THE BIOLOGY OF ASCIDIANS
19
-
30Q.
.- CI
CI
3. Behaviour
The ascidian larva has a characteristic pattern of behaviour, consisting of an initial period when it swims upwards (positive phototropism and negative geotropism) followed by a period when it swims
or sinks downwards (Grave, 1920, 1926; Mast, 1921 ; Grave and Woodbridge, 1924 ; Sebastian, 1953). The initial phase serves to distribute
larvae, and it is in the second phase that the critical reaction is elicited
in response to a decrease in light. The larva then swims towards dark
areas, which in nature tend to be the vertical or lower surfaces of rocks
etc. At this time a large tailed larva is advantageous since it is able to
turn, swim quickly and attach to the substratum (Berrill, 1955), and
Berrill (1931) has shown that the larger the larva, the faster it swims
0
1 I I.
0
. 0 .
I
I
1
0.1
0.2
0.3
0 4
0.5
0.6
Diameter of egg. mm
Pra. 7. Relationship bctwecn longth of lerval life and size of egg, in various species. The
vcrtical lincs indicate the range of larval life within a spocios, whom this is known
(from data of authors mcntioncd in text).
The duration of free swimming life under laboratory conditions
varies from a few minutes to several days, according to species. With
the exception of a few in which the embryo is nourished from the
parental zooid, large larvae develop only from large yolky eggs, and
might be expected to have correspondingly long swimming periods.
However, Fig. 7, based on information from several sources (Grave,
1921, 1935; Berrill, 1931, 1935a, 1947b, 1948a, b, c, 1950; Millar, 1951,
1954b; Brewin, 1959; Sebastian, 1953, 1954) shows no strict relationship between larval duration and egg size (taken as a measure of larval
A.X.B.--8
2
19
-
30Q.
.- CI
CI
3. Behaviour
The ascidian larva has a characteristic pattern of behaviour, consisting of an initial period when it swims upwards (positive phototropism and negative geotropism) followed by a period when it swims
or sinks downwards (Grave, 1920, 1926; Mast, 1921 ; Grave and Woodbridge, 1924 ; Sebastian, 1953). The initial phase serves to distribute
larvae, and it is in the second phase that the critical reaction is elicited
in response to a decrease in light. The larva then swims towards dark
areas, which in nature tend to be the vertical or lower surfaces of rocks
etc. At this time a large tailed larva is advantageous since it is able to
turn, swim quickly and attach to the substratum (Berrill, 1955), and
Berrill (1931) has shown that the larger the larva, the faster it swims
0
1 I I.
0
. 0 .
I
I
1
0.1
0.2
0.3
0 4
0.5
0.6
Diameter of egg. mm
Pra. 7. Relationship bctwecn longth of lerval life and size of egg, in various species. The
vcrtical lincs indicate the range of larval life within a spocios, whom this is known
(from data of authors mcntioncd in text).
The duration of free swimming life under laboratory conditions
varies from a few minutes to several days, according to species. With
the exception of a few in which the embryo is nourished from the
parental zooid, large larvae develop only from large yolky eggs, and
might be expected to have correspondingly long swimming periods.
However, Fig. 7, based on information from several sources (Grave,
1921, 1935; Berrill, 1931, 1935a, 1947b, 1948a, b, c, 1950; Millar, 1951,
1954b; Brewin, 1959; Sebastian, 1953, 1954) shows no strict relationship between larval duration and egg size (taken as a measure of larval
A.X.B.--8
2