64
R. 11. MILLAR
is oftcii dominated by a few species, as in the Canadian Arctic, whcrc of
27 ascidian species identified, thrcc (Boltenia ovifera, Boltenia echinnla
and Ascidia callosa) accounted for about half of all specimcns collccted
(Trason, 1964).
The Arctic ascidian fauna is, naturally enough, most closely allied
to that of the colder parts of the boreal regions of the Atlantic and
Pacific Oceans; and the eastern Pacific shows more afinity to the
Arctic than does the western Pacific (Tokioka, 1963). The land mass
encircling much of the Arctic Ocean can account for the relatively
uniform nature of the fauna, but for the gap made by the North Atlantic and the Greenland Sea. The submarine ridges linking northern
Europe, the Faeroes, Iceland and Greenland may have formed a bridge
across which species could pass (Huus, 1927, 1936), but Liitzen (1959)
suggests an alternative or additional route of ascidian migration, by
way of the unbroken shelf bordering the north polar sea, possibly during
periods when the Arctic was less cold than now.
Antarctic
The Antarctic region is one of the most clearly defined of the worlds
zoogeographic divisions, and its northern limit coincides rather closely
with the abrupt change in water temperature indicated by the Antarctic Convergence. It is distinguished, too, by the sudden increase in
depth at the edge of the shelf, and the Antarctic shelf fauna is in consequence somewhat isolated from neighbouring shelf faunas. Two
divisions have been recognized : the high-Antarctic sub-region consisting of the continental coastal area and the low Antarctic sub-region
including South Georgia and Shag Rock Bank (Ekman, 1953). Knox
(1960) accepts these provinces, but Kott (1969) in dealing with Antarctic ascidians takes the South Georgian province to include the
Bellingshausen Sea, the Antarctic Peninsula, and the South Shetland,
South Orkney and South Sandwich Islands.
The shelf fauna of the region contains about 73 ascidian species, this
total being based on the most recent accounts by Vinogradova (1962a)
and Kott (1969), but excluding some doubtful species. In other invertebrate groups most species seem to occur fairly generally in suitable
habitats around the continent (Mackintosh, 1960) and Kott (1969)
mentions five ascidians as circumpolar, although others appear to be
more localized. The South Georgian province, as defined by Kott,
contains a mixed fauna, and she named six ascidian species which extend southwards into it from the Subantarctic. Material collected by
the " Discovery " from South Georgia proper contained 15 species also
known Zrom adjacent Subantarctic areas, out of a total of 32 species
R. 11. MILLAR
is oftcii dominated by a few species, as in the Canadian Arctic, whcrc of
27 ascidian species identified, thrcc (Boltenia ovifera, Boltenia echinnla
and Ascidia callosa) accounted for about half of all specimcns collccted
(Trason, 1964).
The Arctic ascidian fauna is, naturally enough, most closely allied
to that of the colder parts of the boreal regions of the Atlantic and
Pacific Oceans; and the eastern Pacific shows more afinity to the
Arctic than does the western Pacific (Tokioka, 1963). The land mass
encircling much of the Arctic Ocean can account for the relatively
uniform nature of the fauna, but for the gap made by the North Atlantic and the Greenland Sea. The submarine ridges linking northern
Europe, the Faeroes, Iceland and Greenland may have formed a bridge
across which species could pass (Huus, 1927, 1936), but Liitzen (1959)
suggests an alternative or additional route of ascidian migration, by
way of the unbroken shelf bordering the north polar sea, possibly during
periods when the Arctic was less cold than now.
Antarctic
The Antarctic region is one of the most clearly defined of the worlds
zoogeographic divisions, and its northern limit coincides rather closely
with the abrupt change in water temperature indicated by the Antarctic Convergence. It is distinguished, too, by the sudden increase in
depth at the edge of the shelf, and the Antarctic shelf fauna is in consequence somewhat isolated from neighbouring shelf faunas. Two
divisions have been recognized : the high-Antarctic sub-region consisting of the continental coastal area and the low Antarctic sub-region
including South Georgia and Shag Rock Bank (Ekman, 1953). Knox
(1960) accepts these provinces, but Kott (1969) in dealing with Antarctic ascidians takes the South Georgian province to include the
Bellingshausen Sea, the Antarctic Peninsula, and the South Shetland,
South Orkney and South Sandwich Islands.
The shelf fauna of the region contains about 73 ascidian species, this
total being based on the most recent accounts by Vinogradova (1962a)
and Kott (1969), but excluding some doubtful species. In other invertebrate groups most species seem to occur fairly generally in suitable
habitats around the continent (Mackintosh, 1960) and Kott (1969)
mentions five ascidians as circumpolar, although others appear to be
more localized. The South Georgian province, as defined by Kott,
contains a mixed fauna, and she named six ascidian species which extend southwards into it from the Subantarctic. Material collected by
the " Discovery " from South Georgia proper contained 15 species also
known Zrom adjacent Subantarctic areas, out of a total of 32 species