170
THE BIOLOGY O F EUPHAUSIIDS
This recombination would prevent the development of new specialized
species unable to adapt to a changing environment. McLaren draws
on the evidence from studies of the three most abundant copepods in
northern seas-the Calanus finmarchicus (Gunn.) group, the Pseudocalanus minutus (Kroyer) group, and the Oithona similis (Claus) group
-to show that 0. similis, the only one of the three which performs little
or no vertical migration, is the least polytypic of these three groups.
He therefore agrees with David himself who concludes that his theory is
" regrettably speculative and what evidence there is which might
support it is both slight and circumstantial ',.
Next, McLaren examines the theory of Wynne-Edwards (1962).
He summarizes Wynne-Edwards' Theory as follows : " animals by
periodically moving vertically concentrate themselves near the surface
by night, although sometimes by day and sometimes a t other levels in
the water column (e.g. at thermoclines), in order to carry out social
(' epideictic ') displays which serve to plot the reproductive strategy
of the population. By sensing their population density, animals limit
the rate of increase to prevent the catastrophic reduction of available
resources." McLaren disagrees completely with this theory and states
" that there is simply no evidence for the social control of the intrinsic
rate of increase of zooplankters ',. Finally, McLaren dispenses with
Harris' (1953) theory which suggested that vertical migration was
simply a means of maintaining an optimum depth within the euphotic
zone and that it had no adaptive significance. An optimal depth could
be maintained much more efficiently in respect to energy expenditure
through sensing of pressure rather than through sensing of light.
There are valid grounds for several of McLaren's criticisms of these
different theories of the significance of vertical migration, but before
we agree or disagree with him let us examine his own theory of the
significance of this phenomenon. He reviews the available data on the
effect of temperature on generation length and adult size and also the
relationship of fecundity (or brood size) to body length in small planktonic organisms, and in the chaetognath, Sagitta elegans Verrill. He
shows that, in general, low temperature increases the time for development and increases the size of adults. Increasing fecundity is associated
with increasing body size. McLaren says that " what we have to consider when vertical migration is carried out in thermally stratified waters
are the consequences of feeding at temperatures which average higher than
the temperatures governing growth and development (assuming them to be
continuous). Qualitatively this can be understood as giving an energy
bonus through the more efficient uptake of food at higher temperatures,
together with the more efficient directing of energy to growth a t lower
THE BIOLOGY O F EUPHAUSIIDS
This recombination would prevent the development of new specialized
species unable to adapt to a changing environment. McLaren draws
on the evidence from studies of the three most abundant copepods in
northern seas-the Calanus finmarchicus (Gunn.) group, the Pseudocalanus minutus (Kroyer) group, and the Oithona similis (Claus) group
-to show that 0. similis, the only one of the three which performs little
or no vertical migration, is the least polytypic of these three groups.
He therefore agrees with David himself who concludes that his theory is
" regrettably speculative and what evidence there is which might
support it is both slight and circumstantial ',.
Next, McLaren examines the theory of Wynne-Edwards (1962).
He summarizes Wynne-Edwards' Theory as follows : " animals by
periodically moving vertically concentrate themselves near the surface
by night, although sometimes by day and sometimes a t other levels in
the water column (e.g. at thermoclines), in order to carry out social
(' epideictic ') displays which serve to plot the reproductive strategy
of the population. By sensing their population density, animals limit
the rate of increase to prevent the catastrophic reduction of available
resources." McLaren disagrees completely with this theory and states
" that there is simply no evidence for the social control of the intrinsic
rate of increase of zooplankters ',. Finally, McLaren dispenses with
Harris' (1953) theory which suggested that vertical migration was
simply a means of maintaining an optimum depth within the euphotic
zone and that it had no adaptive significance. An optimal depth could
be maintained much more efficiently in respect to energy expenditure
through sensing of pressure rather than through sensing of light.
There are valid grounds for several of McLaren's criticisms of these
different theories of the significance of vertical migration, but before
we agree or disagree with him let us examine his own theory of the
significance of this phenomenon. He reviews the available data on the
effect of temperature on generation length and adult size and also the
relationship of fecundity (or brood size) to body length in small planktonic organisms, and in the chaetognath, Sagitta elegans Verrill. He
shows that, in general, low temperature increases the time for development and increases the size of adults. Increasing fecundity is associated
with increasing body size. McLaren says that " what we have to consider when vertical migration is carried out in thermally stratified waters
are the consequences of feeding at temperatures which average higher than
the temperatures governing growth and development (assuming them to be
continuous). Qualitatively this can be understood as giving an energy
bonus through the more efficient uptake of food at higher temperatures,
together with the more efficient directing of energy to growth a t lower