REARING OF BIVAI,VE MOLLUSKS
101
H. Ostreo edulis Linni5
We are interested in this species because its northern races propagate at somewhat lower temperatures than the American oyster,
c. virginica. Because of this we imported 0. edulis into the United
States in 1949 in the belief that certain areas along the shoreline of
Maine and our Pacific coast states were promising for natural propagation of these oysters (Loosanoff, 1951, 1955, 1962b). Since then,
0. edulis have grown and propagated naturally in New England waters
and have become established in Maine. Young oysters reared at
Milford, Connecticut and sent to the State of Washington have also
grown well there.
Literature on the sexuality and propagation of the European flat
oyster, 0. edulis, has been well reviewed by Orton (1937), Korringa
(1941), Walne (1956) and Yonge (1960) and, therefore, need not be
discussed here in detail. Using our methods for ripening mollusks out
of season we have obtained larvae of 0. edulis from the end of January
t o the end of the normal spawning period which, in Milford Harbor,
extends into September. Our attempts to induce spawning of ripe
oysters were not always successful. On several occasions, nevertheless,
by raising the temperature of the water containing conditioned oysters
and simultaneously adding a suspension of gonad material, spawning
has been initiated in both sexes. This was ascertained by finding
sperm balls or recently discharged eggs on the bottom of the aquaria.
Well-developed eggs ranging in size from 114 to 126 p were often lost
when, during the process of spawning, they were released from the
gonad to enter the gill chamber where, normally, they are incubated
until larvae are released a t swarming.
The manner in which eggs are lost was observed on several occasions
during normal spawnings. These eggs were released from the exhalant
chamber in a thin stream, almost in ‘‘ single file ”. Because only a
weak current was carrying them, the eggs settled next to the shells of
the mother oysters, eventually forming a small mound. By a gentle
moving of the spawning oysters from one area of the aquarium to
another and observing the formation of new piles of eggs, i t waa
established that a spawning female may continue a steady discharge of
eggs for as long as 4 hr.
Since we could not always induce spawning in conditioned oyntere,
we depended quite often on unprovoked normal spawning8 and
subsequent release, or swarming of the larvae. Therefore, when a
gravid female was detected, by the presence of a small pile of eggo
near its excurrent side, it was placed in a separate aquarium with
aerated standing water and kept there until larvae were released. The
101
H. Ostreo edulis Linni5
We are interested in this species because its northern races propagate at somewhat lower temperatures than the American oyster,
c. virginica. Because of this we imported 0. edulis into the United
States in 1949 in the belief that certain areas along the shoreline of
Maine and our Pacific coast states were promising for natural propagation of these oysters (Loosanoff, 1951, 1955, 1962b). Since then,
0. edulis have grown and propagated naturally in New England waters
and have become established in Maine. Young oysters reared at
Milford, Connecticut and sent to the State of Washington have also
grown well there.
Literature on the sexuality and propagation of the European flat
oyster, 0. edulis, has been well reviewed by Orton (1937), Korringa
(1941), Walne (1956) and Yonge (1960) and, therefore, need not be
discussed here in detail. Using our methods for ripening mollusks out
of season we have obtained larvae of 0. edulis from the end of January
t o the end of the normal spawning period which, in Milford Harbor,
extends into September. Our attempts to induce spawning of ripe
oysters were not always successful. On several occasions, nevertheless,
by raising the temperature of the water containing conditioned oysters
and simultaneously adding a suspension of gonad material, spawning
has been initiated in both sexes. This was ascertained by finding
sperm balls or recently discharged eggs on the bottom of the aquaria.
Well-developed eggs ranging in size from 114 to 126 p were often lost
when, during the process of spawning, they were released from the
gonad to enter the gill chamber where, normally, they are incubated
until larvae are released a t swarming.
The manner in which eggs are lost was observed on several occasions
during normal spawnings. These eggs were released from the exhalant
chamber in a thin stream, almost in ‘‘ single file ”. Because only a
weak current was carrying them, the eggs settled next to the shells of
the mother oysters, eventually forming a small mound. By a gentle
moving of the spawning oysters from one area of the aquarium to
another and observing the formation of new piles of eggs, i t waa
established that a spawning female may continue a steady discharge of
eggs for as long as 4 hr.
Since we could not always induce spawning in conditioned oyntere,
we depended quite often on unprovoked normal spawning8 and
subsequent release, or swarming of the larvae. Therefore, when a
gravid female was detected, by the presence of a small pile of eggo
near its excurrent side, it was placed in a separate aquarium with
aerated standing water and kept there until larvae were released. The