THE BIOLOGY O F ASCIDJANS
69
Tho ascidians of othcr parts of the world havc been so little investigated or the studies havc bccii so spccializcd-as tho taxonomic
rcview of tho Dideninidan: of tlic lndo-Pncific (Eldredgc, 1967)-that
general spcculation 011 their faunistic relationships is unprofitable.
B. Deepwater aseidium
A number of vertical zones have been recognized in the deep water
beyond the shelf, but the terminology and limits applied to them have
varied considerably (Hedgpeth, 1957 ; Vinogradova, 1962b). Vinogradova (1969a) recognized a bathyal zone from 500-3 000 m mainly on
the slope, an abyssal zone from 3 000-6 000 m constituting most of the
sea-floor, and an ultra-abyssal zone in depths greater than 6 000 m and
corresponding t o the hadal zone of Bruun (1956). The upper limit of
the abyssal zone is somewhat arbitrary, and Ekman (1953) has emphasized that it is not the same in all parts of the oceans. Many abyssal
species certainly extend up to 2 000 m and this is the depth which I am
taking as the upper boundary. Whether the floor below 6 000 m supports a distinctive fauna is not certain. According to Wolff (1960) it
does, but Menzies and George (1967) believe that there is little evidence
supporting the view. Very few ascidians have been found at such
depths, although they do occur down to 8 430 m in the Kuril-Kamchatka Trench (Vinogradova, 1969a, b, 1970). In less extreme depths
there exists a moderately rich ascidian fauna representing several
families (Table IV, below). This fauna is known mainly from the
accounts of Herdman (1882, 1886, 1888), Verrill (1885), Ritter (1907),
Sluiter (1904), Hartmeyer (191 1,1912), Michaelsen (1904), Millar (1955b,
1959a, 1964b), Monniot and Monniot (1968) and Kott (1969), whose
records are incorporated in Fig. 24.
In addition to the bathymetric divisions, geographical divisions are
also to be recognized in the abyssal parts of the sea. Ekman (1953)
broadly divided the deep seas into the Atlantic, Indo-Pan-Pacific,
Antarctic, Arctic, Mediterranean, Red Sea and Sea of Japan. Following
a review of subsequent faunistic studies, Vinogradova (1956, 1962b)
proposed thc scheme used in Fig. 24, which recognizes more subdivisions. The validity of the geographical areas depends on the distinctness of their faunas, and there is some evidence that the abyssal
faunas of the oceans show a measure of independence from one another.
Vinogradova (1962b) concluded that, for a number of invertebrate
groups taken as a whole, the proportion of endemic species in the
Atlantic Ocean is 76%, in the Pacific Ocean 73.2% and in the Indian
Ocean slightly over 50%.
69
Tho ascidians of othcr parts of the world havc been so little investigated or the studies havc bccii so spccializcd-as tho taxonomic
rcview of tho Dideninidan: of tlic lndo-Pncific (Eldredgc, 1967)-that
general spcculation 011 their faunistic relationships is unprofitable.
B. Deepwater aseidium
A number of vertical zones have been recognized in the deep water
beyond the shelf, but the terminology and limits applied to them have
varied considerably (Hedgpeth, 1957 ; Vinogradova, 1962b). Vinogradova (1969a) recognized a bathyal zone from 500-3 000 m mainly on
the slope, an abyssal zone from 3 000-6 000 m constituting most of the
sea-floor, and an ultra-abyssal zone in depths greater than 6 000 m and
corresponding t o the hadal zone of Bruun (1956). The upper limit of
the abyssal zone is somewhat arbitrary, and Ekman (1953) has emphasized that it is not the same in all parts of the oceans. Many abyssal
species certainly extend up to 2 000 m and this is the depth which I am
taking as the upper boundary. Whether the floor below 6 000 m supports a distinctive fauna is not certain. According to Wolff (1960) it
does, but Menzies and George (1967) believe that there is little evidence
supporting the view. Very few ascidians have been found at such
depths, although they do occur down to 8 430 m in the Kuril-Kamchatka Trench (Vinogradova, 1969a, b, 1970). In less extreme depths
there exists a moderately rich ascidian fauna representing several
families (Table IV, below). This fauna is known mainly from the
accounts of Herdman (1882, 1886, 1888), Verrill (1885), Ritter (1907),
Sluiter (1904), Hartmeyer (191 1,1912), Michaelsen (1904), Millar (1955b,
1959a, 1964b), Monniot and Monniot (1968) and Kott (1969), whose
records are incorporated in Fig. 24.
In addition to the bathymetric divisions, geographical divisions are
also to be recognized in the abyssal parts of the sea. Ekman (1953)
broadly divided the deep seas into the Atlantic, Indo-Pan-Pacific,
Antarctic, Arctic, Mediterranean, Red Sea and Sea of Japan. Following
a review of subsequent faunistic studies, Vinogradova (1956, 1962b)
proposed thc scheme used in Fig. 24, which recognizes more subdivisions. The validity of the geographical areas depends on the distinctness of their faunas, and there is some evidence that the abyssal
faunas of the oceans show a measure of independence from one another.
Vinogradova (1962b) concluded that, for a number of invertebrate
groups taken as a whole, the proportion of endemic species in the
Atlantic Ocean is 76%, in the Pacific Ocean 73.2% and in the Indian
Ocean slightly over 50%.