456
N. BALAKRISHNAN NAIR AND M. SARASWATHY
where species sensitive to low salinities cannot survive. Thus the genus
Nausitora effectively occupies an ecological niche and so extends the
zone of opcration of shipworms to river mouths and even farther
upstream. The construction of dams both for hydroelectric power
and irrigation across the rivers and the consequent check on the
river flow may lead t o greater spread of tidal water upstream
and this can result in the extension of shipworm activity. Psiloteredo
healdi is reported as a freshwater form from Miraflores Lake, Panama,
Lake Maracaibo, Venezuela and other freshwater lakes and streams on
the north coast of South America.
References t o salinity tolerance of shipworm larvae are few.
Scheltema and Truitt ( 1954) have reviewed previous observations
indicating that larvae cannot tolerate the lower limits of salinities
which can be endured by adults. M’Gonigle (1926) finds lo%, detrimental t o Teredo navalis larvae. Edmondson (1942) noticed that (‘ 60 h
is about an average period of resistance of various species, of Hawaiian
shipworms to fresh water, and that the larvae could survive only for
much shorter time.” Observations at Cochin on Nausitora hedleyi
suggest (see Table X) that for the early development of the species the
most suitable salinity range is from 11.24-14.54%,. Above and below
this segmentation was abnormal and the percentage of normal embryos
declined. I n salinities lower than 4.36 and above 27.14%, there was no
evidence of development (Saraswathy, 1967). Loosanoff and Davis
(1963) have observed in the case of oysters that the older the larvae
the better they withstood salinity variations.
Most species of shipworms require normal marine conditions for
successful spawning but the adults may withstand unfavourable conditions by closing the burrow with the pallets. They are also able to
utilize stored glycogen under anaerobic conditions.
C. Occurrence in fresh water
Shipworms are not limited to the sea and brackish water. Nausitora
dunlopei has been reported in timbers in the river Comer, a tributory
of the Ganges, 150 miles above the mouth where the water is fresh
(Wright, 1864) and in similar situations in Brisbane, Queensland
(Johnson and Moore, 1950). Mariot (1965) reports from Cochin China
that teredo ” is found in timber structures and in submerged branches
of mangroves in the river Saigon which carries almost fresh water.
Shipworm activity has been reported in almost fresh water up the
Mississippi river (Putnam, 1880), from the Rewe river, Fiji (Steel, 1896),
Brazos river, Texas (McGlone, 1914), Upper Pedro Miguel Locks,
Panama canal (Embree, 1922), Chulalongkorn Lock in the Rangsit
N. BALAKRISHNAN NAIR AND M. SARASWATHY
where species sensitive to low salinities cannot survive. Thus the genus
Nausitora effectively occupies an ecological niche and so extends the
zone of opcration of shipworms to river mouths and even farther
upstream. The construction of dams both for hydroelectric power
and irrigation across the rivers and the consequent check on the
river flow may lead t o greater spread of tidal water upstream
and this can result in the extension of shipworm activity. Psiloteredo
healdi is reported as a freshwater form from Miraflores Lake, Panama,
Lake Maracaibo, Venezuela and other freshwater lakes and streams on
the north coast of South America.
References t o salinity tolerance of shipworm larvae are few.
Scheltema and Truitt ( 1954) have reviewed previous observations
indicating that larvae cannot tolerate the lower limits of salinities
which can be endured by adults. M’Gonigle (1926) finds lo%, detrimental t o Teredo navalis larvae. Edmondson (1942) noticed that (‘ 60 h
is about an average period of resistance of various species, of Hawaiian
shipworms to fresh water, and that the larvae could survive only for
much shorter time.” Observations at Cochin on Nausitora hedleyi
suggest (see Table X) that for the early development of the species the
most suitable salinity range is from 11.24-14.54%,. Above and below
this segmentation was abnormal and the percentage of normal embryos
declined. I n salinities lower than 4.36 and above 27.14%, there was no
evidence of development (Saraswathy, 1967). Loosanoff and Davis
(1963) have observed in the case of oysters that the older the larvae
the better they withstood salinity variations.
Most species of shipworms require normal marine conditions for
successful spawning but the adults may withstand unfavourable conditions by closing the burrow with the pallets. They are also able to
utilize stored glycogen under anaerobic conditions.
C. Occurrence in fresh water
Shipworms are not limited to the sea and brackish water. Nausitora
dunlopei has been reported in timbers in the river Comer, a tributory
of the Ganges, 150 miles above the mouth where the water is fresh
(Wright, 1864) and in similar situations in Brisbane, Queensland
(Johnson and Moore, 1950). Mariot (1965) reports from Cochin China
that teredo ” is found in timber structures and in submerged branches
of mangroves in the river Saigon which carries almost fresh water.
Shipworm activity has been reported in almost fresh water up the
Mississippi river (Putnam, 1880), from the Rewe river, Fiji (Steel, 1896),
Brazos river, Texas (McGlone, 1914), Upper Pedro Miguel Locks,
Panama canal (Embree, 1922), Chulalongkorn Lock in the Rangsit