126
THE BIOLOGY OF EUPHAUSIIDS
setose pleopods ; the notation sp refers to the number of terminal spines
on the telson while PL records the number of pairs of post-lateral
spines. The letters A or B indicate that there are other larvae of
similar appearance with a different history.
Larvae of increasing complexity tend to be larger (Fig. 48,
Table 11). A close examination of Fig. 48 shows that larvae with, for
example, three pairs of non-setose pleopods range from 2.05 to
3.00 mm in total length. These moult to one of the three following types
of larvae : (1) with three pairs of setose pleopods, (2) with three pairs of
setose pleopods and one pair of non-setose pleopods, or (3) with three
pairs of setose pleopods and two pairs of non-setose pleopods. Not all
larvae, however, with three pairs of non-setose pleopods have a choice ;
there is a strong tendency for the smallest larvae to moult to the form
with three pairs of setose pleopods while the medium-sized larvae moult
to the form with three pairs of setose pleopods and one pair of nonsetose pleopods ; the form with three pairs of setose and two pairs of
non-setose pleopods derives mainly from the larger larvae with three
pairs of non-setose pleopods. What appears to be happening is that
moults are being interposed in a path of continuous development
because the state of development is related to size but moulting is not
directly influenced by size. The factors controlling moulting of larvae
are unknown.
This large sample of N . australis consisted of several sub-samples
taken from different sea areas off the southeast coast of Australia at
different times during the years 1938 to 1944. Analyses of individual
sub-samples frequently showed a very different sequence of development to be present in different areas so that restrictive sampling,
geographically and in time, can produce very misleading results. This
problem is amply illustrated in Fig. 49. The development of Thysanoessa raschii was examined in the Firth of Clyde during two years, 1957
and 1963. I n 1957 the developmental path of the majority of larvae,
during the earlier part of the breeding season, was from the form without
pleopods to that with five pairs of non-setose pleopods but this changed
in the latter part of the breeding season and the form without pleopods
moulted, in the majority of cases, to that with four pairs of non-setose
pleopods. The latter path of development was dominant throughout
the entire breeding season in 1963 and was also dominant off northwest
Iceland. Larvae off western Greenland, however, had five pairs of
non-setose pleopods occurring most commonly. Fraser (1 936) found
the development of Euphausia superba to follow the same pathway at
ten sampling stations ; the numbers of larvae caught a t a further nine
stations were too small to determine the dominant pathway but at a
THE BIOLOGY OF EUPHAUSIIDS
setose pleopods ; the notation sp refers to the number of terminal spines
on the telson while PL records the number of pairs of post-lateral
spines. The letters A or B indicate that there are other larvae of
similar appearance with a different history.
Larvae of increasing complexity tend to be larger (Fig. 48,
Table 11). A close examination of Fig. 48 shows that larvae with, for
example, three pairs of non-setose pleopods range from 2.05 to
3.00 mm in total length. These moult to one of the three following types
of larvae : (1) with three pairs of setose pleopods, (2) with three pairs of
setose pleopods and one pair of non-setose pleopods, or (3) with three
pairs of setose pleopods and two pairs of non-setose pleopods. Not all
larvae, however, with three pairs of non-setose pleopods have a choice ;
there is a strong tendency for the smallest larvae to moult to the form
with three pairs of setose pleopods while the medium-sized larvae moult
to the form with three pairs of setose pleopods and one pair of nonsetose pleopods ; the form with three pairs of setose and two pairs of
non-setose pleopods derives mainly from the larger larvae with three
pairs of non-setose pleopods. What appears to be happening is that
moults are being interposed in a path of continuous development
because the state of development is related to size but moulting is not
directly influenced by size. The factors controlling moulting of larvae
are unknown.
This large sample of N . australis consisted of several sub-samples
taken from different sea areas off the southeast coast of Australia at
different times during the years 1938 to 1944. Analyses of individual
sub-samples frequently showed a very different sequence of development to be present in different areas so that restrictive sampling,
geographically and in time, can produce very misleading results. This
problem is amply illustrated in Fig. 49. The development of Thysanoessa raschii was examined in the Firth of Clyde during two years, 1957
and 1963. I n 1957 the developmental path of the majority of larvae,
during the earlier part of the breeding season, was from the form without
pleopods to that with five pairs of non-setose pleopods but this changed
in the latter part of the breeding season and the form without pleopods
moulted, in the majority of cases, to that with four pairs of non-setose
pleopods. The latter path of development was dominant throughout
the entire breeding season in 1963 and was also dominant off northwest
Iceland. Larvae off western Greenland, however, had five pairs of
non-setose pleopods occurring most commonly. Fraser (1 936) found
the development of Euphausia superba to follow the same pathway at
ten sampling stations ; the numbers of larvae caught a t a further nine
stations were too small to determine the dominant pathway but at a