224
HILb&Y B. MOORE
the appropriate proportion with the data for the semidiurnal tides, there
is no marked change in the shape of the curve in Fig. 6. If this twice
monthly variation results in added stress for intertidal organisms, then
such stress is greater in the tropics than in temperate waters, as is the
stress associated with small tidal range.
IV. BIOLOGICAL ASPECTS
A. Temperature tolerance
We have examined a number of environmental factors, several of
which suggest that the tropics are a region of higher stress than
temperate regions. The fact remains, however, that an outstanding
characteristic of tropical waters is that they are hot, and it is worth
considering whether the temperature is above what is optimal for life
in general.
It is a generally accepted principle that departure from optimal value
for a species in one environmental factor tends to reduce the tolerance
range for other factors. This correlation should work in both directions,
so the occurrence of reduced tolerance ranges should tend to indicate
departure of one or more factors from their optimal values, in other
words stress. Experimental determinations of tolerance ranges are
too few, and have employed too wide a range of techniques, to be
usable for an analysis. Good material is, however, available in the
recorded distribution of American molluscs. We have drawn mainly
on Abbott (1967), Johnson (1934) and Warmke and Abbott (1962).
Since we are considering a relationship to water temperature we have
excluded intertidal species and records from below twenty meters.
We have also excluded small species which might not have been adequately covered in some works. Disregarding the existence of physiological races, we have considered the temperature range of a species to
lie between the mean winter minimum at its poleward limit and the
mean summer maximum at its equatorward limit. These values were
obtained for all the species used. The data are so extensive that general
trends are unlikely to be significantly affected by errors in observation
or taxonomy. For each species the mean of its maximum and minimum
temperatures was also recorded. In a first analysis the species were
grouped by mean temperatures, and for each group the mean range
temperature was determined. However, a large range can center only
near the middle of the available temperature scale, while a small one
can center nearer either end. We can calculate what this effect would be
and compare our observed values with what would be predicted if the
effect acted on a population of tolerance ranges which was uniformly
HILb&Y B. MOORE
the appropriate proportion with the data for the semidiurnal tides, there
is no marked change in the shape of the curve in Fig. 6. If this twice
monthly variation results in added stress for intertidal organisms, then
such stress is greater in the tropics than in temperate waters, as is the
stress associated with small tidal range.
IV. BIOLOGICAL ASPECTS
A. Temperature tolerance
We have examined a number of environmental factors, several of
which suggest that the tropics are a region of higher stress than
temperate regions. The fact remains, however, that an outstanding
characteristic of tropical waters is that they are hot, and it is worth
considering whether the temperature is above what is optimal for life
in general.
It is a generally accepted principle that departure from optimal value
for a species in one environmental factor tends to reduce the tolerance
range for other factors. This correlation should work in both directions,
so the occurrence of reduced tolerance ranges should tend to indicate
departure of one or more factors from their optimal values, in other
words stress. Experimental determinations of tolerance ranges are
too few, and have employed too wide a range of techniques, to be
usable for an analysis. Good material is, however, available in the
recorded distribution of American molluscs. We have drawn mainly
on Abbott (1967), Johnson (1934) and Warmke and Abbott (1962).
Since we are considering a relationship to water temperature we have
excluded intertidal species and records from below twenty meters.
We have also excluded small species which might not have been adequately covered in some works. Disregarding the existence of physiological races, we have considered the temperature range of a species to
lie between the mean winter minimum at its poleward limit and the
mean summer maximum at its equatorward limit. These values were
obtained for all the species used. The data are so extensive that general
trends are unlikely to be significantly affected by errors in observation
or taxonomy. For each species the mean of its maximum and minimum
temperatures was also recorded. In a first analysis the species were
grouped by mean temperatures, and for each group the mean range
temperature was determined. However, a large range can center only
near the middle of the available temperature scale, while a small one
can center nearer either end. We can calculate what this effect would be
and compare our observed values with what would be predicted if the
effect acted on a population of tolerance ranges which was uniformly