256
HILARY B. MOORE
even down to polar values. These prove to be species which do not range
outside the tropics. Those which grow fast in the tropics are species
which are able to range out into cooler water. There is the possibility
that the slower growth in cooler waters might result from a shorter
growing season resulting from a shorter season of available phytoplankton. This possibility is improbable because there was no significant
difference found in temperate growth rate between phytoplanktondependent filter feeders and species which had a year-round supply of
available food.
The production of genital products is a serious energy drain, and
under conditions of stress there is likely to be little energy left for
somatic growth. The amount of body growth after sexual maturity is
greatest at mid-temperatures and less in cold and warm waters.
The seasonal pattern of growth shows a rather consistent maximum
in summer in temperate waters. I n the tropics there is a wide range of
pattern from summer maximum to winter maximum growth. Longevity
decreases markedly in warmer waters.
The proportion of large species was shown to decrease under the
stress conditions associated with life in deep water. It decreased
similarly in tropical and polar seas, and is greatest at midtemperatures. Despite this trend the largest species of molluscs occur
in, and are restricted to, warm waters.
Variability tends to increase with departure from optimal conditions.
This was demonstrated in the case of the coefficient of variation of the
size of shallow-water and deep-water molluscs. The coefficient was also
shown to be least at mid-temperatures and to increase towards the
tropics and cold waters in the data on growth rates, growth after
sexual maturity, intertidal zonation and proportion of large species.
Finally, it was suggested that it is advantageous to synchronize
spawning in a species. Spawning is most frequently triggered by rising
temperature, and some specific rise above winter minimum is probably
required. Such a threshold rise will be attained in spring or early
summer where there is a large seasonal temperature range in midlatitudes. I n polar and tropical seas there is a smaller seasonal range
and the threshold will not be reached until the hottest part of summer.
This would be in agreement with observed spawning seasons. It would
also agree with the observed increase in proportion of species which are
unsynchronized and breed throughout the year in the tropics.
A number of biological aspects, then, point to the tropics and cold
seas as being regions of greater stress than those with water of
intermediate temperatures. The same was found in the consideration
of environmental factors. At the same time several aspects point to
HILARY B. MOORE
even down to polar values. These prove to be species which do not range
outside the tropics. Those which grow fast in the tropics are species
which are able to range out into cooler water. There is the possibility
that the slower growth in cooler waters might result from a shorter
growing season resulting from a shorter season of available phytoplankton. This possibility is improbable because there was no significant
difference found in temperate growth rate between phytoplanktondependent filter feeders and species which had a year-round supply of
available food.
The production of genital products is a serious energy drain, and
under conditions of stress there is likely to be little energy left for
somatic growth. The amount of body growth after sexual maturity is
greatest at mid-temperatures and less in cold and warm waters.
The seasonal pattern of growth shows a rather consistent maximum
in summer in temperate waters. I n the tropics there is a wide range of
pattern from summer maximum to winter maximum growth. Longevity
decreases markedly in warmer waters.
The proportion of large species was shown to decrease under the
stress conditions associated with life in deep water. It decreased
similarly in tropical and polar seas, and is greatest at midtemperatures. Despite this trend the largest species of molluscs occur
in, and are restricted to, warm waters.
Variability tends to increase with departure from optimal conditions.
This was demonstrated in the case of the coefficient of variation of the
size of shallow-water and deep-water molluscs. The coefficient was also
shown to be least at mid-temperatures and to increase towards the
tropics and cold waters in the data on growth rates, growth after
sexual maturity, intertidal zonation and proportion of large species.
Finally, it was suggested that it is advantageous to synchronize
spawning in a species. Spawning is most frequently triggered by rising
temperature, and some specific rise above winter minimum is probably
required. Such a threshold rise will be attained in spring or early
summer where there is a large seasonal temperature range in midlatitudes. I n polar and tropical seas there is a smaller seasonal range
and the threshold will not be reached until the hottest part of summer.
This would be in agreement with observed spawning seasons. It would
also agree with the observed increase in proportion of species which are
unsynchronized and breed throughout the year in the tropics.
A number of biological aspects, then, point to the tropics and cold
seas as being regions of greater stress than those with water of
intermediate temperatures. The same was found in the consideration
of environmental factors. At the same time several aspects point to