the blue
around 485 mp, corresponding to the region of maximal transmission
of clear oceanic water. After considering a number of advantages
which the eyes of bathypelagic fish may possess-high pigment density,
relatively large pupil, reduced “ noise ” level-they suggest that the
gain in sensitivity of the eye of a deep-sea fish over that of the human
may be as much as 100 times. This means that in very clear oceanic
water, having an extinction coefficient of, say, 0.04, fish may be able to
just detect daylight down to depths of 900 to 1000 m for a short
time each day,
Recordings made with bathyphotometers in deep waters have shown
that the flashes of luminous animals may be so frequent that they
could conceivably provide background illumination having levels of
lo-’ pW/cmZ or more (see, for example, Clarke and Huhhard, 1969).
Light is heavily absorbed by water ; trammission vrtluo~ mrrgo from
around 65%/m for turbid coaHtnl waters t ( ~ 03%/m for oloar ooeariia
waters. The eyo ie so eenHitive, howovcr, that in oomtal wutor8, hrtving
abeorption c:ocdTioiuntH of arouiid 0. I :I, a fish at 100 IKI o m Htill porooivs
objock froill H I I I ~ ~ ~ H O
to H u i w o t ~ I I
tho darkofh wintor dtty (region of
J’lyi~~o~ith). 1 1 1 111 uoli docyo19 wtdmH, oiilinnood ~,hotoHotiRitivity should
iiioroiwo LIIO t i i t l o outrli day wlioii thylight is still purcoptiblo t80 a fish.
In tho Iiumaii baing visual acuity incromes quickly as the light rises
above absolute threshold. Similarly we may presume that a fish can
begin to distinguish objects at some intensity only a few times brighter
than absolute threshold (Clarke and Denton, 1962). I n deep waters Of
the ocean this means that a fish can begin to perceive objects by f d t e d
daylight at depths slightly less than those when it can just detect
background light. I n some experiments involving rheotactic reapom
of the sunfish (Lepomds) it was shown that it would just respond to
J. A. C. NIOOL
many fishes are equally sensitive. Densities of retinal pigment in the
human eye are only about 0.15 and some 30% of incident blue-green
light is absorbed ; densities in fish retinae are usually much greater than
this, 0.6 in the skipper (Scontberesox t?auru.s), and 0.58 in the conger eel
(Conger conger). The retinae of these fish absorb about 75% of the light
reaching their surfaces, a gain of over twice that of the human. Indeed,
in deep-sea fish such as Gonostoma and Xenodermichthys, densities reach
1.0 or more, permitting absorptions of 90% or greater (Denton, 1959).
It is known that the owl, nocturnal, has an absolute threshold one
tenth lower than that of man, and fishes habitually living in dim
light may have thresholds as low or lower (Pirenne, 1956). Denton
and Warren (1957), following their investigation of visual pigments
of deep-sea fish, have discussed this problem at length. Tho scotopic
pigments or visual golds of such fish absorb maximally in the blue
around 485 mp, corresponding to the region of maximal transmission
of clear oceanic water. After considering a number of advantages
which the eyes of bathypelagic fish may possess-high pigment density,
relatively large pupil, reduced “ noise ” level-they suggest that the
gain in sensitivity of the eye of a deep-sea fish over that of the human
may be as much as 100 times. This means that in very clear oceanic
water, having an extinction coefficient of, say, 0.04, fish may be able to
just detect daylight down to depths of 900 to 1000 m for a short
time each day,
Recordings made with bathyphotometers in deep waters have shown
that the flashes of luminous animals may be so frequent that they
could conceivably provide background illumination having levels of
lo-’ pW/cmZ or more (see, for example, Clarke and Huhhard, 1969).
Light is heavily absorbed by water ; trammission vrtluo~ mrrgo from
around 65%/m for turbid coaHtnl waters t ( ~ 03%/m for oloar ooeariia
waters. The eyo ie so eenHitive, howovcr, that in oomtal wutor8, hrtving
abeorption c:ocdTioiuntH of arouiid 0. I :I, a fish at 100 IKI o m Htill porooivs
objock froill H I I I ~ ~ ~ H O
to H u i w o t ~ I I
tho darkofh wintor dtty (region of
J’lyi~~o~ith). 1 1 1 111 uoli docyo19 wtdmH, oiilinnood ~,hotoHotiRitivity should
iiioroiwo LIIO t i i t l o outrli day wlioii thylight is still purcoptiblo t80 a fish.
In tho Iiumaii baing visual acuity incromes quickly as the light rises
above absolute threshold. Similarly we may presume that a fish can
begin to distinguish objects at some intensity only a few times brighter
than absolute threshold (Clarke and Denton, 1962). I n deep waters Of
the ocean this means that a fish can begin to perceive objects by f d t e d
daylight at depths slightly less than those when it can just detect
background light. I n some experiments involving rheotactic reapom
of the sunfish (Lepomds) it was shown that it would just respond to