11. ECOLOUY O F DISTRIBUTION
349
the sub-populations forming the swarms ; only a brief account of his
results can be given here and the reader must refer to his original text
for detailed data. Marr suggests that these swarms are formed by larval
stages which from hatching onwards appear to form aggregations, these
aggregations once formed being of a relatively permanent nature. This
would explain why one larval stage or two consecutive larval stages are
frequently very dominant by numbers in any one swarm; swarms
comprising older individuals usually have a modal group of individuals
of a small range of body length that far outnumber the other size groups
in the swarm. Nothing is known about the mechanisms that keep the
animals together ; pre-spawning aggregations of Meganyctiphanes
norvegica are probably effected by the photophores but whether the
photophores act to maintain the surface swarms of Euphausia superba
is unknown. There must be some behavioural mechanism to effect this
otherwise the individuals would tend to be dispersed. Marr states that
a swarm can behave like an individual organism and he compares the
lateral movements of a swarm to the flight manoeuvres of starlings.
Consequently, the whole field of behavioural mechanisms of euphausiids
requires investigation and some very interesting results, applicable not
only to euphausiids but probably also to other planktonic animals, may
be obtained.
Finally, brief recourse must be made to the problems of maintenance
of populations in geographical areas. As mentioned previously, a
species may be maintained in an oceanic area by a gyral system or a
current/counter current system. Coastal areas are different ; some
areas have a well-defined coastal water mass circulating within a hydrographically enclosed area so that the rates of exchange, and so of loss
of an euphausiid species, are slow. Intensive sampling of plankton in the
North Atlantic over recent years has shown that individuals of several
different warm water species may be found far to the north of their
general areas of occurrence (Mauchline, 1965b). This indicates that
there is probably, in most populations, a continuous loss of individuals
to probably unfavourable environments. If, as is suggested above,
behavioural mechanisms are important in maintaining aggregations
and act along with hydrographic conditions to maintain a species in a
favourable area then individuals which L ‘ wander )’ may wander
because they are abnormal physiologically. For instance, if during one
season luminescent behaviour is the main mechanism in maintaining
an aggregation and for physiological reasons an individual in the swarm
does not react norinally to the stimulus of the population’s luminescence, then it may not be held in the population but be subject to
factors moving it away from the main population. There is some
349
the sub-populations forming the swarms ; only a brief account of his
results can be given here and the reader must refer to his original text
for detailed data. Marr suggests that these swarms are formed by larval
stages which from hatching onwards appear to form aggregations, these
aggregations once formed being of a relatively permanent nature. This
would explain why one larval stage or two consecutive larval stages are
frequently very dominant by numbers in any one swarm; swarms
comprising older individuals usually have a modal group of individuals
of a small range of body length that far outnumber the other size groups
in the swarm. Nothing is known about the mechanisms that keep the
animals together ; pre-spawning aggregations of Meganyctiphanes
norvegica are probably effected by the photophores but whether the
photophores act to maintain the surface swarms of Euphausia superba
is unknown. There must be some behavioural mechanism to effect this
otherwise the individuals would tend to be dispersed. Marr states that
a swarm can behave like an individual organism and he compares the
lateral movements of a swarm to the flight manoeuvres of starlings.
Consequently, the whole field of behavioural mechanisms of euphausiids
requires investigation and some very interesting results, applicable not
only to euphausiids but probably also to other planktonic animals, may
be obtained.
Finally, brief recourse must be made to the problems of maintenance
of populations in geographical areas. As mentioned previously, a
species may be maintained in an oceanic area by a gyral system or a
current/counter current system. Coastal areas are different ; some
areas have a well-defined coastal water mass circulating within a hydrographically enclosed area so that the rates of exchange, and so of loss
of an euphausiid species, are slow. Intensive sampling of plankton in the
North Atlantic over recent years has shown that individuals of several
different warm water species may be found far to the north of their
general areas of occurrence (Mauchline, 1965b). This indicates that
there is probably, in most populations, a continuous loss of individuals
to probably unfavourable environments. If, as is suggested above,
behavioural mechanisms are important in maintaining aggregations
and act along with hydrographic conditions to maintain a species in a
favourable area then individuals which L ‘ wander )’ may wander
because they are abnormal physiologically. For instance, if during one
season luminescent behaviour is the main mechanism in maintaining
an aggregation and for physiological reasons an individual in the swarm
does not react norinally to the stimulus of the population’s luminescence, then it may not be held in the population but be subject to
factors moving it away from the main population. There is some