THE BIOLOGY OF ASCIDIAN’S
26
out the geographical range of the species, but as a rule two generations
live side by side for much of the time.
Comparisons drawn from populations living in widely separated
regions are suspect because of the possible existence of physiological
races (Runnstram, 1927, 1929, 1936), although Dybern doubts the
evidence for these. The effect of temperature is clearly shown, however,
by a comparison of the growth and annual cycle of C. intestinalis in two
British docks, one of which was subject to heating by a power station
(Millar, 1952; Naylor, 1965a, 1965b) (Fig. 10). In a single population,
too, the growth rate rises with the temperature, and Sentz-Braconnot
(1966) found that Ciona settling in the early part of the summer reached
0
90
@
0
V
d
4 0
2 0
1
I
Jan
Feb
Mar
*Pr
May
Jun
Ju 1
Aue
SeP
Oct
Nov
Dec
-
0
2
4
6
E l 0
Body length, cm
FIQ. 10. Effect of temperature on growth and annual cycle in Ciona inleatidia ; A, in a
dock at Ardrossen, Scotland, and B, in a heated dock at Swansee, Wales (redrawn
from Neylor, 1966b).