172
THE BIOLOGY OF EUPHAUSIIDS
full potential resources of the geographical volume available to it.
Descent of adults from the surface layers during the day no doubt
decreases the amount of predation effected on the adult populations by
surface feeding planktivorous fish and whales, and from the point of
view of natural selection, is a very good reason for euphausiids performing such a migration. Differences in isolume preferences of different
species decrease direct competition of closely related species and again
allow full utilization of the vertical and horizontal areas available. We
know nothing of the " social '' behaviour of plankton organisms
although, in the case of euphausiids which have image-forming eyes
and a set of photophores, a social life probably exists inasmuch as
swarming can be brought about to facilitate mating. Whether they
bring about social control of their rate of increase, as suggested for
various organisms by Wynne-Edwards, is another matter about which
nothing of value can be said here because we know almost nothing
about the behaviour of these animals. The young of many planktonic
organisms are simple filter feeders and omnivorous feeding habits, such
as those of the Euphausiacea, only develop in the later larval or
adolescent stages. Consequently, an advantage accrues to filter-feeding
larvae which maintain themselves in the euphotic zone. Further, a
layering of size classes spreads the organisms in the vertical plane and
also means that the smaller animals have less distance to travel to the
surface than the larger animals ; since the amount of energy expended
by the smaller animals is relatively greater than that expended by the
larger animals this distributional pattern has advantages. We know
little about the behavioural aspects of euphausiids ; for instance, is there
an optimal density4.g. one animal per cubic metre of water-which
the individuals within a population maintain. If so, then a spread in
the vertical as well as the horizontal plane provides a compact interconnected population unit which still allows horizontal displacement of
its peripheral individuals.
It is very probable that no proven reason for diurnal vertical
migration of plankton organisms will be found. It is also probable that
the most important benefits derived by one species or group of organisms
from a migration will differ from those derived by other species or
groups. Further, we consider that the various theories previously
offered to explain vertical migrations may all be true rather than any
one being of paramount importance. Vertical migration of organisms
in a three-dimensional environment seems to provide the best chances
of utilizing the full resources of that environment and the species are
separated in space, and frequently time, because the movements of
each species are probably related to those of isolumes and the limits of
THE BIOLOGY OF EUPHAUSIIDS
full potential resources of the geographical volume available to it.
Descent of adults from the surface layers during the day no doubt
decreases the amount of predation effected on the adult populations by
surface feeding planktivorous fish and whales, and from the point of
view of natural selection, is a very good reason for euphausiids performing such a migration. Differences in isolume preferences of different
species decrease direct competition of closely related species and again
allow full utilization of the vertical and horizontal areas available. We
know nothing of the " social '' behaviour of plankton organisms
although, in the case of euphausiids which have image-forming eyes
and a set of photophores, a social life probably exists inasmuch as
swarming can be brought about to facilitate mating. Whether they
bring about social control of their rate of increase, as suggested for
various organisms by Wynne-Edwards, is another matter about which
nothing of value can be said here because we know almost nothing
about the behaviour of these animals. The young of many planktonic
organisms are simple filter feeders and omnivorous feeding habits, such
as those of the Euphausiacea, only develop in the later larval or
adolescent stages. Consequently, an advantage accrues to filter-feeding
larvae which maintain themselves in the euphotic zone. Further, a
layering of size classes spreads the organisms in the vertical plane and
also means that the smaller animals have less distance to travel to the
surface than the larger animals ; since the amount of energy expended
by the smaller animals is relatively greater than that expended by the
larger animals this distributional pattern has advantages. We know
little about the behavioural aspects of euphausiids ; for instance, is there
an optimal density4.g. one animal per cubic metre of water-which
the individuals within a population maintain. If so, then a spread in
the vertical as well as the horizontal plane provides a compact interconnected population unit which still allows horizontal displacement of
its peripheral individuals.
It is very probable that no proven reason for diurnal vertical
migration of plankton organisms will be found. It is also probable that
the most important benefits derived by one species or group of organisms
from a migration will differ from those derived by other species or
groups. Further, we consider that the various theories previously
offered to explain vertical migrations may all be true rather than any
one being of paramount importance. Vertical migration of organisms
in a three-dimensional environment seems to provide the best chances
of utilizing the full resources of that environment and the species are
separated in space, and frequently time, because the movements of
each species are probably related to those of isolumes and the limits of