26
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
'
conditioned for spawning, formed normal, new shell growth, thus
indicating that even at that age and size the oysters did not lose their
ability to grow (Fig. 12).
IV. CULTIVATION o w Earn A N D IARVAE O Y IJIVAI~VES
A . (Imertd descript.ioii~ (4 flu: dwdopmerd
Kggs of' l,ivdv(!x clitTcr i r i tiitLily w - q j o c t H , i~tclu~litig thcir H i m , color
and spcxific gravity. They also tlifier in thickness of the membrane
surrounding them (Costello et nl., 1957). In oysters and certain other
forms this membrane is only a few microns thick. In others, however,
such as M. mercenaria, the egg proper measures only 70 to 73 p, while
the total diameter of the egg and surrounding gelatinous membrane
is about 170 p. This membrane, in many instances, continues to surround the embryo past blastula stage and, on some occasions, until
late trochophore stage is reached (Loosanoff and Davis, 1950).
We shall describe specific characteristics of the eggs later on,
when discussing cach of the specien Atdied. Here, because the description of a typical hivalve egg and its development to straight-hinge
stage or, as i t is often called, early veliger has been given on many
occasions, including Brooks (1880), MacBride (1914) and others, we shall
present only a general picture of changes occurring from the moment
the egg is discharged, or stripped, until it becomes a straight-hinge
larva. This description is based upon observations made on eggs and
early embryos of Mactra (= Spisula) solidissima, the surf clam, which is
the largest bivalve of our Atlantic coast. It measures up to 79 in long
and can be found in considerable numbers from Labrador to Cape
H atteras. Additional informat,ioti on spawning of these clams and
rcaririg of' their I i i r v i i c in i t i i : l i i i I w l irr f.tw wi:t,icur f l 4 i t i K wi1,li rwwjtig
I tit. diiitii(+v of' IL i t i t ~ i ~ i r c
(:gg of M . .wlidiwirn.ii, avcragw Mi.5 p
(Fig. 13a). Costello et al. (1!)87) give the diameter of the unfertilized
ovum of the same spocies as ranging hetwwn 53 and 56 p, thuti agreeing
with oiir mrnsurements. According to Cahn (1851), who bases his
conclusions o t i thc work of thc Japanese investigators, Kinoshita and
Hinulo ( 1!)34), wtiosc ptq)cr WILS not nviiilable to us for consultation,
the diatrictcr of ttic ~ g g
of a cloxoly rc:lated form, Spisula sachalinensis,
is only 50 p. Another group of ,Japanese workers (Imai et al., 1953),
studying the same species reports that the diameter of the mature
egg of this clam varies from 70 to 75 p , thus being considerably larger
than the size given by Cahn. Jmgensen (1946) states that eggs of
Spisula subtruncata of European waters vary in diameter from 50
O f IHI'VILI! f J 1 ' ~ ~ l f ~ f ! f Y ! l l ~ ~
HlN!(!i(S4.'
I ,
to 55 p.
VICTOR L. LOOSANOFF AND HARRY C. DAVIS
'
conditioned for spawning, formed normal, new shell growth, thus
indicating that even at that age and size the oysters did not lose their
ability to grow (Fig. 12).
IV. CULTIVATION o w Earn A N D IARVAE O Y IJIVAI~VES
A . (Imertd descript.ioii~ (4 flu: dwdopmerd
Kggs of' l,ivdv(!x clitTcr i r i tiitLily w - q j o c t H , i~tclu~litig thcir H i m , color
and spcxific gravity. They also tlifier in thickness of the membrane
surrounding them (Costello et nl., 1957). In oysters and certain other
forms this membrane is only a few microns thick. In others, however,
such as M. mercenaria, the egg proper measures only 70 to 73 p, while
the total diameter of the egg and surrounding gelatinous membrane
is about 170 p. This membrane, in many instances, continues to surround the embryo past blastula stage and, on some occasions, until
late trochophore stage is reached (Loosanoff and Davis, 1950).
We shall describe specific characteristics of the eggs later on,
when discussing cach of the specien Atdied. Here, because the description of a typical hivalve egg and its development to straight-hinge
stage or, as i t is often called, early veliger has been given on many
occasions, including Brooks (1880), MacBride (1914) and others, we shall
present only a general picture of changes occurring from the moment
the egg is discharged, or stripped, until it becomes a straight-hinge
larva. This description is based upon observations made on eggs and
early embryos of Mactra (= Spisula) solidissima, the surf clam, which is
the largest bivalve of our Atlantic coast. It measures up to 79 in long
and can be found in considerable numbers from Labrador to Cape
H atteras. Additional informat,ioti on spawning of these clams and
rcaririg of' their I i i r v i i c in i t i i : l i i i I w l irr f.tw wi:t,icur f l 4 i t i K wi1,li rwwjtig
I tit. diiitii(+v of' IL i t i t ~ i ~ i r c
(:gg of M . .wlidiwirn.ii, avcragw Mi.5 p
(Fig. 13a). Costello et al. (1!)87) give the diameter of the unfertilized
ovum of the same spocies as ranging hetwwn 53 and 56 p, thuti agreeing
with oiir mrnsurements. According to Cahn (1851), who bases his
conclusions o t i thc work of thc Japanese investigators, Kinoshita and
Hinulo ( 1!)34), wtiosc ptq)cr WILS not nviiilable to us for consultation,
the diatrictcr of ttic ~ g g
of a cloxoly rc:lated form, Spisula sachalinensis,
is only 50 p. Another group of ,Japanese workers (Imai et al., 1953),
studying the same species reports that the diameter of the mature
egg of this clam varies from 70 to 75 p , thus being considerably larger
than the size given by Cahn. Jmgensen (1946) states that eggs of
Spisula subtruncata of European waters vary in diameter from 50
O f IHI'VILI! f J 1 ' ~ ~ l f ~ f ! f Y ! l l ~ ~
HlN!(!i(S4.'
I ,
to 55 p.