44
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
conclusion of extreme sensitivity of these larvae to temperature
changes, beakers of 1-liter capacity containing larvae grown at a temperature of about 22°C and measuring about 200 p in length were
placed in a refrigerator at 2°C and returned to room temperature
following 6, 12 and 24 hr of refrigeration. Within a few hours the
predominating majority of larvae exposed to the low temperature for
6 and 12 hr were swimming and feeding normally. However, many of
the larvae which were refrigerated for 24 hr lost a portion of the velum
and eventually died. The mortality in each of the above groups at
the end of one week after return to room temperature was: control,
4.2%; 6-hr chilling, 6.5%; 12-hr chilling, 4.9%; and 24-hr chilling,
44.9%. Thus, even though exposed to a near freezing temperature
for a 24-hr period, more than half of the larvae survived and continued
to develop.
In still another experiment oyster larvae lived and grew when
subjected every 48 hr to a sharp drop in temperature, from 20" to 10°C,
for periods of 15 to 30 min, followed in a few minutes by an equally
abrupt return to 20°C. A majority of these larvae subsequently
reached metamorphosis. It would seem unlikely, therefore, as
claimed by Nelson (1921), that ordinary short-term temperature
fluctuations of only a few degrees, occurring in natural waters, could
be responsible for an appreciable, sometimes total mortality of
larvae.
It is also certain a t this time that bivalve larvae may survive
long periods with Iittle or no food. In many of our experiments several
control cultures survived from 2 to 3 weeks with little or no mortality,
even though they did not receive any food except that which was
present in the filtered sea water where they were kept. Moreover, in
our earlier experiments, before such good food forms as naked flagellates became available, many oyster larvae cultures were kept for more
than 40 days, although they did not show any growth. As already
mentioned, in some of these cultures setting began only after 50 days.
These observations demonstrate that bivalve larvae may tolerate
comparatively long periods of semi-starvation and some may even
reach setting size and metamorphose regardless of poor feeding conditions. It is improbable, therefore, that under natural conditions larval
populations of such mollusks as oystms will die within 2 or 3 days
because of a lack of sufficient quantities of Sood. It is clear, however,
that lack of food will prolong the larval period, thus increaaing the
loss of larvae because of predation and dispersal.
Larvae are also able to tolerate very low oxygen concentrations, at
least for short periods. For example, on several occasions a number of
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
conclusion of extreme sensitivity of these larvae to temperature
changes, beakers of 1-liter capacity containing larvae grown at a temperature of about 22°C and measuring about 200 p in length were
placed in a refrigerator at 2°C and returned to room temperature
following 6, 12 and 24 hr of refrigeration. Within a few hours the
predominating majority of larvae exposed to the low temperature for
6 and 12 hr were swimming and feeding normally. However, many of
the larvae which were refrigerated for 24 hr lost a portion of the velum
and eventually died. The mortality in each of the above groups at
the end of one week after return to room temperature was: control,
4.2%; 6-hr chilling, 6.5%; 12-hr chilling, 4.9%; and 24-hr chilling,
44.9%. Thus, even though exposed to a near freezing temperature
for a 24-hr period, more than half of the larvae survived and continued
to develop.
In still another experiment oyster larvae lived and grew when
subjected every 48 hr to a sharp drop in temperature, from 20" to 10°C,
for periods of 15 to 30 min, followed in a few minutes by an equally
abrupt return to 20°C. A majority of these larvae subsequently
reached metamorphosis. It would seem unlikely, therefore, as
claimed by Nelson (1921), that ordinary short-term temperature
fluctuations of only a few degrees, occurring in natural waters, could
be responsible for an appreciable, sometimes total mortality of
larvae.
It is also certain a t this time that bivalve larvae may survive
long periods with Iittle or no food. In many of our experiments several
control cultures survived from 2 to 3 weeks with little or no mortality,
even though they did not receive any food except that which was
present in the filtered sea water where they were kept. Moreover, in
our earlier experiments, before such good food forms as naked flagellates became available, many oyster larvae cultures were kept for more
than 40 days, although they did not show any growth. As already
mentioned, in some of these cultures setting began only after 50 days.
These observations demonstrate that bivalve larvae may tolerate
comparatively long periods of semi-starvation and some may even
reach setting size and metamorphose regardless of poor feeding conditions. It is improbable, therefore, that under natural conditions larval
populations of such mollusks as oystms will die within 2 or 3 days
because of a lack of sufficient quantities of Sood. It is clear, however,
that lack of food will prolong the larval period, thus increaaing the
loss of larvae because of predation and dispersal.
Larvae are also able to tolerate very low oxygen concentrations, at
least for short periods. For example, on several occasions a number of