THE BIOLOGY O F WOOD-BORING TEREDINID MOLLUSCS
419
south-west Scotland, Owen (1953a) observed that the larvae of Teredo
norvegica Spengler tend to settle in increased numbers in dimly lit
regions and that there is an obvious increase in the number of entry
holes with increase in depth, maximum attack being noticed at the
mud-line about 30 ft below mean tide level. Generally in shallow
waters the strike of teredines increases with depth (Kofoid et al., 1927 ;
Johnson and Miller, 1935 ; Edmondson, 1942 ; Black and Elsey, 1948 ;
Greenfield, 1952; Nair, 1966). Quayle (1953) who studied the distribution of larvae of Bankia setacea in British Columbia found that the
number of larvae increases with depth. Information regarding this
important aspect of activity of marine borers has been scanty from the
Indian Ocean area. At Visakhapatnam, India, Ganapati and Nagabhushanam (1955) studied the vertical distribution of Martesia striata.
They noticed a definite increase in the number of entry holes from the
high tide level down to about 3 ft below the low tide mark below which
there was a sharp decline down to the mud-line. The maximum intensity of attack was in the 3-ft zone above and below low water mark. It
is well known that the relations of closely allied species are different and
even those of the same may vary according to the hydrographic conditions locally prevailing (Clapp, 1935; somme, 1940), each species of
borer having its own characteristic preferences, life history and seasons
of settlement. Earlier studies have been chiefly based on random
observations on piles pulled out from harbours reporting the activity of
either the molluscan or crustacean borers and no effort has been made
to study the combined activity of the different species of borers in a
locality. Generally these animals work together in their destruction of
wood in close association with a variety of fouling organisms. An
attempt to study the nature of destruction by all the boring animals was
made by Nair (1966) at Cochin harbour (Fig. 38), a typical tropical
estuarine habitat on the south-west coast of India. The pattern of
distribution was found to be similar to that observed at Beaufort, at
Loch Ryan, Manati Bay or Oahu, the deepest blocks registering the
most severe attack.
In the case of the estuarine species Nausitora hedleyi, significant
differences have been noticed in the intensity of settlement at the different levels during the different periods of the year (Nair, 1966;
Saraswathy and Nair, 1969). The data presented in Tables VI and VTI
besides providing details regarding seasonal settlement, also give
information on the vertical distribution at intertidal and subtidal levels
and on the bottom for all the three significant periods of the year. The
pre-monsoon is a period when the salinity is fairly uniform and high
with little difference between the surface and the bottom (Ramamir-
419
south-west Scotland, Owen (1953a) observed that the larvae of Teredo
norvegica Spengler tend to settle in increased numbers in dimly lit
regions and that there is an obvious increase in the number of entry
holes with increase in depth, maximum attack being noticed at the
mud-line about 30 ft below mean tide level. Generally in shallow
waters the strike of teredines increases with depth (Kofoid et al., 1927 ;
Johnson and Miller, 1935 ; Edmondson, 1942 ; Black and Elsey, 1948 ;
Greenfield, 1952; Nair, 1966). Quayle (1953) who studied the distribution of larvae of Bankia setacea in British Columbia found that the
number of larvae increases with depth. Information regarding this
important aspect of activity of marine borers has been scanty from the
Indian Ocean area. At Visakhapatnam, India, Ganapati and Nagabhushanam (1955) studied the vertical distribution of Martesia striata.
They noticed a definite increase in the number of entry holes from the
high tide level down to about 3 ft below the low tide mark below which
there was a sharp decline down to the mud-line. The maximum intensity of attack was in the 3-ft zone above and below low water mark. It
is well known that the relations of closely allied species are different and
even those of the same may vary according to the hydrographic conditions locally prevailing (Clapp, 1935; somme, 1940), each species of
borer having its own characteristic preferences, life history and seasons
of settlement. Earlier studies have been chiefly based on random
observations on piles pulled out from harbours reporting the activity of
either the molluscan or crustacean borers and no effort has been made
to study the combined activity of the different species of borers in a
locality. Generally these animals work together in their destruction of
wood in close association with a variety of fouling organisms. An
attempt to study the nature of destruction by all the boring animals was
made by Nair (1966) at Cochin harbour (Fig. 38), a typical tropical
estuarine habitat on the south-west coast of India. The pattern of
distribution was found to be similar to that observed at Beaufort, at
Loch Ryan, Manati Bay or Oahu, the deepest blocks registering the
most severe attack.
In the case of the estuarine species Nausitora hedleyi, significant
differences have been noticed in the intensity of settlement at the different levels during the different periods of the year (Nair, 1966;
Saraswathy and Nair, 1969). The data presented in Tables VI and VTI
besides providing details regarding seasonal settlement, also give
information on the vertical distribution at intertidal and subtidal levels
and on the bottom for all the three significant periods of the year. The
pre-monsoon is a period when the salinity is fairly uniform and high
with little difference between the surface and the bottom (Ramamir-