REAXNU OP BIVALVY MOLLUSKS
113
stages of P . morrhuana. Sullivan (1948) is virtually the only author
to devote some attention to veligers of this bivalve. Costello et al.
(1967), calling the same form Callocardia convexa, gave a good description of procuring gametes of these mollusks.
Our own experience with P. morrhuanc, has been extensive but not
always satisfactory because this clam is one of the few bivalves that
we could not induct? to spawn. All the methods known to us, including
increasing of the water temperature, adtlition of ripe sex products,
changes in p H or salinity, and use of mechanicrtl and electrical stimulation failed to induce spawning. Moreover, wc have never observed
natural spawning among the hundreds of these clams that have been
kept in our laboratory under different conditions, sometimes for periods
of several months.
Animals brought into the laboratory from their natural beds in
January already contained what appeared to be morphologically-ripe
gametes. The color of the female gonad was usually creamy white
and that of the male a light yellow. Even this early in the season
active sperm could be taken from male gonads. The spermatozoa
resembled those of M. mercenaria in shape. Eggs could be easily
obtained by stripping, but they could not be fertilized so early in the
season. This was true even when P . morrhuana was slowly conditioned
in the laboratory by day-by-day increases in temperature. However,
as the season progressed, the eggs became more responsive and the
germinal vesicle in some of them dissolved soon after introduction of
spermatozoa.
Our larval cultures were grown from eggs obtained by stripping.
The size of the eggs taken from females with well-developed gonads
ranged in diameter from 49 to 00 p, with t h c modal size-group betwoen
50 and 5 5 p . These measurements, however, refer only to the egg
proper; if the entire diameter of the ovum, including its thick, gelatinous membrane were measured, it would vary between 92 and 128 p,
with the modal class near 117 p.
On several occasions we tried to improve the condition of stripped
eggs by adding 3 ml of 0.1 N ammonium hydroxide to every 100 ml of
egg suspension. After 25 min of this treatment the eggs were screened
to discard the fluid. The eggs were then rinsed with sea water, resuspended, and sperniatozoa added in proper quantities. In some instances,
after this treatmciit about 40% of all eggs underwent cell division, but
llsually not more than 5% developed to normal straight-hinge larvae.
Early development of eggs and larvtw of P . m r h u a n a is virtually
identical to that of other mollusks of the same group. Progress of
development, as usual, depends upon the temperature at which the
113
stages of P . morrhuana. Sullivan (1948) is virtually the only author
to devote some attention to veligers of this bivalve. Costello et al.
(1967), calling the same form Callocardia convexa, gave a good description of procuring gametes of these mollusks.
Our own experience with P. morrhuanc, has been extensive but not
always satisfactory because this clam is one of the few bivalves that
we could not induct? to spawn. All the methods known to us, including
increasing of the water temperature, adtlition of ripe sex products,
changes in p H or salinity, and use of mechanicrtl and electrical stimulation failed to induce spawning. Moreover, wc have never observed
natural spawning among the hundreds of these clams that have been
kept in our laboratory under different conditions, sometimes for periods
of several months.
Animals brought into the laboratory from their natural beds in
January already contained what appeared to be morphologically-ripe
gametes. The color of the female gonad was usually creamy white
and that of the male a light yellow. Even this early in the season
active sperm could be taken from male gonads. The spermatozoa
resembled those of M. mercenaria in shape. Eggs could be easily
obtained by stripping, but they could not be fertilized so early in the
season. This was true even when P . morrhuana was slowly conditioned
in the laboratory by day-by-day increases in temperature. However,
as the season progressed, the eggs became more responsive and the
germinal vesicle in some of them dissolved soon after introduction of
spermatozoa.
Our larval cultures were grown from eggs obtained by stripping.
The size of the eggs taken from females with well-developed gonads
ranged in diameter from 49 to 00 p, with t h c modal size-group betwoen
50 and 5 5 p . These measurements, however, refer only to the egg
proper; if the entire diameter of the ovum, including its thick, gelatinous membrane were measured, it would vary between 92 and 128 p,
with the modal class near 117 p.
On several occasions we tried to improve the condition of stripped
eggs by adding 3 ml of 0.1 N ammonium hydroxide to every 100 ml of
egg suspension. After 25 min of this treatment the eggs were screened
to discard the fluid. The eggs were then rinsed with sea water, resuspended, and sperniatozoa added in proper quantities. In some instances,
after this treatmciit about 40% of all eggs underwent cell division, but
llsually not more than 5% developed to normal straight-hinge larvae.
Early development of eggs and larvtw of P . m r h u a n a is virtually
identical to that of other mollusks of the same group. Progress of
development, as usual, depends upon the temperature at which the