452
N. BALAKRISHNAN NAIR AND M. SARASWATHY
1861 ; Bartsch, 1922, 1931). For the permanent life of a species in an
area, the prevailing temperature must never be so high or so low at any
time 8s to kill the organism and the temperature must be suitable for a
sufficient period to permit the reproduction, larval development and
growth to sexual maturity of the species. The limitation in the geographical range is brought about by the most susceptible stage and the
boundary of distribution is determined by the weakest link in the life
cycle with respect to temperature tolerance. Other life stages may
sometimes extend temporarily beyond the area of permanent cxistence.
Osler (1 826) noted that though nearly 4 000 vessels arrived at Swansea annually shipworms had not been seen for years. It is probable that
those brought from warmer areas are destroyed by the low winter
temperatures. I n those instances where the adults with their comparatively greater powers of endurance are transported to areas where
conditions are not very satisfactory, the adult animal may live at least
vegetatively for some time though not capable of sexual maturity and
spawning. Thus the absence of sufficient warmth for the susceptible
early stages of development prevents the establishment of a native
breeding population despite the fact that adults may be met with in
the hulls of boats that visit the port or in the drift material cast ashore.
B. The effects of salinity
Salinity affects the organism by influencing the density of the
medium and through variations in osmotic pressure. I n tropical
estuaries the wet season with low salinity is in summer. Though lower
salinities may be tolerated at summer temperatures (Moore, 1966)
where this drops suddenly and then continues at a low level for a long
period stenohaline species are likely to be killed (Nair, 1965a).
The reaction of shipworms to different salinities varies widely. Some
species can tolerate only high salinities, others can tolerate a fairly wide
range of salinities while a few are capable of enduring very low salinities
and even fresh water. Further, the salinity tolerance of the same
species may vary according t o the geographic location depending on
the prevailing temperature and may even vary in the same locality
during the different seasons of the year. This is due to the existence of
a complex correlation between the biological effects of temperature and
salinity, the former having the ability to modify the effects of the latter
and change the salinity range of an organism (Kinne, 1963). Loosanoff
and Davis (1963) have pointed out that in determining minimum,
maximum and optimal salinities, each species, especially those populating estuarine regions, must be studied individually.
N. BALAKRISHNAN NAIR AND M. SARASWATHY
1861 ; Bartsch, 1922, 1931). For the permanent life of a species in an
area, the prevailing temperature must never be so high or so low at any
time 8s to kill the organism and the temperature must be suitable for a
sufficient period to permit the reproduction, larval development and
growth to sexual maturity of the species. The limitation in the geographical range is brought about by the most susceptible stage and the
boundary of distribution is determined by the weakest link in the life
cycle with respect to temperature tolerance. Other life stages may
sometimes extend temporarily beyond the area of permanent cxistence.
Osler (1 826) noted that though nearly 4 000 vessels arrived at Swansea annually shipworms had not been seen for years. It is probable that
those brought from warmer areas are destroyed by the low winter
temperatures. I n those instances where the adults with their comparatively greater powers of endurance are transported to areas where
conditions are not very satisfactory, the adult animal may live at least
vegetatively for some time though not capable of sexual maturity and
spawning. Thus the absence of sufficient warmth for the susceptible
early stages of development prevents the establishment of a native
breeding population despite the fact that adults may be met with in
the hulls of boats that visit the port or in the drift material cast ashore.
B. The effects of salinity
Salinity affects the organism by influencing the density of the
medium and through variations in osmotic pressure. I n tropical
estuaries the wet season with low salinity is in summer. Though lower
salinities may be tolerated at summer temperatures (Moore, 1966)
where this drops suddenly and then continues at a low level for a long
period stenohaline species are likely to be killed (Nair, 1965a).
The reaction of shipworms to different salinities varies widely. Some
species can tolerate only high salinities, others can tolerate a fairly wide
range of salinities while a few are capable of enduring very low salinities
and even fresh water. Further, the salinity tolerance of the same
species may vary according t o the geographic location depending on
the prevailing temperature and may even vary in the same locality
during the different seasons of the year. This is due to the existence of
a complex correlation between the biological effects of temperature and
salinity, the former having the ability to modify the effects of the latter
and change the salinity range of an organism (Kinne, 1963). Loosanoff
and Davis (1963) have pointed out that in determining minimum,
maximum and optimal salinities, each species, especially those populating estuarine regions, must be studied individually.