5. VERTICAL DISTRIBUTION AND MIGRATION
169
at light intensities of
times the values of daylight at the surface
yet he considers escape from predators “ the essential reason for descent
by day rather than by night ”, and he is probably correct in this
supposition. Many of the important predators of euphausiids feed on
them in the surface layers. The whales and planktivorous fish are, for
the most part, surface feeders and a migration of the euphausiids to
deeper and darker depths during the day no doubt decreases the amount
of predation suffered by them.
McLaren severely criticizes the hypothesis first proposed by Hardy
and Gunther (1935). They suggested that vertically migrating animals
could be displaced horizontally because they moved upwards into a
surface layer which was moving at a different rate relative to the deeper
layers, so that when they descended they arrived in a different part of
the deeper layer and vertical migration the next night enabled them to
enter a new area of the upper water layer; thus, they could avoid
the toxic effects of a denso patch of phytoplankton to which they
had been subjected the previous night. Hardy (1956), however, has
modified this view and now considers vertical migration from a deeper
slower-moving water layer into a faster-moving surface layer to be the
means whereby herbivores can find patches of phytoplankton to feed
on. Most euphausiids, however, are omnivorous, Euphausia superba
being the only probable exception (see Chapter 6), and they, according
to this theory, would simply follow the herbivores. This means that
vertical migration is carried out for the purpose of feeding and in the
case of the euphausiids, and other organisms, there is no evidence that
this is true. McLaren’s main arguments for this are strong : “ In order
to congregate in dense patches of phytoplankton, the animals have to
respond to the amount of food by modifying the extent of the vertical
migration, even by giving it up. There is no unequivocal evidence that
vertical migration is most pronounced when food is locally scarce or that
it ceases when food is most abundant. The general objection to these
ideas is that vertical migration seems a remarkably regular and widespread phenomenon to have arisen under what must be spatially and
temporally a most irregular and elusive selective demand.” He points
out that vertical migration of freshwater plankton is unlikely to benefit
the migrants by bringing them into contact with patches of phytoplankton because phytoplankton patchiness is frequently absent in the
thoroughly mixed euphotic zone.
David (1961) suggests that vertical migration and consequent shift
in horizontal position of organisms in the sub-populations brings about
genetic recombination because it is unlikely that enough recombination
would be brought about by the forces of diffusion in the sea or lake.
169
at light intensities of
times the values of daylight at the surface
yet he considers escape from predators “ the essential reason for descent
by day rather than by night ”, and he is probably correct in this
supposition. Many of the important predators of euphausiids feed on
them in the surface layers. The whales and planktivorous fish are, for
the most part, surface feeders and a migration of the euphausiids to
deeper and darker depths during the day no doubt decreases the amount
of predation suffered by them.
McLaren severely criticizes the hypothesis first proposed by Hardy
and Gunther (1935). They suggested that vertically migrating animals
could be displaced horizontally because they moved upwards into a
surface layer which was moving at a different rate relative to the deeper
layers, so that when they descended they arrived in a different part of
the deeper layer and vertical migration the next night enabled them to
enter a new area of the upper water layer; thus, they could avoid
the toxic effects of a denso patch of phytoplankton to which they
had been subjected the previous night. Hardy (1956), however, has
modified this view and now considers vertical migration from a deeper
slower-moving water layer into a faster-moving surface layer to be the
means whereby herbivores can find patches of phytoplankton to feed
on. Most euphausiids, however, are omnivorous, Euphausia superba
being the only probable exception (see Chapter 6), and they, according
to this theory, would simply follow the herbivores. This means that
vertical migration is carried out for the purpose of feeding and in the
case of the euphausiids, and other organisms, there is no evidence that
this is true. McLaren’s main arguments for this are strong : “ In order
to congregate in dense patches of phytoplankton, the animals have to
respond to the amount of food by modifying the extent of the vertical
migration, even by giving it up. There is no unequivocal evidence that
vertical migration is most pronounced when food is locally scarce or that
it ceases when food is most abundant. The general objection to these
ideas is that vertical migration seems a remarkably regular and widespread phenomenon to have arisen under what must be spatially and
temporally a most irregular and elusive selective demand.” He points
out that vertical migration of freshwater plankton is unlikely to benefit
the migrants by bringing them into contact with patches of phytoplankton because phytoplankton patchiness is frequently absent in the
thoroughly mixed euphotic zone.
David (1961) suggests that vertical migration and consequent shift
in horizontal position of organisms in the sub-populations brings about
genetic recombination because it is unlikely that enough recombination
would be brought about by the forces of diffusion in the sea or lake.