8
R. € I .
IKJLLAR
Diehl, 1957 ; Dybern, 1965,1969b ; Millar, 1952, 1954b, 1960 ; Sabbadin,
1955, 1957). Statistical analysis of size distribution in the adult
populations, using the probability paper method (Harding, 1949 ;
Cassie, 1950), should further refine estimates of the dates of larval
settlement, but the method has not yet been used in ascidian studies.
Temperature is generally recognized as a major factor controlling
the sexual reproduction of marine invertebrates. Hutchins (1947), in
discussing the bases for temperature zonation in geographical distribution in the sea, noted two situations in which temperature requirements will limit breeding : at the summer poleward boundary, beyond
which the sea is too cold to permit breeding ; and at the winter equatorial boundary, beyond which it is too warm. Towards these geographical boundaries the timing and duration of the breeding season
can be expected to show variations from the pattern prevailing over
the main part of the range of a species. In only a few ascidian species is
sufficient information available to test these ideas. The influence of
temperature on the breeding season may be seen in Ciona intestinalis,
a particularly favourable species since it occupies such a wide latitudinal
range, and Dybern (1965) has summarized the observations of other
workers (Berrill, 1935a; Orton, 1914, 1920; Millar, 1952; Sabbadin,
1957 ; Komarovsky and Schwartz, 1957 ; Millard, 1952 ; Runnstrclm,
1929, 1936) and added fresh evidence from Swedish populations. He
showed that f. typica breeds during all or most of the year in the
Mediterranean, and that the season is progressively restricted to the
summer months towards the northern parts of its range. Runnstrclm
(1927, 1929, 1936) had concluded that f. typicu is divided into a number
of races each with its own breeding temperature characteristics, but
Dybern doubts whether Runnstrclm’s experiments and hypothesis were
sound. This, however, is a disagreement about genetic differentiation ;
the controlling influence of temperature on the breeding season is not
in dispute.
Botryllw schlosseri is another species of wide distribution whose
breeding season in a number of localities is known (Lo Bianco, 1909;
Millar, 1952 ; Sabbadin, 1955 ; L’Hardy, 1962 ; Polk, 1962). It is evident
from Fig. 2 that the duration of breeding is progressively restricted,
presumably by temperature, towards the cooler more northerly parts
of the geographical range.
An instance of the very restricted breeding season at the distributional limit of a species was investigated in Pelonuia corrwata
Goodsir and Forbes (Millar, 1954a). At its southern boundary this
boreo-arctic species breeds over a period of only 2-4 weeks in January
and February, when the sea temperature is near its yearly minimum.
R. € I .
IKJLLAR
Diehl, 1957 ; Dybern, 1965,1969b ; Millar, 1952, 1954b, 1960 ; Sabbadin,
1955, 1957). Statistical analysis of size distribution in the adult
populations, using the probability paper method (Harding, 1949 ;
Cassie, 1950), should further refine estimates of the dates of larval
settlement, but the method has not yet been used in ascidian studies.
Temperature is generally recognized as a major factor controlling
the sexual reproduction of marine invertebrates. Hutchins (1947), in
discussing the bases for temperature zonation in geographical distribution in the sea, noted two situations in which temperature requirements will limit breeding : at the summer poleward boundary, beyond
which the sea is too cold to permit breeding ; and at the winter equatorial boundary, beyond which it is too warm. Towards these geographical boundaries the timing and duration of the breeding season
can be expected to show variations from the pattern prevailing over
the main part of the range of a species. In only a few ascidian species is
sufficient information available to test these ideas. The influence of
temperature on the breeding season may be seen in Ciona intestinalis,
a particularly favourable species since it occupies such a wide latitudinal
range, and Dybern (1965) has summarized the observations of other
workers (Berrill, 1935a; Orton, 1914, 1920; Millar, 1952; Sabbadin,
1957 ; Komarovsky and Schwartz, 1957 ; Millard, 1952 ; Runnstrclm,
1929, 1936) and added fresh evidence from Swedish populations. He
showed that f. typica breeds during all or most of the year in the
Mediterranean, and that the season is progressively restricted to the
summer months towards the northern parts of its range. Runnstrclm
(1927, 1929, 1936) had concluded that f. typicu is divided into a number
of races each with its own breeding temperature characteristics, but
Dybern doubts whether Runnstrclm’s experiments and hypothesis were
sound. This, however, is a disagreement about genetic differentiation ;
the controlling influence of temperature on the breeding season is not
in dispute.
Botryllw schlosseri is another species of wide distribution whose
breeding season in a number of localities is known (Lo Bianco, 1909;
Millar, 1952 ; Sabbadin, 1955 ; L’Hardy, 1962 ; Polk, 1962). It is evident
from Fig. 2 that the duration of breeding is progressively restricted,
presumably by temperature, towards the cooler more northerly parts
of the geographical range.
An instance of the very restricted breeding season at the distributional limit of a species was investigated in Pelonuia corrwata
Goodsir and Forbes (Millar, 1954a). At its southern boundary this
boreo-arctic species breeds over a period of only 2-4 weeks in January
and February, when the sea temperature is near its yearly minimum.