56
R. 11. MILLAR
coasts of South America. Moreover, the accounts of these faunas which
have been extensively studied reflect the varying viewpoints of different
systematists, some of whom have been more ready than others to create
new species.
A. Shallow-water ascidians
Assuming that most of the species are now known in the areas which
have been reasonably well studied, it is of interest to look at the relative
richness of the faunas. In doing so it is necessary to compare like with
like, by taking areas defined by the homogeneity of their faunas and
having regard to the hydrographic conditions, sudden changes in which,
over a short distance, often determine faunistic boundaries. Figure 22
shows the approximate number of ascidian species believed to occur in
the main shallow-water zoogeographical regions. The regions are
broadly those defined by Ekman (1953), but with some local modifications adopted by students of particular areas.
It is evident that certain faunas are much more diverse than others,
the Mediterranean, Indo-Malayan, southern Japanese and northwestern Pacific faunas being notably rich in species, the polar seas,
subantarctic and temperate North American Atlantic less so, and in
general the warm-water faunas have most species, although not
necessarily most individuals. It should be noted, incidentally, that
the total number of species throughout the world is not the sum of the
numbers of all the regions, because by no means all species are confined
to one faunistic area, many being shared by adjacent or even by several
more distant areas.
The warm-water regions, which will be considered first, are remarkable both for their richness and for the number of species of wide
distribution. And some genera, such as Eudistomu, although not confined to warm regions, find by far their greatest development there.
Indo- West-Pacijc
This is a very large region comprising the coasts of the Indian Ocean,
the Indonesian archipelago, the western Pacific north to about Tokyo,
and the northern, as well as much of the western and eastern coasts of
Australia. Although relatively homogeneous, the region can be divided
into a number of faunistic areas.
Southern Japan
The ascidian fauna of Japanese and immediately adjacent waters is
very diverse, Tokioka (1963) having listed 277 species. These are divisible into four faunistic groups :
R. 11. MILLAR
coasts of South America. Moreover, the accounts of these faunas which
have been extensively studied reflect the varying viewpoints of different
systematists, some of whom have been more ready than others to create
new species.
A. Shallow-water ascidians
Assuming that most of the species are now known in the areas which
have been reasonably well studied, it is of interest to look at the relative
richness of the faunas. In doing so it is necessary to compare like with
like, by taking areas defined by the homogeneity of their faunas and
having regard to the hydrographic conditions, sudden changes in which,
over a short distance, often determine faunistic boundaries. Figure 22
shows the approximate number of ascidian species believed to occur in
the main shallow-water zoogeographical regions. The regions are
broadly those defined by Ekman (1953), but with some local modifications adopted by students of particular areas.
It is evident that certain faunas are much more diverse than others,
the Mediterranean, Indo-Malayan, southern Japanese and northwestern Pacific faunas being notably rich in species, the polar seas,
subantarctic and temperate North American Atlantic less so, and in
general the warm-water faunas have most species, although not
necessarily most individuals. It should be noted, incidentally, that
the total number of species throughout the world is not the sum of the
numbers of all the regions, because by no means all species are confined
to one faunistic area, many being shared by adjacent or even by several
more distant areas.
The warm-water regions, which will be considered first, are remarkable both for their richness and for the number of species of wide
distribution. And some genera, such as Eudistomu, although not confined to warm regions, find by far their greatest development there.
Indo- West-Pacijc
This is a very large region comprising the coasts of the Indian Ocean,
the Indonesian archipelago, the western Pacific north to about Tokyo,
and the northern, as well as much of the western and eastern coasts of
Australia. Although relatively homogeneous, the region can be divided
into a number of faunistic areas.
Southern Japan
The ascidian fauna of Japanese and immediately adjacent waters is
very diverse, Tokioka (1963) having listed 277 species. These are divisible into four faunistic groups :