112
THE BIOLOGY OF EUPHAUSIIDS
found euphausiid eggs in the plankton of the Barents Sea before commencement of the phytoplankton increase nor by our observations in
the Clyde where this can also be true. Further, in the Clyde, Thysanoessa
raschii starts to spawn some two weeks after Meganyctiphanes norvegica
and similar differences in timing have been found in other sea areas.
Consequently, the factors which cause the onset of spawning of the
various species require much further investigation before any useful
conclusions can be drawn.
The eggs of euphausiids, between the time they leave the ovary and
the time they are fertilized and pass out of the oviducts, are invested
with membranes. Eggs which are freely laid into the sea, like those of
Thysanoessa raschii and Meganyctiphanes norvegica, have a vitelline
membrane round the embryo, a perivitelline space, large in Thysanoessa
raschii but smaller in Meganyctiphanes norvegica, and an outer membrane which is thought to be really two membranes stuck together.
The walls of the oviduct are glandular and a series of glands (Fig. 37,
shell glands) develop, prior to egg laying, around the mouths of the
oviducts and open into the oviducts. Although there is lack of conclusive evidence, it is probable that the oviduct wall secretes the inner
of the two membranes and the glands supply the outer membrane. In
the case of eggs laid together in a mass attached to the female,
Mauchline thinks that the glands around the mouths of the oviducts
secrete material which forms the second membrane around the eggs
and also binds the eggs together and to the thoracic limbs. The egg
masses are attached to the endopodites of the sixth and seventh thoracic
legs and to the exopodites of the eighth thoracic legs.
There is considerable variation in both the diameter of the embryo
and its capsule in samples of eggs of the same species taken a t the same
time and place. Zelikman (1958a) for Thysanoessa inermis and
T . raschii in the Barents Sea, Naumov (1962) for Euphausia superba,
Wang (1 965) for Pseudeuphausia sinica, Komaki (1 967b) for Nemtoscelis dificilis, and Mauchline (1968) for Meganyctiphanes norvegica and
Thysanoessa raschii found, however, that the differences in the sizes of
the eggs produced by any one species are not directly related to the
differences in the sizes of the females producing them. There is a
further variation in the diameters of eggs of the same species taken
from different geographical locations. The ranges of size of capsules in
Meganyctiphanes norvegica are 0.67-0.75 mm (Lebour, 1924), 0.590.85 mm (Macdonald, 1927b), and 0.36-0.41 (Ruud, 1932). Euphausia
superba has an embryo whose range of size is 0.51-0*83mm (Ruud,
1932) and numerous other examples of similar variation in other species
can be quoted (Lebour, 1924, 1926a,c; Macdonald, 1928; Ruud, 1932;
THE BIOLOGY OF EUPHAUSIIDS
found euphausiid eggs in the plankton of the Barents Sea before commencement of the phytoplankton increase nor by our observations in
the Clyde where this can also be true. Further, in the Clyde, Thysanoessa
raschii starts to spawn some two weeks after Meganyctiphanes norvegica
and similar differences in timing have been found in other sea areas.
Consequently, the factors which cause the onset of spawning of the
various species require much further investigation before any useful
conclusions can be drawn.
The eggs of euphausiids, between the time they leave the ovary and
the time they are fertilized and pass out of the oviducts, are invested
with membranes. Eggs which are freely laid into the sea, like those of
Thysanoessa raschii and Meganyctiphanes norvegica, have a vitelline
membrane round the embryo, a perivitelline space, large in Thysanoessa
raschii but smaller in Meganyctiphanes norvegica, and an outer membrane which is thought to be really two membranes stuck together.
The walls of the oviduct are glandular and a series of glands (Fig. 37,
shell glands) develop, prior to egg laying, around the mouths of the
oviducts and open into the oviducts. Although there is lack of conclusive evidence, it is probable that the oviduct wall secretes the inner
of the two membranes and the glands supply the outer membrane. In
the case of eggs laid together in a mass attached to the female,
Mauchline thinks that the glands around the mouths of the oviducts
secrete material which forms the second membrane around the eggs
and also binds the eggs together and to the thoracic limbs. The egg
masses are attached to the endopodites of the sixth and seventh thoracic
legs and to the exopodites of the eighth thoracic legs.
There is considerable variation in both the diameter of the embryo
and its capsule in samples of eggs of the same species taken a t the same
time and place. Zelikman (1958a) for Thysanoessa inermis and
T . raschii in the Barents Sea, Naumov (1962) for Euphausia superba,
Wang (1 965) for Pseudeuphausia sinica, Komaki (1 967b) for Nemtoscelis dificilis, and Mauchline (1968) for Meganyctiphanes norvegica and
Thysanoessa raschii found, however, that the differences in the sizes of
the eggs produced by any one species are not directly related to the
differences in the sizes of the females producing them. There is a
further variation in the diameters of eggs of the same species taken
from different geographical locations. The ranges of size of capsules in
Meganyctiphanes norvegica are 0.67-0.75 mm (Lebour, 1924), 0.590.85 mm (Macdonald, 1927b), and 0.36-0.41 (Ruud, 1932). Euphausia
superba has an embryo whose range of size is 0.51-0*83mm (Ruud,
1932) and numerous other examples of similar variation in other species
can be quoted (Lebour, 1924, 1926a,c; Macdonald, 1928; Ruud, 1932;