4. THE LARVAE
133
developing embryos. The major problem faced by Marr is the scarcity
of eggs of Euphausia superba in the plankton hauls made in the Antarctic
over a period of many years. Gravid females have been found in large
numbers in the surface waters but eggs appear to be very scarce and
Marr dwells considerably on the hypothesis that this euphausiid may
swim down to deeper layers to lay its eggs. The present authors consider this unlikely because if the eggs were laid at say 1 500 m then
hatching would take place at a considerably greater depth, owing to
the eggs sinking, and the nauplii, metanauplii, and calyptopes would
have a very long climb to the surface layers where the food for these
filter-feeding larvae is present.
Mauchline (unpublished) found,
experimentally, that eggs of Thysanoesssa raschii and Meganyctiphanes
norvegica, no matter at what stage of development, sink at comparable
rates, namely 5-5-76 m per hr in water of 33x0 salinity and at a
temperature of 15°C. The rate of sinking, of course, decreases in waters
of the same salinity but of lower temperatures and the eggs sink at a rate
of 4-5 m per hr in water a t 0°C. Marshall and Orr (1953) found that
the much smaller eggs of the copepod, Calanus finmarchicus (Gunn.)
sink at a rate of 2 m per hr in water at 15°C and 1 m per hr in water
at 0°C Since the eggs of Euphausia superba are larger than these it is
probable that their rate of sinking is higher than that of the two northern
euphausiids. On present evidence from northern species, the time
elapsing between the egg being laid and the nauplius hatching out is
five to six days, that is about 150 hr. A sinking rate of 10 m per hr in
still conditions would cause eggs at the surface to sink 1 500 m in
that time. It is probable, however, that the sinking rate may be less
than this, and that the eggs are laid not only in the upper 50 m of
Antarctic water but also as deep as the 500 m layer. If so, then natural
sinking rates of the order of 7 m per hr would cause most of these eggs
to hatch at a depth of about 1 500 m or below (Figs 50, 51). Eggs of
species living in shallower water or, indeed, of E. superba spawning
over the continental shelf, tend to sink to the bottom and hatch out
there. Whether they can survive conditions on the sea bed is open to
question because the mortality of such eggs caused by predation of
bottom living organisms, including the euphausiids themselves, and
also cumaceans, amphipods and mysids, must be high. The highest
survival of eggs might be expected among those which are kept afloat
by the vertical movements within the water column.
Mauchline (unpublished) found that unsegmented embryos of
Thysanoessa raschii and Neganyctiphanes norvegiea occurred close to
the surface, segmented embryos deeper, and developing nauplii within
the egg membranes occurred deepest of all. Hatching of the nauplii
133
developing embryos. The major problem faced by Marr is the scarcity
of eggs of Euphausia superba in the plankton hauls made in the Antarctic
over a period of many years. Gravid females have been found in large
numbers in the surface waters but eggs appear to be very scarce and
Marr dwells considerably on the hypothesis that this euphausiid may
swim down to deeper layers to lay its eggs. The present authors consider this unlikely because if the eggs were laid at say 1 500 m then
hatching would take place at a considerably greater depth, owing to
the eggs sinking, and the nauplii, metanauplii, and calyptopes would
have a very long climb to the surface layers where the food for these
filter-feeding larvae is present.
Mauchline (unpublished) found,
experimentally, that eggs of Thysanoesssa raschii and Meganyctiphanes
norvegica, no matter at what stage of development, sink at comparable
rates, namely 5-5-76 m per hr in water of 33x0 salinity and at a
temperature of 15°C. The rate of sinking, of course, decreases in waters
of the same salinity but of lower temperatures and the eggs sink at a rate
of 4-5 m per hr in water a t 0°C. Marshall and Orr (1953) found that
the much smaller eggs of the copepod, Calanus finmarchicus (Gunn.)
sink at a rate of 2 m per hr in water at 15°C and 1 m per hr in water
at 0°C Since the eggs of Euphausia superba are larger than these it is
probable that their rate of sinking is higher than that of the two northern
euphausiids. On present evidence from northern species, the time
elapsing between the egg being laid and the nauplius hatching out is
five to six days, that is about 150 hr. A sinking rate of 10 m per hr in
still conditions would cause eggs at the surface to sink 1 500 m in
that time. It is probable, however, that the sinking rate may be less
than this, and that the eggs are laid not only in the upper 50 m of
Antarctic water but also as deep as the 500 m layer. If so, then natural
sinking rates of the order of 7 m per hr would cause most of these eggs
to hatch at a depth of about 1 500 m or below (Figs 50, 51). Eggs of
species living in shallower water or, indeed, of E. superba spawning
over the continental shelf, tend to sink to the bottom and hatch out
there. Whether they can survive conditions on the sea bed is open to
question because the mortality of such eggs caused by predation of
bottom living organisms, including the euphausiids themselves, and
also cumaceans, amphipods and mysids, must be high. The highest
survival of eggs might be expected among those which are kept afloat
by the vertical movements within the water column.
Mauchline (unpublished) found that unsegmented embryos of
Thysanoessa raschii and Neganyctiphanes norvegiea occurred close to
the surface, segmented embryos deeper, and developing nauplii within
the egg membranes occurred deepest of all. Hatching of the nauplii