ASPEOTS OF STRESS IN THE TROPICAL MARINE ENVIRONMENT
266
and towards the equator, although with a possible small rise close to
the equator. A wide intertidal zone is advantageous in carrying a large
population of algae and animals whose spores and eggs or larvae are an
important source of food to filter feeding animals in shallow waters.
A large tidal range also results in strong tidal currents which flush
both the intertidal zone and the sublittoral with resulting good food
supply, oxygenation and removal of waste products.
In the intertidal zone, except in the neighborhood of mid-tide,
conditions remain relatively constant at any given level when there is
little difference in range between neap and spring tides, whereas, when
the ranges differ much, conditions are less stable. The spring:neap
ratio is high in the tropics and the Arctic and low in mid-latitudes.
Incident radiation is not greatly different in summer in the tropics
and in temperate latitudes, but the total for the year is less in the
latter, and the tropics lack the seasonal fluctuation of the temperate
regions.
Most of these environmental factors suggest the most favorable
conditions in mid-latitudes, with poorer conditions in the tropics and,
usually also, in polar seas.
The first biological indicator of stress considered was temperature
tolerance, on the theory that tolerances tend to decrease under stress.
Tolerance ranges in the tropics were only about half those in temperate
waters, and in polar waters also there was a decrease, although not as
great. The next indicator taken was the percentage of the intertidal
zone occupied by organisms. Since this is already a region of considerable stress, the results of any additional stress may be exaggerated.
The average tropical species occupies only about half as much of the
intertidal zone as the average temperate species. Polar species, although
too few to yield adequate data, apparently are similarly restricted in
their zonation. There are more species in the tropics than in temperate
regions, so restriction of zonation might be due to increased competition
for space or food but our impression is that there is, if anything, less
crowding on tropical shores. In any case, zones appear to be restricted
also in cold seas, and these have fewer species. The other aspect of
intertidal zonation is the zone more or less defined by high and low
water respectively of neap tides. This is displaced downwards in
warmer waters. At the warmest localities the intertidal zonation
widens out again somewhat as if some species there have become
specially adapted to tropic life.
Growth rates before sexual maturity increase with increasing
temperature, but here also some species in the tropics appear to have
become adapted to the conditions and have reduced their growth rate,
266
and towards the equator, although with a possible small rise close to
the equator. A wide intertidal zone is advantageous in carrying a large
population of algae and animals whose spores and eggs or larvae are an
important source of food to filter feeding animals in shallow waters.
A large tidal range also results in strong tidal currents which flush
both the intertidal zone and the sublittoral with resulting good food
supply, oxygenation and removal of waste products.
In the intertidal zone, except in the neighborhood of mid-tide,
conditions remain relatively constant at any given level when there is
little difference in range between neap and spring tides, whereas, when
the ranges differ much, conditions are less stable. The spring:neap
ratio is high in the tropics and the Arctic and low in mid-latitudes.
Incident radiation is not greatly different in summer in the tropics
and in temperate latitudes, but the total for the year is less in the
latter, and the tropics lack the seasonal fluctuation of the temperate
regions.
Most of these environmental factors suggest the most favorable
conditions in mid-latitudes, with poorer conditions in the tropics and,
usually also, in polar seas.
The first biological indicator of stress considered was temperature
tolerance, on the theory that tolerances tend to decrease under stress.
Tolerance ranges in the tropics were only about half those in temperate
waters, and in polar waters also there was a decrease, although not as
great. The next indicator taken was the percentage of the intertidal
zone occupied by organisms. Since this is already a region of considerable stress, the results of any additional stress may be exaggerated.
The average tropical species occupies only about half as much of the
intertidal zone as the average temperate species. Polar species, although
too few to yield adequate data, apparently are similarly restricted in
their zonation. There are more species in the tropics than in temperate
regions, so restriction of zonation might be due to increased competition
for space or food but our impression is that there is, if anything, less
crowding on tropical shores. In any case, zones appear to be restricted
also in cold seas, and these have fewer species. The other aspect of
intertidal zonation is the zone more or less defined by high and low
water respectively of neap tides. This is displaced downwards in
warmer waters. At the warmest localities the intertidal zonation
widens out again somewhat as if some species there have become
specially adapted to tropic life.
Growth rates before sexual maturity increase with increasing
temperature, but here also some species in the tropics appear to have
become adapted to the conditions and have reduced their growth rate,