HEARING OF BIVALVE MOLLUSKS
73
temperature of 30°C setting usually began on the 10th or 12th day
but, again, it extended for several days, as was the case with clam
larvae. At lower temperatures the beginning of setting was delayed
and its duration prolonged. For example, in one culture kept at a
temperature of 24°C the larvae metamorphosed continuously for
27 days.
The number of days needed for larvae of the same cultures to reach
setting stage is, therefore, not sharply dejined even for a culture reared
under standard, well-controlled conditions. In poorly fed, crowded
or generally neglected cultures the beginning of setting is usually
delayed and the time difference between beginning and end of setting
is considerably extended. Sometimes, in cultures kept under poor
conditions, towards the end of the experiment large numbers of larvae
were found which were apparently unable to grow beyond a certain
stage and eventually died without metamorphosing.
In earlier studies (Loosanoff and Davis, 1950) we were already
aware that at the time of metamorphosis the size of larvae varied,
even among those of the same species. At first, the smallest metamorphosing larvae of M . mercenaria were thought to be 210 p long
but, later, larvae measuring only about 175 p but already going
through this process were seen in the cultures. However, setting
occurs most commonly when larvae measure between 200 and 210 p
in length.
Often, in
the same cultures comparatively small individuals only about 180 p
in length will complete metamorphosis, while many larger larvae,
exceeding 220 p in length, still continue to swim.
Since there are several suggestions in the literature that invertebrate
larvae reach a larger size before metamorphosis if they develop at low
temperatures, we conducted a number of experiments to verify these
suggestions (Loosanoff, 1959). Thjs information was desired because
if it were true that there was a definite relationship between setting
size and temperature at which the larvae were grown, then it fihould
be possible to devise a formula to predict setting size for each temperature.
The studies were conducted with larvae of M . mercenaria a t temPeratures ranging from 18" to 30°C. Within this temperature range
there were no significant"differences with respect to mean length of
larvae at time of setting. Thus, although larvae grown at lower temPeratures required more time before reaching metamorphosis, in all
Cases the larvae attained approximately the same mean lengths before
setting. Furthermore, there was no apparent relationship between
The largest larvae do not always metamorphose first.
73
temperature of 30°C setting usually began on the 10th or 12th day
but, again, it extended for several days, as was the case with clam
larvae. At lower temperatures the beginning of setting was delayed
and its duration prolonged. For example, in one culture kept at a
temperature of 24°C the larvae metamorphosed continuously for
27 days.
The number of days needed for larvae of the same cultures to reach
setting stage is, therefore, not sharply dejined even for a culture reared
under standard, well-controlled conditions. In poorly fed, crowded
or generally neglected cultures the beginning of setting is usually
delayed and the time difference between beginning and end of setting
is considerably extended. Sometimes, in cultures kept under poor
conditions, towards the end of the experiment large numbers of larvae
were found which were apparently unable to grow beyond a certain
stage and eventually died without metamorphosing.
In earlier studies (Loosanoff and Davis, 1950) we were already
aware that at the time of metamorphosis the size of larvae varied,
even among those of the same species. At first, the smallest metamorphosing larvae of M . mercenaria were thought to be 210 p long
but, later, larvae measuring only about 175 p but already going
through this process were seen in the cultures. However, setting
occurs most commonly when larvae measure between 200 and 210 p
in length.
Often, in
the same cultures comparatively small individuals only about 180 p
in length will complete metamorphosis, while many larger larvae,
exceeding 220 p in length, still continue to swim.
Since there are several suggestions in the literature that invertebrate
larvae reach a larger size before metamorphosis if they develop at low
temperatures, we conducted a number of experiments to verify these
suggestions (Loosanoff, 1959). Thjs information was desired because
if it were true that there was a definite relationship between setting
size and temperature at which the larvae were grown, then it fihould
be possible to devise a formula to predict setting size for each temperature.
The studies were conducted with larvae of M . mercenaria a t temPeratures ranging from 18" to 30°C. Within this temperature range
there were no significant"differences with respect to mean length of
larvae at time of setting. Thus, although larvae grown at lower temPeratures required more time before reaching metamorphosis, in all
Cases the larvae attained approximately the same mean lengths before
setting. Furthermore, there was no apparent relationship between
The largest larvae do not always metamorphose first.