REARING O F BIVALVE MOLLUSKS
107
The general behavior, food requirements and growth rate of larvae
of C. gigas also appear to be identical to those of larvae of the American
oyster. Even the size of the eggs arid the size at which larvae of the
tmo species begin to metamorphose are alike. A comparison of the
length-width relationship of larvae of the two groups will still further
indicate their similarity. Yet, regardless of this similarity, the attempt8
of Davis (1950) and Imai et al. (1950b) to hybridize these two species
were unsuccessful. Although fertilization occurred and the larvae
developed into veligers, they all died before reaching metamorphosis.
Gonad development and spawning of Japanese oysters kept at
Milford have not been as thoroughly investigated as those of the
American oyster. Nevertheless, there appear to be several important
differences. First, Japanese oysters require higher temperatures to
begin spawning. Groups of these oysters kept in the laboratory all
summer did not spawn, although the temperature at times was over
26°C. Furthermore, C. gigas either does not resorb the gonad material
that remains undischarged at the end of the spawning season or does
so at a much later date and at lower temperatures than C. virginim.
Japanese oysters, brought to Milford from Piiget Sound in October
with undischarged gonads, held this material far into the winter and
active sperm was found in some oysters late in December.
I n our waters growth of post-setting stages of Japanese oysters
is much more rapid than that of the local species. C. gigas that set
in the laboratory in the spring and then were planted in Milford Harbor
were 25 to 3 in long by the end of the growing season, while native
oysters, which had set, at the same time and were grown under identical
conditions, were only 1 to 14 in long.
K. Lnevicardium wwrtoni (Conrad)
This bivalve, which is called Cardium nmrtoni by some authors and
is also known as Morton's cockle, is quite common in the comparatively shallow waters of Long Island Sound. If brought into the
laboratory in late January and conditioned at temperatures between
15" arid 20°C for several weeks, these cockles are easily induced to
spawn by temperature stimulation alone. On several occasions
spawning of ripe cockles began in less than 5 min after the water
temperature was raised only 1" or 2" above 20°C. The majority spawned
within fr hr and, &s a rule, all began spawning before the water reached
30°C. Some cockles spawned at least three times during a single week.
The hermaphroditic cockles often acted first as males and then as
females in the same spawning. Sometimes, after first discharging
sperm, they released large quantities of eggs and finally ejected sperm
107
The general behavior, food requirements and growth rate of larvae
of C. gigas also appear to be identical to those of larvae of the American
oyster. Even the size of the eggs arid the size at which larvae of the
tmo species begin to metamorphose are alike. A comparison of the
length-width relationship of larvae of the two groups will still further
indicate their similarity. Yet, regardless of this similarity, the attempt8
of Davis (1950) and Imai et al. (1950b) to hybridize these two species
were unsuccessful. Although fertilization occurred and the larvae
developed into veligers, they all died before reaching metamorphosis.
Gonad development and spawning of Japanese oysters kept at
Milford have not been as thoroughly investigated as those of the
American oyster. Nevertheless, there appear to be several important
differences. First, Japanese oysters require higher temperatures to
begin spawning. Groups of these oysters kept in the laboratory all
summer did not spawn, although the temperature at times was over
26°C. Furthermore, C. gigas either does not resorb the gonad material
that remains undischarged at the end of the spawning season or does
so at a much later date and at lower temperatures than C. virginim.
Japanese oysters, brought to Milford from Piiget Sound in October
with undischarged gonads, held this material far into the winter and
active sperm was found in some oysters late in December.
I n our waters growth of post-setting stages of Japanese oysters
is much more rapid than that of the local species. C. gigas that set
in the laboratory in the spring and then were planted in Milford Harbor
were 25 to 3 in long by the end of the growing season, while native
oysters, which had set, at the same time and were grown under identical
conditions, were only 1 to 14 in long.
K. Lnevicardium wwrtoni (Conrad)
This bivalve, which is called Cardium nmrtoni by some authors and
is also known as Morton's cockle, is quite common in the comparatively shallow waters of Long Island Sound. If brought into the
laboratory in late January and conditioned at temperatures between
15" arid 20°C for several weeks, these cockles are easily induced to
spawn by temperature stimulation alone. On several occasions
spawning of ripe cockles began in less than 5 min after the water
temperature was raised only 1" or 2" above 20°C. The majority spawned
within fr hr and, &s a rule, all began spawning before the water reached
30°C. Some cockles spawned at least three times during a single week.
The hermaphroditic cockles often acted first as males and then as
females in the same spawning. Sometimes, after first discharging
sperm, they released large quantities of eggs and finally ejected sperm