10. QROWTH, MATURITY AND MORTALITY
295
body ; it varied in Meganyctiphanes norvegica and Thysanoessa raschii
between 8 and 12%. The spent ovary has a volume of less than half
that of the gravid ovary. The volume of eggs laid must therefore be
equal to or greater than half the volume of the gravid ovary of the
female. The diameters of the spherical eggs of several species are
known and their volumes were calculated, neglecting the perivitelline
spaces which develop after the eggs are laid. Knowing the length
of gravid females, and thus the volume of the body (Fig. 110), the
number of eggs whose total volume equals 5% of the body volume
of the respective female was calculated (Table XXIX) and this is the
number laid during any one breeding season. Different species have
different sizes of eggs, a species such as Pseudeuphausia sinica having
an egg whose volume is 1 x
ml while the egg of Euphausia
superba has a volume of 1 x
neglecting the perivitelline spaces.
Body volumes of mature euphausiids range from about 0.001 ml,
Stylocheiron suhmii, up to over 1.5 ml in Euphausia superba. Thus body
volumes range through three orders of magnitude while egg volumes
only range through one order of magnitude. From these calculations,
the number of eggs laid is greater in larger females of a species than in
smaller females of the same species ; for example, a E . superba of 40 mm
body length lays about 300 eggs while one of 55 mm lays about 800
eggs. Komaki (196713) found that spent Nematoscelis dificilis weighing
about 60 mg wet wt carried about 200 eggs, those weighing about 110 mg
carried about 400 eggs, while one specimen of 143-6 mg wet wt carried
562 eggs; thus, the number of eggs doubles as the weight of females
doubles. Lasker (personal communication) supplied the measurement
of the diameter of eggs of Euphuusia pacijca and also a count of the
number of eggs laid by E . pacijca in the laboratory; a large
discrepancy is evident between the calculated brood size, 31, and the
actual numbers of eggs laid, 230.
Estimations of fecundity of euphausiids calculated in this way are
more likely to underestimate the true fecundity whereas counting of
stage IV eggs in the ovary overestimates it. Taking account of (1)
maximum errors in the regression equation for body volume/body
length, (2) variations in the volume of the gravid ovary relative to body
volume and (3) variations in the percentage of the volume of the ovary
representing eggs laid it is possible that the actual number of eggs laid
by some species could be twice that calculated here. Values twice as
high as those calculated here are still much less than many of those
obtained by counting stage IV eggs in the ovary.
The majority of species, about 60, lay their eggs freely in the sea.
According to these calculations, the numbers of eggs laid vary between
295
body ; it varied in Meganyctiphanes norvegica and Thysanoessa raschii
between 8 and 12%. The spent ovary has a volume of less than half
that of the gravid ovary. The volume of eggs laid must therefore be
equal to or greater than half the volume of the gravid ovary of the
female. The diameters of the spherical eggs of several species are
known and their volumes were calculated, neglecting the perivitelline
spaces which develop after the eggs are laid. Knowing the length
of gravid females, and thus the volume of the body (Fig. 110), the
number of eggs whose total volume equals 5% of the body volume
of the respective female was calculated (Table XXIX) and this is the
number laid during any one breeding season. Different species have
different sizes of eggs, a species such as Pseudeuphausia sinica having
an egg whose volume is 1 x
ml while the egg of Euphausia
superba has a volume of 1 x
neglecting the perivitelline spaces.
Body volumes of mature euphausiids range from about 0.001 ml,
Stylocheiron suhmii, up to over 1.5 ml in Euphausia superba. Thus body
volumes range through three orders of magnitude while egg volumes
only range through one order of magnitude. From these calculations,
the number of eggs laid is greater in larger females of a species than in
smaller females of the same species ; for example, a E . superba of 40 mm
body length lays about 300 eggs while one of 55 mm lays about 800
eggs. Komaki (196713) found that spent Nematoscelis dificilis weighing
about 60 mg wet wt carried about 200 eggs, those weighing about 110 mg
carried about 400 eggs, while one specimen of 143-6 mg wet wt carried
562 eggs; thus, the number of eggs doubles as the weight of females
doubles. Lasker (personal communication) supplied the measurement
of the diameter of eggs of Euphuusia pacijca and also a count of the
number of eggs laid by E . pacijca in the laboratory; a large
discrepancy is evident between the calculated brood size, 31, and the
actual numbers of eggs laid, 230.
Estimations of fecundity of euphausiids calculated in this way are
more likely to underestimate the true fecundity whereas counting of
stage IV eggs in the ovary overestimates it. Taking account of (1)
maximum errors in the regression equation for body volume/body
length, (2) variations in the volume of the gravid ovary relative to body
volume and (3) variations in the percentage of the volume of the ovary
representing eggs laid it is possible that the actual number of eggs laid
by some species could be twice that calculated here. Values twice as
high as those calculated here are still much less than many of those
obtained by counting stage IV eggs in the ovary.
The majority of species, about 60, lay their eggs freely in the sea.
According to these calculations, the numbers of eggs laid vary between