THE BIOLOGY OF WOOD-BORING TEREDINID MOLLUSCS
451
1935). Thus in shipworms, as in oysters (see Stauber, 1950) local
physiological races may exist in which the spawning temperature,
dthough relatively constant at any given place, may differ markedly
dong different parts of the coast. The allied species B. gouldi in
Chesapeake Bay, Maryland, spawns only when the water temperature is
btween 16-20°C (Scheltema and Truitt, 1954).
Long spawning seasons may be due to various causes. If a species
lives over a considerable vertical range, then the critical temperature
for spawning will be reached at different dates at different depths
(Thorson, 1946), different age groups may spawn at different times or
srtch individual may spawn for short periods over the whole breeding
season and if this is rhythmical, it may result in a continuous production of young ones over a long period (see Fretter and Graham,
1964).
Within the range, growth rate is usually accelerated at higher temperatures. Thus in species from temperate waters such as B. setacea
(Quayle, 1953) and B. gouldi (Sigerfoos, 1908) the free swimming veliger
$age lasts for 3-4 weeks while in tropical forms such as B. indica (= B.
mrinata) it is only about 17 days (Nair, 1956a). However, a low tempersture, if not lethal, may prolong the pelagic life of the larva and this
may indirectly help it to find a piece of timber on which to settle and
metrimorphose and also facilitate wider dispersal of the larvae under
the influence of currents. But this extension of pelagic life also increases
the period during which the larvae are exposed to predation (Thorson,
1950). According to Imai et al. (1950) the duration of the larval period of
Teredo navalis in Onagawa Bay, Japan, is from 22 days to 1 month, the
shorter period corresponding to the higher temperatures. The most
sitive stage of shipworms is the embryo (6oth-8oth h) about the
time of shell formation (Anon., 1927; Cheney and Searles, 1935;
Zvorykin, 1941). Quayle (1956) reports that during summer the larvae
of Bankia setacea occur at deeper levels than in the winter, apparently
a reflection of temperature preference.
It has been claimed that the distribution of shipworms has been
effected to a larger extent through the passive dispersal of these within
driftwood (Moll, 1941a), floating seeds (Moll, 1936), wooden hulls of
boats (Kofoid et al., 1927 ; Brown, 1935), sea-water tanks of ships and
log booms (Kirkbride, 1922). Brown (1816) states that tropical species
have been carried to temperate or even polar regions. As a matter of
fact such dispersal has not been so widespread and extensive as the
claims indicate. The fact is that many tropical species have not been
able to establish themselves when transported by such agencies to
areas where the environmental conditions are not favourable (;Teffreys,
451
1935). Thus in shipworms, as in oysters (see Stauber, 1950) local
physiological races may exist in which the spawning temperature,
dthough relatively constant at any given place, may differ markedly
dong different parts of the coast. The allied species B. gouldi in
Chesapeake Bay, Maryland, spawns only when the water temperature is
btween 16-20°C (Scheltema and Truitt, 1954).
Long spawning seasons may be due to various causes. If a species
lives over a considerable vertical range, then the critical temperature
for spawning will be reached at different dates at different depths
(Thorson, 1946), different age groups may spawn at different times or
srtch individual may spawn for short periods over the whole breeding
season and if this is rhythmical, it may result in a continuous production of young ones over a long period (see Fretter and Graham,
1964).
Within the range, growth rate is usually accelerated at higher temperatures. Thus in species from temperate waters such as B. setacea
(Quayle, 1953) and B. gouldi (Sigerfoos, 1908) the free swimming veliger
$age lasts for 3-4 weeks while in tropical forms such as B. indica (= B.
mrinata) it is only about 17 days (Nair, 1956a). However, a low tempersture, if not lethal, may prolong the pelagic life of the larva and this
may indirectly help it to find a piece of timber on which to settle and
metrimorphose and also facilitate wider dispersal of the larvae under
the influence of currents. But this extension of pelagic life also increases
the period during which the larvae are exposed to predation (Thorson,
1950). According to Imai et al. (1950) the duration of the larval period of
Teredo navalis in Onagawa Bay, Japan, is from 22 days to 1 month, the
shorter period corresponding to the higher temperatures. The most
sitive stage of shipworms is the embryo (6oth-8oth h) about the
time of shell formation (Anon., 1927; Cheney and Searles, 1935;
Zvorykin, 1941). Quayle (1956) reports that during summer the larvae
of Bankia setacea occur at deeper levels than in the winter, apparently
a reflection of temperature preference.
It has been claimed that the distribution of shipworms has been
effected to a larger extent through the passive dispersal of these within
driftwood (Moll, 1941a), floating seeds (Moll, 1936), wooden hulls of
boats (Kofoid et al., 1927 ; Brown, 1935), sea-water tanks of ships and
log booms (Kirkbride, 1922). Brown (1816) states that tropical species
have been carried to temperate or even polar regions. As a matter of
fact such dispersal has not been so widespread and extensive as the
claims indicate. The fact is that many tropical species have not been
able to establish themselves when transported by such agencies to
areas where the environmental conditions are not favourable (;Teffreys,