448
N. BALAKRISHNAN NAIR AND M. SARASWATHY
process. This has often been cited as the reason why the tropical zone is
richer in genera and in species than elsewhere. The existence of numerous species along tropical coasts poses a serious problem regarding
specific identification and accurate assessment of the destruction each
species inflicts.
The majority of the known species of shipworms are active and
breed during the warmer months of the year (Kindle, 1918, M'Gonigle,
1926 ; Mackenzie, 1927 ; Nelson, 1928 ; Cheney and Searles, 1935;
Zvorykin, 1941 ; Tarasov, 1943; Dons, 1945, 1949; Mawatari, 1950;
Nagabhushanam, 1962). This is particularly conspicuous in the higher
latitudes where the seasons are well marked with wide temperature
variations. Here during the late spring and early summer, the temperature rises above the threshold and remains above that value sufficiently to allow active growth and reproduction. For the developmental and larval stages to survive, the water must be sufficiently
warm. Therefore, the parent is stimulated t o expel the gametes or the
larvae when the temperature of the water reaches a specific point for
each species. In the Adriatic, Teredo navalis, T . utriculus (larviparous)
and Bankia minima are summer breeders, Teredo pedicellata is a winter
breeder when the female phase occurs in T. utriculus (Roch, 1940).
However, there are such species as T . norvegica (Nair, 1959, 1962)
which can breed during the cold months of the year.
In higher latitudes a slight increase in the normal summer temperatures may lead to an increase in activity (Cheney and Searles, 1935;
Clapp, 1935 ; Tarasov, 1943 ; Riabchikov, 1957). Similarly a perceivable
retardation of their activity occurs during unusually long and severe
winters (Anon., 1943a) which may even completely destroy these pests
in certain areas (Smollett, 1790). Extremes of heat or cold may completely wipe out entire populations at whatever season the most
vulnerable stage of development occurs (Anon, 1943a). Shipworm
attacks are not necessarily annual ; they are periodically recurring
phenomena. One among the several reasons for this especially in higher
latitudes is temperature.
Adult shipworms are known to survive wide variations of temperature and salinity (Anon, 1927 ; Chenney and Searles, 1935). This
enables them to be transported through wide areas where they are
likely to experience without serious ill effects temperatures either above
or below the optimum for the species. For T. navalis of the Atlantic
coast of Canada 22-5OC appears the optimum and a temperature of
7.5"C is unfavourable (Anon, 1927). At Kristineberg (Sweden), T.
navalis is most active in the temperature range between 15-25OC and
can tolerate temperatures from 5 to 30°C. It can even survive for some
N. BALAKRISHNAN NAIR AND M. SARASWATHY
process. This has often been cited as the reason why the tropical zone is
richer in genera and in species than elsewhere. The existence of numerous species along tropical coasts poses a serious problem regarding
specific identification and accurate assessment of the destruction each
species inflicts.
The majority of the known species of shipworms are active and
breed during the warmer months of the year (Kindle, 1918, M'Gonigle,
1926 ; Mackenzie, 1927 ; Nelson, 1928 ; Cheney and Searles, 1935;
Zvorykin, 1941 ; Tarasov, 1943; Dons, 1945, 1949; Mawatari, 1950;
Nagabhushanam, 1962). This is particularly conspicuous in the higher
latitudes where the seasons are well marked with wide temperature
variations. Here during the late spring and early summer, the temperature rises above the threshold and remains above that value sufficiently to allow active growth and reproduction. For the developmental and larval stages to survive, the water must be sufficiently
warm. Therefore, the parent is stimulated t o expel the gametes or the
larvae when the temperature of the water reaches a specific point for
each species. In the Adriatic, Teredo navalis, T . utriculus (larviparous)
and Bankia minima are summer breeders, Teredo pedicellata is a winter
breeder when the female phase occurs in T. utriculus (Roch, 1940).
However, there are such species as T . norvegica (Nair, 1959, 1962)
which can breed during the cold months of the year.
In higher latitudes a slight increase in the normal summer temperatures may lead to an increase in activity (Cheney and Searles, 1935;
Clapp, 1935 ; Tarasov, 1943 ; Riabchikov, 1957). Similarly a perceivable
retardation of their activity occurs during unusually long and severe
winters (Anon., 1943a) which may even completely destroy these pests
in certain areas (Smollett, 1790). Extremes of heat or cold may completely wipe out entire populations at whatever season the most
vulnerable stage of development occurs (Anon, 1943a). Shipworm
attacks are not necessarily annual ; they are periodically recurring
phenomena. One among the several reasons for this especially in higher
latitudes is temperature.
Adult shipworms are known to survive wide variations of temperature and salinity (Anon, 1927 ; Chenney and Searles, 1935). This
enables them to be transported through wide areas where they are
likely to experience without serious ill effects temperatures either above
or below the optimum for the species. For T. navalis of the Atlantic
coast of Canada 22-5OC appears the optimum and a temperature of
7.5"C is unfavourable (Anon, 1927). At Kristineberg (Sweden), T.
navalis is most active in the temperature range between 15-25OC and
can tolerate temperatures from 5 to 30°C. It can even survive for some