82
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
based on our observations and experiments on larvae of C. virginica
and M . mercenaria, these species will be considered first. However, to
avoid repetition of the material already discussed, information on
rearing of larvae of t,hese two bivalves will be presented in the form of a
comparatively brief review.
A. Crassostrea virginica (Gmelin)
A discussion of the literature on various aspects of propagation of
this species has already been given in the earlier part of this article.
By using different methods these oysters can be conditioned for
spawning throughout the year and, if necessary, several times annually
(Loosanoff and Davis, 1952a). Spawning is induced by raising the
temperature within the range from 20" to 32"C, depending upon the
previous temperature r6gime. Addition of suspensions of sex products
helps to stimulate the spawning. Eggs can be stripped and fertilized.
Eggs of C . virginica vary in size from 45 to 62 p, but the majority
measure between 50 and 55 p. Costello et al. (1957) stated that the size
of the oyster egg varied between 45 and 54 p. The size of the egg is not
influenced by the size of the mother oyster. For example, eggs discharged by females 30 to 40 years old and over 9 in long averaged
50.4 p and ranged between 48 and 54 p in diameter, while the eggs
from much younger and smaller females, approximately 6 years old and
4 in long, a#veraged about 51 p, the difference being insignificant.
Under favorable conditions 90 to 957" of ripe, properly handled
eggs can develop within 24 hr to shelled veliger stage. The smallest
normal straight-hinge larva,e measure approximately 68 x 55 p. In
48 hr, at a temperature of about, 22"C, the larvae measure about
75 x 67 p. At this stage they already begin to take in particulate
food.
The length of the larvae, measured parallel with the hinge line,
continues to be approximately 5 to 10 p greater than the width,
measured from the hinge to the ventral edge of the shell, until the larvae
reach the size of 85 x 80 p. A t 95 to 100 p the length and width are
approximately equal, but after that stage the increase in width is more
rapid. When the length reaches 125 to 130 p the width already e x d
it by about 10 p. This 10 p disparity between the two measurements
remains virtually constant until the larvae reach metamorphosis.
The length-width ratio of larvae of C. virginica is almost linear,
except for the section roughly confined to the length between 105 and
130 p where it displays a tendency to form a flat curve. It is of interest
that the length-width curve of the closely related species, C. qigae, the
Japanese oyster, clearly shows the same tendency.
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
based on our observations and experiments on larvae of C. virginica
and M . mercenaria, these species will be considered first. However, to
avoid repetition of the material already discussed, information on
rearing of larvae of t,hese two bivalves will be presented in the form of a
comparatively brief review.
A. Crassostrea virginica (Gmelin)
A discussion of the literature on various aspects of propagation of
this species has already been given in the earlier part of this article.
By using different methods these oysters can be conditioned for
spawning throughout the year and, if necessary, several times annually
(Loosanoff and Davis, 1952a). Spawning is induced by raising the
temperature within the range from 20" to 32"C, depending upon the
previous temperature r6gime. Addition of suspensions of sex products
helps to stimulate the spawning. Eggs can be stripped and fertilized.
Eggs of C . virginica vary in size from 45 to 62 p, but the majority
measure between 50 and 55 p. Costello et al. (1957) stated that the size
of the oyster egg varied between 45 and 54 p. The size of the egg is not
influenced by the size of the mother oyster. For example, eggs discharged by females 30 to 40 years old and over 9 in long averaged
50.4 p and ranged between 48 and 54 p in diameter, while the eggs
from much younger and smaller females, approximately 6 years old and
4 in long, a#veraged about 51 p, the difference being insignificant.
Under favorable conditions 90 to 957" of ripe, properly handled
eggs can develop within 24 hr to shelled veliger stage. The smallest
normal straight-hinge larva,e measure approximately 68 x 55 p. In
48 hr, at a temperature of about, 22"C, the larvae measure about
75 x 67 p. At this stage they already begin to take in particulate
food.
The length of the larvae, measured parallel with the hinge line,
continues to be approximately 5 to 10 p greater than the width,
measured from the hinge to the ventral edge of the shell, until the larvae
reach the size of 85 x 80 p. A t 95 to 100 p the length and width are
approximately equal, but after that stage the increase in width is more
rapid. When the length reaches 125 to 130 p the width already e x d
it by about 10 p. This 10 p disparity between the two measurements
remains virtually constant until the larvae reach metamorphosis.
The length-width ratio of larvae of C. virginica is almost linear,
except for the section roughly confined to the length between 105 and
130 p where it displays a tendency to form a flat curve. It is of interest
that the length-width curve of the closely related species, C. qigae, the
Japanese oyster, clearly shows the same tendency.