THE BIOLOGY O F WOOD-BORING TEREDINID MOLLUSCS
397
employed for determining the breeding season of invertebrates, those
widely used for shipworms have been (1) to observe the incidence of
larvae based on regular collection of a given species from a known
volume of water and the assessment of the density of larval populations
during the different months in the plankton (Lebour, 1938 ; Nair, 1957 ;
Quayle, 1953), and (2) t o determine the time of settlement based on a
study of the relative abundance of post settlement stages in methodically operated test panels (Fraser, 1925; Johnson and Miller, 1935;
Neave, 1943; Black and Elsey, 1947, 1947a, 1948; Ralph and Hurley,
1952 ; Nair, 1957, 1965, 1966 ; Saraswathy and Nair, 1969). This second
method is of great practical significance since this alone provides a
reliable measure of breeding success, there being the possibility of
spawning without settlement. These methods are fairly reliable and
useful in the case of marine forms and those that live in enclosed waters
of a harbour. Nevertheless, they are not very reliable in special situations such as a harbour with typical estuarine conditions having
oscillatory water movements with the influence of the tide at one end
and the inflow of fresh water at the other complicating the water movements. To determine the exact breeding season of such estuarine forms
(e.g. Nausitora hedleyi) Saraswathy (1967) employed a quantitative
assessment of the condition of the gonad using the technique of gonad
index developed by Giese and his collaborators (Bennet and Giese,
1955; Giese et al., 1959; Tucker and Giese, 1962; Giese et al., 1964).
In this method possibilities of errors due to recruitment of larvae from
marine sources are also excluded. A systematic examination of the
gonad condition gives a fairly accurate picture of the breeding season.
This method is based on the assumption that in individuals large enough
to be mature, a spent or immature gonad is small, whereas a ripe gonad
is large. A gonad which decreases in size suddenly is assumed to have
lost its gametes-" spawned " (Giese, 1959). The gonad index provides
accurate information on the breeding condition of individual animals
and represents a measure of the reproductive condition of the p o p -
lation. I n the calculation of the index the gonad weight is expressed in
terms of the percentage of the body weight. By the application of this
method, not only can the individual reproductive condition be quantitatively assessed, but a graphic representation of the reproductive cycle
of the species in question is possible. The data thus obtained may be
supplemented with those drawn from the two other methods. The
average values for the different months in the case of N . hedleyi (Table
V) show that the gonad index is low during the period January to June.
For the rest of the period the values were above 15. The lowest value
recorded was for June (4.99). The sudden decrease was probably owing
397
employed for determining the breeding season of invertebrates, those
widely used for shipworms have been (1) to observe the incidence of
larvae based on regular collection of a given species from a known
volume of water and the assessment of the density of larval populations
during the different months in the plankton (Lebour, 1938 ; Nair, 1957 ;
Quayle, 1953), and (2) t o determine the time of settlement based on a
study of the relative abundance of post settlement stages in methodically operated test panels (Fraser, 1925; Johnson and Miller, 1935;
Neave, 1943; Black and Elsey, 1947, 1947a, 1948; Ralph and Hurley,
1952 ; Nair, 1957, 1965, 1966 ; Saraswathy and Nair, 1969). This second
method is of great practical significance since this alone provides a
reliable measure of breeding success, there being the possibility of
spawning without settlement. These methods are fairly reliable and
useful in the case of marine forms and those that live in enclosed waters
of a harbour. Nevertheless, they are not very reliable in special situations such as a harbour with typical estuarine conditions having
oscillatory water movements with the influence of the tide at one end
and the inflow of fresh water at the other complicating the water movements. To determine the exact breeding season of such estuarine forms
(e.g. Nausitora hedleyi) Saraswathy (1967) employed a quantitative
assessment of the condition of the gonad using the technique of gonad
index developed by Giese and his collaborators (Bennet and Giese,
1955; Giese et al., 1959; Tucker and Giese, 1962; Giese et al., 1964).
In this method possibilities of errors due to recruitment of larvae from
marine sources are also excluded. A systematic examination of the
gonad condition gives a fairly accurate picture of the breeding season.
This method is based on the assumption that in individuals large enough
to be mature, a spent or immature gonad is small, whereas a ripe gonad
is large. A gonad which decreases in size suddenly is assumed to have
lost its gametes-" spawned " (Giese, 1959). The gonad index provides
accurate information on the breeding condition of individual animals
and represents a measure of the reproductive condition of the p o p -
lation. I n the calculation of the index the gonad weight is expressed in
terms of the percentage of the body weight. By the application of this
method, not only can the individual reproductive condition be quantitatively assessed, but a graphic representation of the reproductive cycle
of the species in question is possible. The data thus obtained may be
supplemented with those drawn from the two other methods. The
average values for the different months in the case of N . hedleyi (Table
V) show that the gonad index is low during the period January to June.
For the rest of the period the values were above 15. The lowest value
recorded was for June (4.99). The sudden decrease was probably owing