30
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
B. Abnormal eggs and larvae
Abnormal development of eggs and larvae of bivalve mollusks may
be due to any one of a variety of factors or to a combination of such
factors. It is our practice, however, to discard cultures in which less
than 50% of the eggs develop into normal straight-hinge larvae. This
is done because batches of eggs giving a low percentage of larvae may
be abnormal in some respects, and these abnormalities may lead to
aberrant experimental results and wrong conclusions. We do not
know, as yet, what factors are responsible for poor eggs and feeble
embryos. I n some instances abnormal larvae, or failure of eggs to
develop to straight-hinge larval stage, may be the result of incompatible genetic combinations. Our experience indicates, however,
that such combinations are comparatively rare.
Some abnormalities of larvae may be ascribed, no doubt, to the
poor physical condition of spawners. Several investigators have
believed that eggs released late in the season were less viable and produced less vigorous larvae than those from earlier spawnings. Loosanoff
and Davis (1950) were under the impression that the last batches of
eggs discharged by virtually spent females gave feeble larvae that grew
slowly and showed high mortality. Cole (1941) offered evidence that
the brood strength of Ostrea edulis may decline during the course of ti
breeding season and he believed that this was due to a depletion of
food reserves in the bodies of parent mollusks. Walne (1956) thinks
that the lack of " vigour " in larvae may result from poor condition
of the parent oyster and believes it possible that the vigour of larvae
may be affected by the quantity of food reserves laid down in the eggs.
More recently, Davis and Chanley (1956b) have shown conclusively
that the last batches of eggs of both clams, M . mereenaria, and oysters,
C. virginica, discharged by virtually spent females were cultured with
no apparent diminution of either percentage of eggs developing into
straight-hinge larvae or rate of growth of these larvae. Accordingly,
we now believe that abnormal or feeble larvae do not occur more
frequently in later spawnings than in other spawnings throughout the
season. As has already been mentioned, our experiments have shown
that there is no correlation between viability of spawn and age of
parents.
There is evidence that bivalves can be induced, by strong chemical
and thermal stimulations, to abort eggs even though they are not
fully ripe (Fig. 15). In some cases such spawnings appear to be quite
normal and a large number of eggs may be released. More often,
however, comparatively few eggs are shed. In the case of C. virginica
immature oyster eggs usually develop only to late gastrula or early
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
B. Abnormal eggs and larvae
Abnormal development of eggs and larvae of bivalve mollusks may
be due to any one of a variety of factors or to a combination of such
factors. It is our practice, however, to discard cultures in which less
than 50% of the eggs develop into normal straight-hinge larvae. This
is done because batches of eggs giving a low percentage of larvae may
be abnormal in some respects, and these abnormalities may lead to
aberrant experimental results and wrong conclusions. We do not
know, as yet, what factors are responsible for poor eggs and feeble
embryos. I n some instances abnormal larvae, or failure of eggs to
develop to straight-hinge larval stage, may be the result of incompatible genetic combinations. Our experience indicates, however,
that such combinations are comparatively rare.
Some abnormalities of larvae may be ascribed, no doubt, to the
poor physical condition of spawners. Several investigators have
believed that eggs released late in the season were less viable and produced less vigorous larvae than those from earlier spawnings. Loosanoff
and Davis (1950) were under the impression that the last batches of
eggs discharged by virtually spent females gave feeble larvae that grew
slowly and showed high mortality. Cole (1941) offered evidence that
the brood strength of Ostrea edulis may decline during the course of ti
breeding season and he believed that this was due to a depletion of
food reserves in the bodies of parent mollusks. Walne (1956) thinks
that the lack of " vigour " in larvae may result from poor condition
of the parent oyster and believes it possible that the vigour of larvae
may be affected by the quantity of food reserves laid down in the eggs.
More recently, Davis and Chanley (1956b) have shown conclusively
that the last batches of eggs of both clams, M . mereenaria, and oysters,
C. virginica, discharged by virtually spent females were cultured with
no apparent diminution of either percentage of eggs developing into
straight-hinge larvae or rate of growth of these larvae. Accordingly,
we now believe that abnormal or feeble larvae do not occur more
frequently in later spawnings than in other spawnings throughout the
season. As has already been mentioned, our experiments have shown
that there is no correlation between viability of spawn and age of
parents.
There is evidence that bivalves can be induced, by strong chemical
and thermal stimulations, to abort eggs even though they are not
fully ripe (Fig. 15). In some cases such spawnings appear to be quite
normal and a large number of eggs may be released. More often,
however, comparatively few eggs are shed. In the case of C. virginica
immature oyster eggs usually develop only to late gastrula or early