8. VISION AND BIOLUMINESCENCE
253
There is not enough evidence to suggest that euphausiids have a diurnal
rhythm of luminescence and that they tend to luminesce more a t dusk
and dawn than a t other times of the day, nor is it known if there are
seasonal changes in their luminescent behaviour. Answers to these
problems will only be obtained once a means of identifying the animals
producing the luminescence down in the sea has been devised. Breslau
and Edgerton (1958) designed a camera which is triggered by a luminescent flash originating in its field of vision but the photographs obtained
did not provide as much useful information as hoped, probably
because the camera was swinging slightly a t the end of the wire and
frightening animals such as euphausiids away (Clarke and Hubbard,
1959).
The ambient light in which euphausiids normally live has a spectral
distribution peaking rather sharply around 470-500 mp and the
luminescence produced by the euphausiids has a similar spectral
distribution but it is not known to what extent bioluminescence of these
organisms in the sea raises the general intensity of light around individuals within a population. The data on levels of light intensity associated
with the vertical positions of deep scattering layers suggest that the
isolumes followed by euphausiids may be of the order of 1 x
p W /
cm2, the isolume probably associated with Euphausia paci$ca in the
region of the San Diego Trough (Kampa and Boden, 1957). The
intensity of the luminescence of one euphausiid is about 2 x
pWlcm2
at a distance of 1 m. If the animals are evenly distributed at a density
of one per cubic metre of water and all are luminescing then the
intensity of luminescence affecting one animal from the surrounding
euphausiids will be of the order of
or
pWIcm2 which, when
compared to the levels of ambient illumination, represents an increase
in total illumination of about 10%. This is a significant addition and,
since euphausiids frequently occur a t densities greater than one per
cubic metre, higher contributions from luminescent activity are
possible a t these relatively low levels of ambient illumination. If
simultaneous light production by organisms other than euphausiids is
considered then, of course, the luminous flux produced could equal or
exceed the sunlight penetrating from the sea surface, as has been found
on several occasions. If the luminescence increased the general light
intensity from 1 to 2 x
pWlcm2 a t a depth of 620 m or so in a
clear oceanic area then this layer will have the same light intensity as
the layer approximately 20m above it where light of this intensity
results from downwelling sunlight alone (Fig. 93). I n trying to place in
perspective increases, from bioluminescent sources, in general illumination level8 it must not be forgotten that luminescing organisms passing
253
There is not enough evidence to suggest that euphausiids have a diurnal
rhythm of luminescence and that they tend to luminesce more a t dusk
and dawn than a t other times of the day, nor is it known if there are
seasonal changes in their luminescent behaviour. Answers to these
problems will only be obtained once a means of identifying the animals
producing the luminescence down in the sea has been devised. Breslau
and Edgerton (1958) designed a camera which is triggered by a luminescent flash originating in its field of vision but the photographs obtained
did not provide as much useful information as hoped, probably
because the camera was swinging slightly a t the end of the wire and
frightening animals such as euphausiids away (Clarke and Hubbard,
1959).
The ambient light in which euphausiids normally live has a spectral
distribution peaking rather sharply around 470-500 mp and the
luminescence produced by the euphausiids has a similar spectral
distribution but it is not known to what extent bioluminescence of these
organisms in the sea raises the general intensity of light around individuals within a population. The data on levels of light intensity associated
with the vertical positions of deep scattering layers suggest that the
isolumes followed by euphausiids may be of the order of 1 x
p W /
cm2, the isolume probably associated with Euphausia paci$ca in the
region of the San Diego Trough (Kampa and Boden, 1957). The
intensity of the luminescence of one euphausiid is about 2 x
pWlcm2
at a distance of 1 m. If the animals are evenly distributed at a density
of one per cubic metre of water and all are luminescing then the
intensity of luminescence affecting one animal from the surrounding
euphausiids will be of the order of
or
pWIcm2 which, when
compared to the levels of ambient illumination, represents an increase
in total illumination of about 10%. This is a significant addition and,
since euphausiids frequently occur a t densities greater than one per
cubic metre, higher contributions from luminescent activity are
possible a t these relatively low levels of ambient illumination. If
simultaneous light production by organisms other than euphausiids is
considered then, of course, the luminous flux produced could equal or
exceed the sunlight penetrating from the sea surface, as has been found
on several occasions. If the luminescence increased the general light
intensity from 1 to 2 x
pWlcm2 a t a depth of 620 m or so in a
clear oceanic area then this layer will have the same light intensity as
the layer approximately 20m above it where light of this intensity
results from downwelling sunlight alone (Fig. 93). I n trying to place in
perspective increases, from bioluminescent sources, in general illumination level8 it must not be forgotten that luminescing organisms passing