310
THE BIOLOQY OF EUPHAUSIIDS
similarity between this analysis of growth of a species and those
described by the curves in Figs 115 and 116. The growth rate of
individuals in swarms from the cold East Wind area is less than that
in swarms from the warmer Weddell Sea region.
Marr has circled the modal values for seven swarms of adolescents
in February and March of the second year in Fig. 117. They were found
in the warm area of the South Georgia whaling ground. The values are
much higher than the average values and although the animals in the
seven swarms may simply have grown a t a faster rate in the warmer
waters of South Georgia, Marr suggests that they might not be l-yearolds at all, but 2-year-old animals which have not matured but will
mature in their third year. Baker (1959) suspects that a small portion
of the population of Euphausia triacantha and Nemoto (1957) of
Thysanoessa inermis may have a %year cycle. If conditions are not
suitable then maturity may be delayed. The conditions which are not
suitable are probably temperature and the amount of food available,
but experimental work is required to define optimal conditions for
allowing euphausiids to mature.
The maximum size attained by euphausiids is apparently influenced
by environmental temperature. Meganyctiphanes norvegiea is larger in
the Clyde than a t Iceland, and 0-group and I-group individuals off
Cadiz are larger than the corresponding age groups in the Clyde
(Fig. 115). Thysanoessa inermis from southern and southwestern
Iceland are larger than those from northern and eastern Iceland and if
these latter animals lived to their third year then, on the basis of their
growth curves, they would probably be larger than ones of a comparable age found off western Greenland. Ponomareva (1963) shows
that this species attains a larger size in the Sea of Japan than in the
Barents Sea. Marr finds that Euphausia superba in the Weddell Sea
grows bigger than in the East Wind drift. No comparisons have been
made of size attained in different years in the same area and under the
differing annual conditions of food and temperature but such an
investigation should be made.
The age a t which sexual maturity is attained can vary in a species
from one latitudinal area to another (Table XXX). The two Antarctic
species, E . superba and E. triacantha require 2 years to attain sexual
maturity. Species such as Thysanoessa raschii, T . inermis, and T .
longipes, when living close to Arctic regions, require 2 years to become
mature but when living farther south in warmer waters become mature
in 1 year. According to Nemoto (1957), T . inermis may require 3 years
to become sexually mature in the sea area northwest of the Aleutians.
Consequently, it is possible that Antarctic species such as Euphausia
THE BIOLOQY OF EUPHAUSIIDS
similarity between this analysis of growth of a species and those
described by the curves in Figs 115 and 116. The growth rate of
individuals in swarms from the cold East Wind area is less than that
in swarms from the warmer Weddell Sea region.
Marr has circled the modal values for seven swarms of adolescents
in February and March of the second year in Fig. 117. They were found
in the warm area of the South Georgia whaling ground. The values are
much higher than the average values and although the animals in the
seven swarms may simply have grown a t a faster rate in the warmer
waters of South Georgia, Marr suggests that they might not be l-yearolds at all, but 2-year-old animals which have not matured but will
mature in their third year. Baker (1959) suspects that a small portion
of the population of Euphausia triacantha and Nemoto (1957) of
Thysanoessa inermis may have a %year cycle. If conditions are not
suitable then maturity may be delayed. The conditions which are not
suitable are probably temperature and the amount of food available,
but experimental work is required to define optimal conditions for
allowing euphausiids to mature.
The maximum size attained by euphausiids is apparently influenced
by environmental temperature. Meganyctiphanes norvegiea is larger in
the Clyde than a t Iceland, and 0-group and I-group individuals off
Cadiz are larger than the corresponding age groups in the Clyde
(Fig. 115). Thysanoessa inermis from southern and southwestern
Iceland are larger than those from northern and eastern Iceland and if
these latter animals lived to their third year then, on the basis of their
growth curves, they would probably be larger than ones of a comparable age found off western Greenland. Ponomareva (1963) shows
that this species attains a larger size in the Sea of Japan than in the
Barents Sea. Marr finds that Euphausia superba in the Weddell Sea
grows bigger than in the East Wind drift. No comparisons have been
made of size attained in different years in the same area and under the
differing annual conditions of food and temperature but such an
investigation should be made.
The age a t which sexual maturity is attained can vary in a species
from one latitudinal area to another (Table XXX). The two Antarctic
species, E . superba and E. triacantha require 2 years to attain sexual
maturity. Species such as Thysanoessa raschii, T . inermis, and T .
longipes, when living close to Arctic regions, require 2 years to become
mature but when living farther south in warmer waters become mature
in 1 year. According to Nemoto (1957), T . inermis may require 3 years
to become sexually mature in the sea area northwest of the Aleutians.
Consequently, it is possible that Antarctic species such as Euphausia