REARING OF BIVALVE MOLLUSKS
41
maximum sizes recorded on the 2lst day were 107 and 221 p. Our
colleagues working in the same field, especially Imai et al. (1954),
fully share our experiences.
At present, no well-based explanation can be advanced for these
differences. Perhaps, as has been suggested in connection with survival
and growth of certain fish, vitality of the individual eggs and larvae
that emerge from them depends to some extent upon the position of the
eggs in the ovaries and the amounts of nutritive materials that have
been stored in the individual eggs before they are discharged.
Chanley (1955) assumed that differences in sizes of larvae in the
same cultures must be due, a t least in part, to inherited characteristics.
He also reported evidence of significantly different rates of growth of
larvae originating from eggs of the samt' female crossed with different
males and larvae grown from eggs of two females individually crossed
with the same male. He tentatively concluded that inherited differences
from either parent may he responsible for differences in rate of growth
of different larvae.
In some cultures, especially those kept at comparatively high
temperatures, the range of larval sizes usually diminishes several days
after beginning of metamorphosis. This is due to the disappearance of
larger individuals because of setting and, partly, because abnormal,
undeveloped, slow-growing larvae are rapidly dying.
There were periods in our practice, for example, in growing larvae
of Ostrea edulis, when regardless of all efforts they would not grow at
all or ceased growing soon after reaching a size of about 220 p. The
reasons for cessation of growth still remain an enigma because, at
times, these larvae refused to grow wen when given foods on which,
in previous experiments, they grew well.
E. Hardiness of eggs icnd larvae
According to Nelson (1921) larvae of C. virginicu are extremely
sensitive to a sudden change in water temperature. A drop of only
3" to 5°C within 24 hr may be followed by the disappearance of a
majority of the larvae. According to the same author rain storms, as
well as strong winds, cause death of large numberu of bivalve larvae.
Nelson, however, failed to offer experimental evidence to support his
contention of the unusual sensitivity of bivalve larvae to relatively
minor changes in their environment. Our observations, reported partly
in this section and partly in the sections to follow, lead us to disagree
with Nelson's point of view because they have clearly demonstrated.
that bivalve eggs and larvae, if protected against disease-causing
organisms and toxic substances, are rather hardy.
41
maximum sizes recorded on the 2lst day were 107 and 221 p. Our
colleagues working in the same field, especially Imai et al. (1954),
fully share our experiences.
At present, no well-based explanation can be advanced for these
differences. Perhaps, as has been suggested in connection with survival
and growth of certain fish, vitality of the individual eggs and larvae
that emerge from them depends to some extent upon the position of the
eggs in the ovaries and the amounts of nutritive materials that have
been stored in the individual eggs before they are discharged.
Chanley (1955) assumed that differences in sizes of larvae in the
same cultures must be due, a t least in part, to inherited characteristics.
He also reported evidence of significantly different rates of growth of
larvae originating from eggs of the samt' female crossed with different
males and larvae grown from eggs of two females individually crossed
with the same male. He tentatively concluded that inherited differences
from either parent may he responsible for differences in rate of growth
of different larvae.
In some cultures, especially those kept at comparatively high
temperatures, the range of larval sizes usually diminishes several days
after beginning of metamorphosis. This is due to the disappearance of
larger individuals because of setting and, partly, because abnormal,
undeveloped, slow-growing larvae are rapidly dying.
There were periods in our practice, for example, in growing larvae
of Ostrea edulis, when regardless of all efforts they would not grow at
all or ceased growing soon after reaching a size of about 220 p. The
reasons for cessation of growth still remain an enigma because, at
times, these larvae refused to grow wen when given foods on which,
in previous experiments, they grew well.
E. Hardiness of eggs icnd larvae
According to Nelson (1921) larvae of C. virginicu are extremely
sensitive to a sudden change in water temperature. A drop of only
3" to 5°C within 24 hr may be followed by the disappearance of a
majority of the larvae. According to the same author rain storms, as
well as strong winds, cause death of large numberu of bivalve larvae.
Nelson, however, failed to offer experimental evidence to support his
contention of the unusual sensitivity of bivalve larvae to relatively
minor changes in their environment. Our observations, reported partly
in this section and partly in the sections to follow, lead us to disagree
with Nelson's point of view because they have clearly demonstrated.
that bivalve eggs and larvae, if protected against disease-causing
organisms and toxic substances, are rather hardy.