60
R. H. MILLAR
15”N to the Straits of Gibraltar (Ekman, 1953). Over 70 species are
known from the tropical part (Michaelsen, 1915; Millar, 19538, 1965),
and although this relatively small number may be to some extent the
result of incomplete knowledge it nevertheless indicates less favourable
conditions than those of tropical areas of the Indo-West-Pacific region.
Michaelsen (1915) did not stress a faunistic connection between the
Guinea fauna and that of the West Indies, but Millar (1 953b) found a
collection of ascidians from Ghana to consist of a large endemic element,
a small pan-tropical element and a small but interesting element comprising species also recorded from the West Indies. A similar result was
shown by material from the “Atlantide ” Expedition (Millar, 1965), and
PQrAs (1949) noted that 25% of the ascidians which he examined from
Senegal were also known from the West Indian area.
Tropical and subtropical American Atlantic
Hydrographic conditions indicate Cape Hatteras (35ON) as the
northern limit of the eastern American warm-water region and a point
south of Rio de Janeiro as the southern limit. Over much of the
southern part of this area, however, the fauna is poorly known and it is
difficult to determine how closely the faunistic boundaries correspond
to the hydrographic ones.
My estimate of about 83 species is based on the accounts of Van
Name (1945), Rodrigues (1962, 1966) and Millar (1958b, 1961a, 1962~).
This is only a modest fauna for a warm-water region, being scarcely
greater than the relatively poor ascidian fauna of West Africa, and
contrasts with the richness of other systematic groups. It has already
been noted above that several species are common to the warm-water
faunas of the western and eastern Atlantic.
The Mediterranean
Although geographically the Mediterranean looks like an entity,
hydrographically and faunistically it is not sharply separated from the
eastern Atlantic. This has long been recognized in other animal groups,
and was confirmed by PQrAs (1958,1967) for the ascidians. He listed 130
Mediterranean species, of which 32% occur also in the temperate part
of the Atlantic. More than half of these Atlanto-Mediterranean species
do not extend into the eastern basin of the Mediterranean and are
presumably Atlantic in origin. Within the Mediterranean there is a
marked faunal division between the western and eastern basins, confirmed by the low percentage of ascidian species known from both
basins. But half of all the species are endemic, and the fact that this
proportion is much higher than amongst hydroids is explained by P&As
R. H. MILLAR
15”N to the Straits of Gibraltar (Ekman, 1953). Over 70 species are
known from the tropical part (Michaelsen, 1915; Millar, 19538, 1965),
and although this relatively small number may be to some extent the
result of incomplete knowledge it nevertheless indicates less favourable
conditions than those of tropical areas of the Indo-West-Pacific region.
Michaelsen (1915) did not stress a faunistic connection between the
Guinea fauna and that of the West Indies, but Millar (1 953b) found a
collection of ascidians from Ghana to consist of a large endemic element,
a small pan-tropical element and a small but interesting element comprising species also recorded from the West Indies. A similar result was
shown by material from the “Atlantide ” Expedition (Millar, 1965), and
PQrAs (1949) noted that 25% of the ascidians which he examined from
Senegal were also known from the West Indian area.
Tropical and subtropical American Atlantic
Hydrographic conditions indicate Cape Hatteras (35ON) as the
northern limit of the eastern American warm-water region and a point
south of Rio de Janeiro as the southern limit. Over much of the
southern part of this area, however, the fauna is poorly known and it is
difficult to determine how closely the faunistic boundaries correspond
to the hydrographic ones.
My estimate of about 83 species is based on the accounts of Van
Name (1945), Rodrigues (1962, 1966) and Millar (1958b, 1961a, 1962~).
This is only a modest fauna for a warm-water region, being scarcely
greater than the relatively poor ascidian fauna of West Africa, and
contrasts with the richness of other systematic groups. It has already
been noted above that several species are common to the warm-water
faunas of the western and eastern Atlantic.
The Mediterranean
Although geographically the Mediterranean looks like an entity,
hydrographically and faunistically it is not sharply separated from the
eastern Atlantic. This has long been recognized in other animal groups,
and was confirmed by PQrAs (1958,1967) for the ascidians. He listed 130
Mediterranean species, of which 32% occur also in the temperate part
of the Atlantic. More than half of these Atlanto-Mediterranean species
do not extend into the eastern basin of the Mediterranean and are
presumably Atlantic in origin. Within the Mediterranean there is a
marked faunal division between the western and eastern basins, confirmed by the low percentage of ascidian species known from both
basins. But half of all the species are endemic, and the fact that this
proportion is much higher than amongst hydroids is explained by P&As