140
THE BIOLOGY O F EUPHAUSIIDS
movement of the larvae below 5 0 m takes place and these larvae
become dispersed vertically throughout the whole water column instead
of in the lower two thirds of it. This would account, in part, for the
substantial decrease in numbers during the hours of darkness of larvae
caught per haul a t depths of 5 0 m or more. There may also be a
horizontal dispersion of larvae a t night ; during the day they are found
in the water column of the deeper areas of the Firth of Clyde but at
night they may disperse horizontally over shallower areas and this
would account for a further part of the decrease in numbers caught. An
examination of Lacroix's (1961) data on the vertical migration of
furciliae I-V suggests that he too obtained fewer larvae in his hauls at
night than in those during the day. Chaleur Bay also has shallower water
close to the deeper water and once again horizontal dispersion during
the hours of darkness may be taking place. Much further work on the
behaviour of these larvae in relation to light and other factors is
required.
The behaviour of the older furciliae in the Clyde is much more conventional (Fig. 53). These larvae aggregate close to the surface film
but there is no unexplainable decrease in numbers caught a t greater
depths as in the case of furciliae 1-111. These late larvae are stronger
swimmers than the earlier larvae but there is no evidence that individual larvae migrate through distances greater than the range
50-100 m and a large proportion of those living close to the sea
bottom during the day remain there a t night (Fig. 53). Lacroix (1961)
found that the fifth furciliae of Meganyctiphanes norvegica were absent
from his deepest hauls (44-70 m) during the night although present
there during the day; since this involves a vertical journey of about
30 m, his pattern of migration is in agreement with that found in the
Clyde. He found a similar movement in the later furciliae of M .
norvegica and Thysanoessa raschii, namely that very few of these
larvae remained close to the bottom during the hours of darkness.
I n the deeper areas of the Firth of Clyde (150 m) a notablevproportion of the furciliae of both species living close to the bottom
during the day remain there during the night. The vertical distribution
of the furciliae during daylight hours (Fig. 53) is as follows : all furciliae
of Meganyctiphanes norvegica and Thysanoessa raschii, except the
largest one or two stages, are usually distributed throughout the water
column (Fig. 53) although there is frequently a tendency for the smaller
furciliae to occur nearer the surface than the larger stages. The last
one or two furciliae and the adolescents tend to live a t the same depths
as the adults and to be absent from the surface layers during hours of
daylight.
THE BIOLOGY O F EUPHAUSIIDS
movement of the larvae below 5 0 m takes place and these larvae
become dispersed vertically throughout the whole water column instead
of in the lower two thirds of it. This would account, in part, for the
substantial decrease in numbers during the hours of darkness of larvae
caught per haul a t depths of 5 0 m or more. There may also be a
horizontal dispersion of larvae a t night ; during the day they are found
in the water column of the deeper areas of the Firth of Clyde but at
night they may disperse horizontally over shallower areas and this
would account for a further part of the decrease in numbers caught. An
examination of Lacroix's (1961) data on the vertical migration of
furciliae I-V suggests that he too obtained fewer larvae in his hauls at
night than in those during the day. Chaleur Bay also has shallower water
close to the deeper water and once again horizontal dispersion during
the hours of darkness may be taking place. Much further work on the
behaviour of these larvae in relation to light and other factors is
required.
The behaviour of the older furciliae in the Clyde is much more conventional (Fig. 53). These larvae aggregate close to the surface film
but there is no unexplainable decrease in numbers caught a t greater
depths as in the case of furciliae 1-111. These late larvae are stronger
swimmers than the earlier larvae but there is no evidence that individual larvae migrate through distances greater than the range
50-100 m and a large proportion of those living close to the sea
bottom during the day remain there a t night (Fig. 53). Lacroix (1961)
found that the fifth furciliae of Meganyctiphanes norvegica were absent
from his deepest hauls (44-70 m) during the night although present
there during the day; since this involves a vertical journey of about
30 m, his pattern of migration is in agreement with that found in the
Clyde. He found a similar movement in the later furciliae of M .
norvegica and Thysanoessa raschii, namely that very few of these
larvae remained close to the bottom during the hours of darkness.
I n the deeper areas of the Firth of Clyde (150 m) a notablevproportion of the furciliae of both species living close to the bottom
during the day remain there during the night. The vertical distribution
of the furciliae during daylight hours (Fig. 53) is as follows : all furciliae
of Meganyctiphanes norvegica and Thysanoessa raschii, except the
largest one or two stages, are usually distributed throughout the water
column (Fig. 53) although there is frequently a tendency for the smaller
furciliae to occur nearer the surface than the larger stages. The last
one or two furciliae and the adolescents tend to live a t the same depths
as the adults and to be absent from the surface layers during hours of
daylight.