252
THE BIOLOGY O F EUPHAUSIIDS
attenuation coefficients of the water columns. Consequently, a specified
isolume (layer of equal light intensity) can occur at significantly
different depths in different sea areas. During the day most euphausiids
are living at a depth such that the downwelling light has a spectrum
peaking rather sharply between 470 and 500 mp, the waveband of light
penetrating deepest into the sea (Fig. 92). Boden and Kampa (1965)
report that at depths between 85 and 100 m in Saanich Inlet, British
Columbia, Canada the transmitted sun- and sky-light had a peak
between 494 and 502 mp and the intensity of irradiance measured at
494mp was about 1.2 x 10-1pW/cm2. There is a twilight shift of
5-6 mp in the peak of the spectrum of irradiance at depth (Boden and
Kampa, 1965) and Boden (196lb) has made a special study of changes
in the spectral characteristics of downwelling light during the twilight
period. This, of course, is at the time when the euphausiids are actively
swimming towards the surface layers and the problem of relating their
vertical migration to changing intensities and spectral characteristics
of downwelling light is complicated.
Most of the papers cited above, and several others, present measurements of bioluminescent activity in the sea but the major difficulty in
these studies is to determine what organism or organisms are producing
the light. The photometer is attached to the end of a cable freely
suspended from the ship and, in many cases, will swing slightly at the
end of the cable so causing agitation of the water around it. This will
induce organisms such as dinoflagellates to luminesce. Nevertheless,
Clarke and Kelly (1 964) and Clarke and Backus (1 964) have been able
to demonstrate that the highest frequencies of bright flashes of luminescence occur near or just below the water layer, which changes depth
during the course of the day, having an ambient light of the same
intensity as the luminescence. They thus find interrelations between
vertical migration of the organisms, the rate and magnitude of luminescent flashing and changes in the intensity, diurnally with depth, of
downwelling sun- and sky-light; they do not, however, identify the
organisms responsible for the luminescence except to state that they
migrated upwards at dusk and downwards at dawn a t rates as great as
10m per minute. Similar studies by Kampa and Boden (1957) and
Boden and Kampa (1957)) although presenting circumstantial evidence
especially in the former paper, that euphausiids may be responsible for
much of the luminescence recorded at depth, could not prove that these
animals are indeed the source of their records. Consequently, although
a considerable body of data relating to luminescence in the sea
geographically, temporally, and in relation to depth, is being gathered
it is early yet to correlate the results with behaviour of euphausiids.
THE BIOLOGY O F EUPHAUSIIDS
attenuation coefficients of the water columns. Consequently, a specified
isolume (layer of equal light intensity) can occur at significantly
different depths in different sea areas. During the day most euphausiids
are living at a depth such that the downwelling light has a spectrum
peaking rather sharply between 470 and 500 mp, the waveband of light
penetrating deepest into the sea (Fig. 92). Boden and Kampa (1965)
report that at depths between 85 and 100 m in Saanich Inlet, British
Columbia, Canada the transmitted sun- and sky-light had a peak
between 494 and 502 mp and the intensity of irradiance measured at
494mp was about 1.2 x 10-1pW/cm2. There is a twilight shift of
5-6 mp in the peak of the spectrum of irradiance at depth (Boden and
Kampa, 1965) and Boden (196lb) has made a special study of changes
in the spectral characteristics of downwelling light during the twilight
period. This, of course, is at the time when the euphausiids are actively
swimming towards the surface layers and the problem of relating their
vertical migration to changing intensities and spectral characteristics
of downwelling light is complicated.
Most of the papers cited above, and several others, present measurements of bioluminescent activity in the sea but the major difficulty in
these studies is to determine what organism or organisms are producing
the light. The photometer is attached to the end of a cable freely
suspended from the ship and, in many cases, will swing slightly at the
end of the cable so causing agitation of the water around it. This will
induce organisms such as dinoflagellates to luminesce. Nevertheless,
Clarke and Kelly (1 964) and Clarke and Backus (1 964) have been able
to demonstrate that the highest frequencies of bright flashes of luminescence occur near or just below the water layer, which changes depth
during the course of the day, having an ambient light of the same
intensity as the luminescence. They thus find interrelations between
vertical migration of the organisms, the rate and magnitude of luminescent flashing and changes in the intensity, diurnally with depth, of
downwelling sun- and sky-light; they do not, however, identify the
organisms responsible for the luminescence except to state that they
migrated upwards at dusk and downwards at dawn a t rates as great as
10m per minute. Similar studies by Kampa and Boden (1957) and
Boden and Kampa (1957)) although presenting circumstantial evidence
especially in the former paper, that euphausiids may be responsible for
much of the luminescence recorded at depth, could not prove that these
animals are indeed the source of their records. Consequently, although
a considerable body of data relating to luminescence in the sea
geographically, temporally, and in relation to depth, is being gathered
it is early yet to correlate the results with behaviour of euphausiids.