4. THE LARVAE
139
a few hours are probable but greater distances are impossible because
they cannot swim fast enough.
A similar statement on the lack of swimming prowess also applies
to the early furciliae in which the pleopods are developing. An aggregation of furciliae I, 11, and I11 as well as of calyptopes of Thysanoessa
raschii and Meganyctiphanes norvegica was present under the surface
film in the Firth of Clyde, but the number of these larvae caught a t
Day
Night
Day
Night
Day
Night
I - = - . rn
f
(3
Furciliae I-IU
Furciliae IV-VIU Furcillae M-XI
,501 W
I
I
I
I I DD
Cal.
F I
TI
lU
IV
V
VI
W
FIG. 53. Upper graph : the vertical distribution of the furciliae of Thysanoessa raschii
Lower graph : the vertical distribution of calyptopes and furciliae of MegaThe sizes of the blocks are proportional to the numbers caught. (After Mauchby day and night in the Firth of Clyde.
fiyctiphanes norvegica during the day.
line, 1959, 1965a.)
greater depth decreased from several hundred per haul to 20-30
individuals, as illustrated for Thysanoessa raschii in Fig. 53. This
is what would happen if an active vertical movement of the population
took place but it seems very unlikely that these relatively poor swimmers could or would migrate vertically through a range of a 100 or
more metres. It seems more likely that the larvae in the upper 50 m
of the water column move right up to the surface and that a vertical