406
N. BALAKRISHNAN NAIR AND M. SARASWATHY
physiological changes (Lane, 1955). Observed swimming rates suggest
that peak short-term velocity may approximate 7 mm per second. The
shell valves do not contribute to swimming nor does the muscular foot.
C. Duration of the larval period
The average duration of the free swimming stage of the veliger is
apparently constant for a given species at a given locality. Naturally
this period is shorter in the warmer waters of tropical and sub-tropical
regions.
The duration of the free swimming period in the non-incubatory
species has been determined in a few cases. Sigerfoos (1908) suggested
that veligers of Bankia gouldi swim about for a month or more at
Beaufort and the same may be said of B. setacea (Tryon) occurring along
the coast of British Columbia (Quayle, 1952). Nair (1956a) noted that in
Bankia indica free swimming life is about 17 days in the tropical waters
of Madras. For B. campanellata this period, according to Nagabhushanam (1959) is about 13 days.
In incubatory species, the duration of free swimming may range
from a few hours to several days. According to Coe (1941) Lyrodus
diegensis (=pedicellatus) has a brief free swimming period and some
attached themselves to the substrate soon after release. Others in the
same brood remained on the surface for 2 weeks or more before metamorphosis. Menzies (1951) concluded that larvae of T . diegensis must
settle on wood within 24 h to develop into normal individuals. Failing
this the shell becomes abnormal and they die within 2-6 weeks. The
larvae of T. bartschi near Miami, Florida are free swimming for not more
than 72 h (Lane, Tierney and Hennacy, 1954). Larvae of T . pedicellata
remain free swimming for 3-4 days only at Miami (Isham and Tierney,
1953), while in Saint Mato and Dinard (English Channel) they do so
only a few hours or days during which time there is no sign of growth
(Rancurel, 1951). I n the waters of Barnegat Bay, New Jersey the free
swimming stage of shipworm larvae is estimated to last 3-4 weeks
(Nelson, 1924). In the tropical incubatory species T . furcifera larvae
attack timber normally between 24-72 h (Karande et al., 1968). For
T . navalis the free swimming stage is reported as between 24-34 days
in Japanese waters (Imai et al., 1950) and about 24 weeks at Woods Hole
(Grave, 1928). Loosanoff and Davis (1963) cultured to metamorphosis,
eggs and early stages of development taken from the gill chamber of
T . navalis in 28 days at a temperature of 20°C. However, the time
required to reach metamorphosis varied, in some this occurred within
20 days after swarming when grown at room temperatures. I n better
growing conditions the free swimming period is shorter.
N. BALAKRISHNAN NAIR AND M. SARASWATHY
physiological changes (Lane, 1955). Observed swimming rates suggest
that peak short-term velocity may approximate 7 mm per second. The
shell valves do not contribute to swimming nor does the muscular foot.
C. Duration of the larval period
The average duration of the free swimming stage of the veliger is
apparently constant for a given species at a given locality. Naturally
this period is shorter in the warmer waters of tropical and sub-tropical
regions.
The duration of the free swimming period in the non-incubatory
species has been determined in a few cases. Sigerfoos (1908) suggested
that veligers of Bankia gouldi swim about for a month or more at
Beaufort and the same may be said of B. setacea (Tryon) occurring along
the coast of British Columbia (Quayle, 1952). Nair (1956a) noted that in
Bankia indica free swimming life is about 17 days in the tropical waters
of Madras. For B. campanellata this period, according to Nagabhushanam (1959) is about 13 days.
In incubatory species, the duration of free swimming may range
from a few hours to several days. According to Coe (1941) Lyrodus
diegensis (=pedicellatus) has a brief free swimming period and some
attached themselves to the substrate soon after release. Others in the
same brood remained on the surface for 2 weeks or more before metamorphosis. Menzies (1951) concluded that larvae of T . diegensis must
settle on wood within 24 h to develop into normal individuals. Failing
this the shell becomes abnormal and they die within 2-6 weeks. The
larvae of T. bartschi near Miami, Florida are free swimming for not more
than 72 h (Lane, Tierney and Hennacy, 1954). Larvae of T . pedicellata
remain free swimming for 3-4 days only at Miami (Isham and Tierney,
1953), while in Saint Mato and Dinard (English Channel) they do so
only a few hours or days during which time there is no sign of growth
(Rancurel, 1951). I n the waters of Barnegat Bay, New Jersey the free
swimming stage of shipworm larvae is estimated to last 3-4 weeks
(Nelson, 1924). In the tropical incubatory species T . furcifera larvae
attack timber normally between 24-72 h (Karande et al., 1968). For
T . navalis the free swimming stage is reported as between 24-34 days
in Japanese waters (Imai et al., 1950) and about 24 weeks at Woods Hole
(Grave, 1928). Loosanoff and Davis (1963) cultured to metamorphosis,
eggs and early stages of development taken from the gill chamber of
T . navalis in 28 days at a temperature of 20°C. However, the time
required to reach metamorphosis varied, in some this occurred within
20 days after swarming when grown at room temperatures. I n better
growing conditions the free swimming period is shorter.