118
R. K. DBLL
the specialized CnemicEomrpa zenkevitchi Vinogradova has been recorded
from less than 60 m. The greatest depth range for any of these species
is 200 m (Aplidium vastaturn (Sluiter)).
3. Species with a limited distribution in the antarctic subregion,
Georgian province. I n Kott's sense this province extends from the
Bellingshausen Sea, along the Antarctic Peninsula, and through the
South Shetlands, South Orkneys and South Sandwich Islands to South
Georgia. This is an " overlap '' zone where six subantarctic species
extend south to the South Shetlands or the Antarctic Peninsula, while
nine antarctic species extend to varying degrees to the north.
Kott (1969b) concluded that Antarctic ascidians are more diverse
than would be expected considering the physical conditions. Some 92
species are recorded from shelf and slope, of which 16 extend north of
the Subtropical Convergence.
Between 2 000 and 6 000 m, 21 species have been obtained. Only
six of these have been recorded from other abyssal basins.
Kott suspected that certain of the obvious adaptations seen in the
Antarctic ascidians have assisted in their success in the area. There
are large populations of single species. Some species obviously live for
a long period resulting in giant forms, e.g. Molgula gigantea Herdman
and Paramolgula gregaria (Lesson), both from the Antarctic, are the
largest simple ascidians known, Paramolgula attaining a length of
330mm. The grmter prevalence of viviparous species, and special
adaptations in relation t o this are discussed in the general section on
viviparity and brooding amongst Antarctic animals. Most successful
species in the Antarctic are obviously able to tolerate a wide bathymetric range, a facility which is important in a region where shallow
water is usually untenable for fixed forms, and where the shelf in places
may be exceptionally deep, and in others be almost lacking. Kott
suggested that one of the responses to the physical conditions for life
in Antarctic seas may well be the general convergence in external
characters which many ascidians from very diverse families exhibit.
Amongst the Antarctic ascidians there is a high degree of speciiic
and generic endemism. Thus the abyssal genera Protoholozoa, Pharyngodictyon, Abyssascidea and Fungulua, and the shallower water Bathypera, Pareugyrioides, Caenagneaia, Alloeocarpa and Oligocarpa are all
endemic. The genus Sycoxoa may well have developed in Antarctic
waters, but has since spread widely in the Southern Hemisphere.
Many of the species are adapted for life on the open sea bed of the
Antarctic continental shelf. Large numbers of sand or mud dwelling
members of the family Molgulidae occur, and some species of the
families Ppridae, Styelidae and Agnesiidae all show adaptations for
R. K. DBLL
the specialized CnemicEomrpa zenkevitchi Vinogradova has been recorded
from less than 60 m. The greatest depth range for any of these species
is 200 m (Aplidium vastaturn (Sluiter)).
3. Species with a limited distribution in the antarctic subregion,
Georgian province. I n Kott's sense this province extends from the
Bellingshausen Sea, along the Antarctic Peninsula, and through the
South Shetlands, South Orkneys and South Sandwich Islands to South
Georgia. This is an " overlap '' zone where six subantarctic species
extend south to the South Shetlands or the Antarctic Peninsula, while
nine antarctic species extend to varying degrees to the north.
Kott (1969b) concluded that Antarctic ascidians are more diverse
than would be expected considering the physical conditions. Some 92
species are recorded from shelf and slope, of which 16 extend north of
the Subtropical Convergence.
Between 2 000 and 6 000 m, 21 species have been obtained. Only
six of these have been recorded from other abyssal basins.
Kott suspected that certain of the obvious adaptations seen in the
Antarctic ascidians have assisted in their success in the area. There
are large populations of single species. Some species obviously live for
a long period resulting in giant forms, e.g. Molgula gigantea Herdman
and Paramolgula gregaria (Lesson), both from the Antarctic, are the
largest simple ascidians known, Paramolgula attaining a length of
330mm. The grmter prevalence of viviparous species, and special
adaptations in relation t o this are discussed in the general section on
viviparity and brooding amongst Antarctic animals. Most successful
species in the Antarctic are obviously able to tolerate a wide bathymetric range, a facility which is important in a region where shallow
water is usually untenable for fixed forms, and where the shelf in places
may be exceptionally deep, and in others be almost lacking. Kott
suggested that one of the responses to the physical conditions for life
in Antarctic seas may well be the general convergence in external
characters which many ascidians from very diverse families exhibit.
Amongst the Antarctic ascidians there is a high degree of speciiic
and generic endemism. Thus the abyssal genera Protoholozoa, Pharyngodictyon, Abyssascidea and Fungulua, and the shallower water Bathypera, Pareugyrioides, Caenagneaia, Alloeocarpa and Oligocarpa are all
endemic. The genus Sycoxoa may well have developed in Antarctic
waters, but has since spread widely in the Southern Hemisphere.
Many of the species are adapted for life on the open sea bed of the
Antarctic continental shelf. Large numbers of sand or mud dwelling
members of the family Molgulidae occur, and some species of the
families Ppridae, Styelidae and Agnesiidae all show adaptations for