10. GROWTH, MATURITY AND MORTALITY
293
The only information on the rate of development of the spermatophores of males is that of Mauchline (1960, 1968). Recourse to Fig. 41
shows that spermatozoa first appear in the vasa deferentia of Meganyctiphanes norvegica in early November and fully formed spermatophores
first appear in early January suggesting that 8-10 weeks are required
for the development of spermatophores. Larger males carry larger
spermatophores than smaller males (Fig. 112); the volume of the
spermatophore was calculated by treating it as the volume of a cylinder,
a reasonable approximation to its shape in this species.
Consideration was given to the number of eggs produced by different
species and different sizes of euphausiids (Mauchline, 1966c, 1968).
The fecundity, or more correctly, the brood size of species in the genera
Pseudeuphausia, Nyctiphanes, Nematobrachion, Nematoscelis, and
Stylocheiron can be estimated simply by counting the eggs carried by
the females but unfortunately few females of many of these species
have been found carrying eggs, and when they have, the investigators
have often failed to record the number carried. The majority of
euphausiids, however, lay their eggs freely into the sea and so other
methods of estimating fecundity or brood size have to be used. Individuals maintained in the laboratory lay only a few eggs and there is
always the suspicion that the numbers laid under these conditions may
bear little relation to the numbers laid in the sea. Bargmann (1937),
Zelikman (1958a)) Ponomareva (1959a)) and Naumov (1962) estimated
the fecundity of euphausiids by counting eggs in the ovaries. Bargmann
counted between 11 000 and 11 500 eggs in the ovary of Euphausia
superba and these figures have been quoted frequently in the literature.
Some of the other authors restricted their counts to late stage eggs,
stage IV (Chapter 4), and obtained lower estimations, of the order of
500. An examination of spent ovaries of Thysanoessa raschii and
Meganyctiphanes norvegica confirmed the idea that not all stage IV
eggs in an ovary are laid, a feature evident in Zelikman’s drawing of
the eggs in spent ovaries of Thysanoessa raschii and T . inermis. Thus
a count of all stage IV eggs overestimates fecundity. There is an
indirect method of estimating fecundity of euphausiids. The majority
of workers on euphausiids agree that most euphausiids lay their eggs
in a short period of time and that the occurrence of eggs in the plankton
over long periods is because not all members of the adult population
attain maturity a t the same time. This being so, then the total
volume of eggs laid by any one euphausiid must be related to the
volume of the ovary.
An examination of the ovaries of different species showed that the
volume of the gravid ovary is approximately 10% of the volume of the
293
The only information on the rate of development of the spermatophores of males is that of Mauchline (1960, 1968). Recourse to Fig. 41
shows that spermatozoa first appear in the vasa deferentia of Meganyctiphanes norvegica in early November and fully formed spermatophores
first appear in early January suggesting that 8-10 weeks are required
for the development of spermatophores. Larger males carry larger
spermatophores than smaller males (Fig. 112); the volume of the
spermatophore was calculated by treating it as the volume of a cylinder,
a reasonable approximation to its shape in this species.
Consideration was given to the number of eggs produced by different
species and different sizes of euphausiids (Mauchline, 1966c, 1968).
The fecundity, or more correctly, the brood size of species in the genera
Pseudeuphausia, Nyctiphanes, Nematobrachion, Nematoscelis, and
Stylocheiron can be estimated simply by counting the eggs carried by
the females but unfortunately few females of many of these species
have been found carrying eggs, and when they have, the investigators
have often failed to record the number carried. The majority of
euphausiids, however, lay their eggs freely into the sea and so other
methods of estimating fecundity or brood size have to be used. Individuals maintained in the laboratory lay only a few eggs and there is
always the suspicion that the numbers laid under these conditions may
bear little relation to the numbers laid in the sea. Bargmann (1937),
Zelikman (1958a)) Ponomareva (1959a)) and Naumov (1962) estimated
the fecundity of euphausiids by counting eggs in the ovaries. Bargmann
counted between 11 000 and 11 500 eggs in the ovary of Euphausia
superba and these figures have been quoted frequently in the literature.
Some of the other authors restricted their counts to late stage eggs,
stage IV (Chapter 4), and obtained lower estimations, of the order of
500. An examination of spent ovaries of Thysanoessa raschii and
Meganyctiphanes norvegica confirmed the idea that not all stage IV
eggs in an ovary are laid, a feature evident in Zelikman’s drawing of
the eggs in spent ovaries of Thysanoessa raschii and T . inermis. Thus
a count of all stage IV eggs overestimates fecundity. There is an
indirect method of estimating fecundity of euphausiids. The majority
of workers on euphausiids agree that most euphausiids lay their eggs
in a short period of time and that the occurrence of eggs in the plankton
over long periods is because not all members of the adult population
attain maturity a t the same time. This being so, then the total
volume of eggs laid by any one euphausiid must be related to the
volume of the ovary.
An examination of the ovaries of different species showed that the
volume of the gravid ovary is approximately 10% of the volume of the