52
VICTOR I,. LOORANOFF A N D HARRY C. D A V I S
sp. (680) or D?LnaZidln euchloru. I n gcwrritl, 1,1~~,1lg]1 t h rangt from
20" to 3 0 T growth incrcastcl paraIIc~1 with tho iticrcvisc* iri tcmpcrature.
At 30" and 33"C, however, tho Iarvirc. grc'w virtually th(. samt: and metamorphosis at both temperatures bcgari consistently between the 10th
and 12th days.
G. Effects of salinity on eggs and larvae
Bivalves, even though they belong to the same class of mollusks,
display extremely wide differences in their salinity requirements and
in ability to withstand sharp or gradual changes in salt content of sea
water. Therefore, in determining minimum, maximum and optimal
salinities for their existence each species, especially those populating
estuarine regions, must be studied individually. For example, deltas
of rivers, where salinities are relatively high most of the time, may be
populated by both C. virginica and M . mercenaria, while a short
distance above this line, where the salinity of the water is considerably lower than 20 parts per thousand, only oyster beds can be found
because clams are unable to survive under such brackish conditions.
To demonstrate the differences that, may exist betwecn two upecies that
often may be found in the samc cwvironment, we HhalI hiefly tliscum
the diffcrencos in salinity rcquircmrntR of larvae of (,'. virginicn and
M . mercenaria.
Loosanoff (1952) found that the lowe& salinity at which normal
development of gonads of C. virginica of Long Island Sound max
proceed is near 7.5 ppt. Continuing the study of various aspects of
variations in salinity on propagation of American oysters, Davis (1958)
demonstrated that 22.5 ppt was the optimum salinity for development
of eggs of oysters that had grown in Long Island Sound and had
developed gonads at a salinity of about 27 ppt. Some normal larvae
developed, nevertheless, in salinities as low as 15 ppt and as high as
85 ppt. At salinities below 22.5 ppt the percentage of eggs that developed to straight-hinge larval stage steadily decreased until, at 15 ppt,
only 50 to 60% of the eggs developed normally. At 12.5 ppt practically
none of the eggs developed into normal shelled larvae.
In another experiment Davis used Maryland oysters that had grown
and developed gonads in the upper parts of Chesapeake Bay where
the salinity, at the time the oysters were collected, was only 8.7 ppt.
These oysters were spawned at Milford Laboratory in Salinities of
7.5, 10 and 15 ppt. Under these conditions some eggn developed into
normal larvae even at 10 ppt and 7.5 ppt although, in the latter,
slightly smaller than normal larvaca were common. In ganeral, the
optimal salinity for normal devetopment of eggs of these oysters from
VICTOR I,. LOORANOFF A N D HARRY C. D A V I S
sp. (680) or D?LnaZidln euchloru. I n gcwrritl, 1,1~~,1lg]1 t h rangt from
20" to 3 0 T growth incrcastcl paraIIc~1 with tho iticrcvisc* iri tcmpcrature.
At 30" and 33"C, however, tho Iarvirc. grc'w virtually th(. samt: and metamorphosis at both temperatures bcgari consistently between the 10th
and 12th days.
G. Effects of salinity on eggs and larvae
Bivalves, even though they belong to the same class of mollusks,
display extremely wide differences in their salinity requirements and
in ability to withstand sharp or gradual changes in salt content of sea
water. Therefore, in determining minimum, maximum and optimal
salinities for their existence each species, especially those populating
estuarine regions, must be studied individually. For example, deltas
of rivers, where salinities are relatively high most of the time, may be
populated by both C. virginica and M . mercenaria, while a short
distance above this line, where the salinity of the water is considerably lower than 20 parts per thousand, only oyster beds can be found
because clams are unable to survive under such brackish conditions.
To demonstrate the differences that, may exist betwecn two upecies that
often may be found in the samc cwvironment, we HhalI hiefly tliscum
the diffcrencos in salinity rcquircmrntR of larvae of (,'. virginicn and
M . mercenaria.
Loosanoff (1952) found that the lowe& salinity at which normal
development of gonads of C. virginica of Long Island Sound max
proceed is near 7.5 ppt. Continuing the study of various aspects of
variations in salinity on propagation of American oysters, Davis (1958)
demonstrated that 22.5 ppt was the optimum salinity for development
of eggs of oysters that had grown in Long Island Sound and had
developed gonads at a salinity of about 27 ppt. Some normal larvae
developed, nevertheless, in salinities as low as 15 ppt and as high as
85 ppt. At salinities below 22.5 ppt the percentage of eggs that developed to straight-hinge larval stage steadily decreased until, at 15 ppt,
only 50 to 60% of the eggs developed normally. At 12.5 ppt practically
none of the eggs developed into normal shelled larvae.
In another experiment Davis used Maryland oysters that had grown
and developed gonads in the upper parts of Chesapeake Bay where
the salinity, at the time the oysters were collected, was only 8.7 ppt.
These oysters were spawned at Milford Laboratory in Salinities of
7.5, 10 and 15 ppt. Under these conditions some eggn developed into
normal larvae even at 10 ppt and 7.5 ppt although, in the latter,
slightly smaller than normal larvaca were common. In ganeral, the
optimal salinity for normal devetopment of eggs of these oysters from