428
N. BALAKRISHNAN NAIR AND M. SARASWATHY
relations have been obtained with reference to burrow length and time,
number of cones on pallets and time, wet weight and length, dry weight
and length and between the number of pallet cones and the number
of ridges on the shell, no such relation was evident between pallet
length and burrow length as obtained by Hurley (1959) for Bankia
australis wherein 1 mm of pallet length was found to represent 1 cm of
total length (as indicated by burrow length). Failure to obtain valid
correlation between these two characters was probably due t o the
slackening of growth in length owing to overcrowding, while the pallets
continued growth after that of the body had ceased. During the period
of maximum growth, the salinity remained between 8 and 20%,. Below
and above this range growth was negligible.
XIV. DISPERSAL OF SHIPWORMS
Notwithstanding the stationary, hidden life within the confines of
their wooden burrows, shipworms are distributed far and wide through
their free-swimming larval stages. While some species liberate eggs into
the water, others brood the eggs, the veligers being releasedwhenready.
During the free-swimming period which may last from a few hours
to even a month depending on the species and the region, the larvae
are drifted about and widely transported in the surface currents.
Three types of larval life could be recognized among shipworms:
(1) oviparous ; (2) short-term larviparous ; (3) long-term larviparous.
The pattern of distribution of many species is based on the temperature
and salinity requirements of the larvae during their planktotrophic life.
The members of the genus Nausitora, generally denizens of brackish
water habitats, are oviparous and fertilization is external (Watson et al.,
1936; Smith, 1963; Saraswathy, 1967). Since the larvae are incapable
of tolerating high salinities (Saraswathy, 1967), those larvae transported by the medium into areas of high salinities would perish. This
accounts for their occurrence in isolated pockets of brackish water. In
this case colonization of new areas could be effected only by the adults
which are more tolerant of higher salinities than their planktonic larvae
and are transported through the agency of driftwood etc., to a suitable
brackish-water environment within the lifetime of the adult. The
picture of the distribution of this genus seems to support this contention.
Nausitora hedleyi and N . dunlopei have so far been reported only from
isolated brackish-water areas along the tropical Indo-west Pacific,
N . dryas and N . excolpa along the tropical eastern Pacific and N .
fusticula along the tropical western Atlantic.
Species of Bankia are also oviparous with protracted planktotrophic
larval stages but their habitat is normally marine. In this case the
N. BALAKRISHNAN NAIR AND M. SARASWATHY
relations have been obtained with reference to burrow length and time,
number of cones on pallets and time, wet weight and length, dry weight
and length and between the number of pallet cones and the number
of ridges on the shell, no such relation was evident between pallet
length and burrow length as obtained by Hurley (1959) for Bankia
australis wherein 1 mm of pallet length was found to represent 1 cm of
total length (as indicated by burrow length). Failure to obtain valid
correlation between these two characters was probably due t o the
slackening of growth in length owing to overcrowding, while the pallets
continued growth after that of the body had ceased. During the period
of maximum growth, the salinity remained between 8 and 20%,. Below
and above this range growth was negligible.
XIV. DISPERSAL OF SHIPWORMS
Notwithstanding the stationary, hidden life within the confines of
their wooden burrows, shipworms are distributed far and wide through
their free-swimming larval stages. While some species liberate eggs into
the water, others brood the eggs, the veligers being releasedwhenready.
During the free-swimming period which may last from a few hours
to even a month depending on the species and the region, the larvae
are drifted about and widely transported in the surface currents.
Three types of larval life could be recognized among shipworms:
(1) oviparous ; (2) short-term larviparous ; (3) long-term larviparous.
The pattern of distribution of many species is based on the temperature
and salinity requirements of the larvae during their planktotrophic life.
The members of the genus Nausitora, generally denizens of brackish
water habitats, are oviparous and fertilization is external (Watson et al.,
1936; Smith, 1963; Saraswathy, 1967). Since the larvae are incapable
of tolerating high salinities (Saraswathy, 1967), those larvae transported by the medium into areas of high salinities would perish. This
accounts for their occurrence in isolated pockets of brackish water. In
this case colonization of new areas could be effected only by the adults
which are more tolerant of higher salinities than their planktonic larvae
and are transported through the agency of driftwood etc., to a suitable
brackish-water environment within the lifetime of the adult. The
picture of the distribution of this genus seems to support this contention.
Nausitora hedleyi and N . dunlopei have so far been reported only from
isolated brackish-water areas along the tropical Indo-west Pacific,
N . dryas and N . excolpa along the tropical eastern Pacific and N .
fusticula along the tropical western Atlantic.
Species of Bankia are also oviparous with protracted planktotrophic
larval stages but their habitat is normally marine. In this case the