62
R. H. MILLAR
the Mediterranean. Polyclinum aurantium, Aplidium nordmanni,
Corella parallelogramma, Pyura tesselata and Microcosmus claudicuns are
perhaps the species most nearly endemic, since none extends into the
Arctic and, although all occur in the Mediterranean, P6rBs (1958) notes
them as rare.
Boreal species living on the continental slope may penetrate southwards beyond the latitudinal limit applying in littoral and inshore
areas. Thus at depths of a few hundred metres in the Bay of Biscay an
ascidian fauna exists which has more affinity with that of Scandinavia
than with the fauna of the adjacent French coast (C. Monniot, 1969).
Amongst the large Boreal-Mediterranean and Arctic-Boreal elements the limits for individual species in some cases at least appear to
depend on temperatures which limit breeding (Millar, 1954a, 195413).
A number of European ascidians are known also from the western
Atlantic. Many of these are northern species with a more or less continuous distribution including Iceland and southern Greenland. The
bridge formed by these islands together with the Wyville-Thompson
Ridge accounts for the presence of species on each side of the otherwise
impassable barrier of the deep Atlantic Ocean (Huus, 1927, 1936).
Huus and Knudsen (1950) remarked that all of the 33 species known
from Iceland, with a single exception, occur also on both the western
and eastern sides of the Atlantic. Similarly, Lutzen (1959) showed that
a high proportion of ascidians from Greenland are also known from
both European and American coasts, although only some penetrate
southwards into European boreal waters.
Within the region the Baltic Sea constitutes an area of highly
specialized conditions, characterized by the low and variable salinity.
As noted elsewhere (p. 42) few ascidians can withstand very reduced
salinities, and their limited powers of osmotic and ionic regulation
(Barrington, 1965) account for a failure of the group as a whole to
penetrate much beyond the transitional zone of the Belt-sea. Thus only
three out of ten species collected from the Oresund and Kattegatt extended to areas of markedly reduced salinity (Millar, 1959b), and Dybern (1967) states that of about 30 species in the more saline approaches
to the Baltic only three can be found in the southern Baltic proper. The
distributional maps of Hartmeyer (1923, 1924) confirm the poverty of
the Baltic ascidian fauna.
Boreal American Atlantic
Ekman (1953) accepts Cape Hatteras as approximately the southern
limit of the temperate fauna of the Atlantic coast of North America.
The northern limit, although more difficult to establish, is taken to be
R. H. MILLAR
the Mediterranean. Polyclinum aurantium, Aplidium nordmanni,
Corella parallelogramma, Pyura tesselata and Microcosmus claudicuns are
perhaps the species most nearly endemic, since none extends into the
Arctic and, although all occur in the Mediterranean, P6rBs (1958) notes
them as rare.
Boreal species living on the continental slope may penetrate southwards beyond the latitudinal limit applying in littoral and inshore
areas. Thus at depths of a few hundred metres in the Bay of Biscay an
ascidian fauna exists which has more affinity with that of Scandinavia
than with the fauna of the adjacent French coast (C. Monniot, 1969).
Amongst the large Boreal-Mediterranean and Arctic-Boreal elements the limits for individual species in some cases at least appear to
depend on temperatures which limit breeding (Millar, 1954a, 195413).
A number of European ascidians are known also from the western
Atlantic. Many of these are northern species with a more or less continuous distribution including Iceland and southern Greenland. The
bridge formed by these islands together with the Wyville-Thompson
Ridge accounts for the presence of species on each side of the otherwise
impassable barrier of the deep Atlantic Ocean (Huus, 1927, 1936).
Huus and Knudsen (1950) remarked that all of the 33 species known
from Iceland, with a single exception, occur also on both the western
and eastern sides of the Atlantic. Similarly, Lutzen (1959) showed that
a high proportion of ascidians from Greenland are also known from
both European and American coasts, although only some penetrate
southwards into European boreal waters.
Within the region the Baltic Sea constitutes an area of highly
specialized conditions, characterized by the low and variable salinity.
As noted elsewhere (p. 42) few ascidians can withstand very reduced
salinities, and their limited powers of osmotic and ionic regulation
(Barrington, 1965) account for a failure of the group as a whole to
penetrate much beyond the transitional zone of the Belt-sea. Thus only
three out of ten species collected from the Oresund and Kattegatt extended to areas of markedly reduced salinity (Millar, 1959b), and Dybern (1967) states that of about 30 species in the more saline approaches
to the Baltic only three can be found in the southern Baltic proper. The
distributional maps of Hartmeyer (1923, 1924) confirm the poverty of
the Baltic ascidian fauna.
Boreal American Atlantic
Ekman (1953) accepts Cape Hatteras as approximately the southern
limit of the temperate fauna of the Atlantic coast of North America.
The northern limit, although more difficult to establish, is taken to be