THE BIOLOGY OF ASCIDIANS
66
(Millar, 1960). The faunistic relationship between South Georgia and
the Magellanic and Falklaiid Islands area of the Subantarctic is also
confirmed by the results of the Norwegian Antarctic Expeditions of
1928-30 (Millar, 1968). The mixed nature of the South Georgian fauna
corresponds to a temperature regime intermediate between the Antarctic and cold temperate (Knox, 1960). As a whole the Antarctic has
a large endemic element (Kott, 1969).
A comparison has sometimes been drawn between the richness in
species of the Antarctic and the relative poverty of the Arctic (Ekman,
1953). This contrast is not borne out by the ascidians, which havc about
as many known species in the north as in the south polar areas ; it is not
likely that a very unequal collecting effort is responsible for this result,
because the ascidians of both regions have been described from most of
the major expeditions.
Kott (1969) speculated on the characteristics of the ascidian fauna
of the Antarctic and Subantarctic. She concluded that viviparity is
especially common, as shown by the prevalence of the Molgulidae,
Agnesiidae and Polyzoinae. The evidence is rather slight since for
instance only a few of the southern molgulids have been proved to be
viviparous, although the disposition of the oviducts sometimes suggests
that the eggs may be retained. Moreover the Polyzoinae are widely
distributed in temperate and warm waters and their viviparous habit
seems to be unrelated to polar conditions. Longevity and the large size
of individuals or coIonies are frequent characters (Millar, 1960 ; Kott,
1969) and may be related to the richness of phytoplankton as well as to
low temperature. Protective siphonal closing mechanisms are also
mentioned by Kott, but it is doubtful whether they are particularly
frequent in Antarctic ascidians, and indeed their relevance to Antarctic
conditions is not clear. Kott also considered tho fauna to have many
primitive elements, the families Agnesiidae and Corellidae being cited.
The former has few species and is certainly well represented in south
polar seas, but the Corellidae, with the possible exception of abyssal
species, are as well represented in other areas. There is, too, little reason
to regard the Corellidae or Agnesiidae as more primitive than the
Ascidiidae, which has few Antarctic species.
Antiboreal
Between the Antarctic Convergence and the Anti-boreal Convergence (or Subtropical Convergence) is an extensive area, containing
a number of small isolated islands. In addition, the south-eastern and
south-western coasts of South America and thc South Island of New
66
(Millar, 1960). The faunistic relationship between South Georgia and
the Magellanic and Falklaiid Islands area of the Subantarctic is also
confirmed by the results of the Norwegian Antarctic Expeditions of
1928-30 (Millar, 1968). The mixed nature of the South Georgian fauna
corresponds to a temperature regime intermediate between the Antarctic and cold temperate (Knox, 1960). As a whole the Antarctic has
a large endemic element (Kott, 1969).
A comparison has sometimes been drawn between the richness in
species of the Antarctic and the relative poverty of the Arctic (Ekman,
1953). This contrast is not borne out by the ascidians, which havc about
as many known species in the north as in the south polar areas ; it is not
likely that a very unequal collecting effort is responsible for this result,
because the ascidians of both regions have been described from most of
the major expeditions.
Kott (1969) speculated on the characteristics of the ascidian fauna
of the Antarctic and Subantarctic. She concluded that viviparity is
especially common, as shown by the prevalence of the Molgulidae,
Agnesiidae and Polyzoinae. The evidence is rather slight since for
instance only a few of the southern molgulids have been proved to be
viviparous, although the disposition of the oviducts sometimes suggests
that the eggs may be retained. Moreover the Polyzoinae are widely
distributed in temperate and warm waters and their viviparous habit
seems to be unrelated to polar conditions. Longevity and the large size
of individuals or coIonies are frequent characters (Millar, 1960 ; Kott,
1969) and may be related to the richness of phytoplankton as well as to
low temperature. Protective siphonal closing mechanisms are also
mentioned by Kott, but it is doubtful whether they are particularly
frequent in Antarctic ascidians, and indeed their relevance to Antarctic
conditions is not clear. Kott also considered tho fauna to have many
primitive elements, the families Agnesiidae and Corellidae being cited.
The former has few species and is certainly well represented in south
polar seas, but the Corellidae, with the possible exception of abyssal
species, are as well represented in other areas. There is, too, little reason
to regard the Corellidae or Agnesiidae as more primitive than the
Ascidiidae, which has few Antarctic species.
Antiboreal
Between the Antarctic Convergence and the Anti-boreal Convergence (or Subtropical Convergence) is an extensive area, containing
a number of small isolated islands. In addition, the south-eastern and
south-western coasts of South America and thc South Island of New