REAXINQ OF BIVALVE MOLLUSKS
26
were available to conduct critical experiments on development of eggs
and growth of larvae to setting or post-setting stage. Since development of these methods, such studies havo become poaeible and recently
were undertaken at our laboratory.
Three groups of oysters of different ages and sizes were conditioned
for spawning (Fig. 12). The average age of individuals of the oldest
group was estimated to be between 30 and 40 years, some of them being
over 9 in long and over 4 in wide. The intermediate group of oysters
of marketable size was from 5 to 7 years old, while the youngest p u p
was composed of small oysters approximately 2 years old. These groups
were conditioned and induced to spawn under controlled conditions,
their larvae grown to setting stage, and rates of survival and growth
of larvae from the three size groups compared.
The results showed no significant difference between oysters of
the different age groups in the time needed to develop ripe gonads.
We were somewhat surprised, however, to find that oysters of the oldest
group responded to spawning stimuli more rapidly than individuals
of the two younger groups.
There wks also no significant difference in percentage of fertilizable
eggs because almost 100% of the eggs of all three groups became
fertilized. Furthermore, the percentage of fertilized eggs developing
to straight-hinge larval stage showed no consistent variation that could
be ascribed to size or age of parent oysters. Finally, no consistent
difference was found either in the sizes of the early straight-hinge
larvae originating from eggs of different age-group oysters or in
survival and rate of growth of their larvae.
Similar studies on hard shell clams, N. mercenaria, measuring from
37 to 110 mm in length, also showed that there was no significant
difference in viability of spawn produced by clams of Werent sizes
and ages. Often the differences between the progeny of individuals
of the same size groups were as great m the differences between those
of Werent ages and sizes. Larvae grown from eggs of clams of all
three sizes were successfully carried to setting stage.
On the basis of the above-described experiments we came to the
conclusion that since there was no Significant difference in the quality
of spawn developed by individuals of different ages or sizes, mature
oysters and clams of all age groups may be safely used as spawners.
Of special biological interest was the observation that the sexes
among the oldest oysters were about evenly divided. This discovery
was contrary to the old conception that in the oldest groups females
should decidedly predominate in numbers. We also noticed that many
of the largest and oldest oysters, while kcpt in the laboratory to be
,
26
were available to conduct critical experiments on development of eggs
and growth of larvae to setting or post-setting stage. Since development of these methods, such studies havo become poaeible and recently
were undertaken at our laboratory.
Three groups of oysters of different ages and sizes were conditioned
for spawning (Fig. 12). The average age of individuals of the oldest
group was estimated to be between 30 and 40 years, some of them being
over 9 in long and over 4 in wide. The intermediate group of oysters
of marketable size was from 5 to 7 years old, while the youngest p u p
was composed of small oysters approximately 2 years old. These groups
were conditioned and induced to spawn under controlled conditions,
their larvae grown to setting stage, and rates of survival and growth
of larvae from the three size groups compared.
The results showed no significant difference between oysters of
the different age groups in the time needed to develop ripe gonads.
We were somewhat surprised, however, to find that oysters of the oldest
group responded to spawning stimuli more rapidly than individuals
of the two younger groups.
There wks also no significant difference in percentage of fertilizable
eggs because almost 100% of the eggs of all three groups became
fertilized. Furthermore, the percentage of fertilized eggs developing
to straight-hinge larval stage showed no consistent variation that could
be ascribed to size or age of parent oysters. Finally, no consistent
difference was found either in the sizes of the early straight-hinge
larvae originating from eggs of different age-group oysters or in
survival and rate of growth of their larvae.
Similar studies on hard shell clams, N. mercenaria, measuring from
37 to 110 mm in length, also showed that there was no significant
difference in viability of spawn produced by clams of Werent sizes
and ages. Often the differences between the progeny of individuals
of the same size groups were as great m the differences between those
of Werent ages and sizes. Larvae grown from eggs of clams of all
three sizes were successfully carried to setting stage.
On the basis of the above-described experiments we came to the
conclusion that since there was no Significant difference in the quality
of spawn developed by individuals of different ages or sizes, mature
oysters and clams of all age groups may be safely used as spawners.
Of special biological interest was the observation that the sexes
among the oldest oysters were about evenly divided. This discovery
was contrary to the old conception that in the oldest groups females
should decidedly predominate in numbers. We also noticed that many
of the largest and oldest oysters, while kcpt in the laboratory to be
,