5. VERTICAL DISTRIBUTION AND MIGRATION
163
luminescence in the water column and concluded that the deepscattering layer was a source of such bioluminescence. This luminescence is blue-green, with a maximal emission near 478 mp. They tested
the spectral emissions of Euphausia paci$ca and Pyrosoma atlantica and
found that they were compatible with the bioluminescence associated
with the scattering layer. Thus, from this work the scattering layers are
of biological origin, but argument still proceeds as to which organisms
give rise to the actual scattering of the sound waves. Fish with swimbladders are the most obvious but the oil contained in euphausiids,
especially during the later summer and the autumn periods of the year,
may have a sufficient differential density to cause sound scattering.
More work is required, but it is very probable that euphausiids, if not
the dominant organisms in these layers, are certainly present.
What parameters of the physical and biological environments
control the vertical migration? Salinity gradients seem to have little
modifying influence on the extent of the migrations and density and
viscosity likewise. Temperature can, as will be shown, act as a limiting
parameter to the upward and downward extent of the vertical distribution of a species. Pressure changes seem to have little effect and Wickstead (1961), working off Singapore, examined the annual and diurnal
cycles of atmospheric pressure variation and found no corresponding
cyclic changes in the vertical movements of the plankton organisms.
Changes of pressure of the order of that to which the animals are
subjected when they move vertically in the water column were detected
by furciliae of Meganyctiphanes norvegiea and therefore these changes
can probably be detected by adult euphausiids in general (Rice, 1964).
He found experimentally that increase in pressure caused the larvae to
move towards a light source (in the natural environment this would be
upwards) and decrease in pressure caused them to move away from the
light source. It is, therefore, probable that the vertical gradient of
pressure is monitored by the migrating animals. Less attention has been
paid to detection of pressure gradients than to light and temperature
gradients.
The discontinuity layer or thermocline, if present, may limit the
upward movement of some species of euphausiids, but little attention
has been paid to this. Hansen (1951) found that adult M . norvegica
never ascended through the thermocline in the Bonnefjord. Lacroix
(1961), on the other hand, found that both Thysanoessa raschii and
T. inermis migrated upwards through the thermocline the presence
of which affected the vertical migration. A sharp thermocline
was associated with slower speeds of ascent, less time spent in the surface layers a t night, and faster speeds of descent a t dawn. A less
163
luminescence in the water column and concluded that the deepscattering layer was a source of such bioluminescence. This luminescence is blue-green, with a maximal emission near 478 mp. They tested
the spectral emissions of Euphausia paci$ca and Pyrosoma atlantica and
found that they were compatible with the bioluminescence associated
with the scattering layer. Thus, from this work the scattering layers are
of biological origin, but argument still proceeds as to which organisms
give rise to the actual scattering of the sound waves. Fish with swimbladders are the most obvious but the oil contained in euphausiids,
especially during the later summer and the autumn periods of the year,
may have a sufficient differential density to cause sound scattering.
More work is required, but it is very probable that euphausiids, if not
the dominant organisms in these layers, are certainly present.
What parameters of the physical and biological environments
control the vertical migration? Salinity gradients seem to have little
modifying influence on the extent of the migrations and density and
viscosity likewise. Temperature can, as will be shown, act as a limiting
parameter to the upward and downward extent of the vertical distribution of a species. Pressure changes seem to have little effect and Wickstead (1961), working off Singapore, examined the annual and diurnal
cycles of atmospheric pressure variation and found no corresponding
cyclic changes in the vertical movements of the plankton organisms.
Changes of pressure of the order of that to which the animals are
subjected when they move vertically in the water column were detected
by furciliae of Meganyctiphanes norvegiea and therefore these changes
can probably be detected by adult euphausiids in general (Rice, 1964).
He found experimentally that increase in pressure caused the larvae to
move towards a light source (in the natural environment this would be
upwards) and decrease in pressure caused them to move away from the
light source. It is, therefore, probable that the vertical gradient of
pressure is monitored by the migrating animals. Less attention has been
paid to detection of pressure gradients than to light and temperature
gradients.
The discontinuity layer or thermocline, if present, may limit the
upward movement of some species of euphausiids, but little attention
has been paid to this. Hansen (1951) found that adult M . norvegica
never ascended through the thermocline in the Bonnefjord. Lacroix
(1961), on the other hand, found that both Thysanoessa raschii and
T. inermis migrated upwards through the thermocline the presence
of which affected the vertical migration. A sharp thermocline
was associated with slower speeds of ascent, less time spent in the surface layers a t night, and faster speeds of descent a t dawn. A less