42
R. € I .
MILIAR
Local differences in the distribution of related species, but on a somewhat larger scale, were shown by a survey of the sea bed off Banyuls
(C. Monniot, 1965b), where Microcosmus sabatieri and 1 1 1 . vulgaris were
dominant in adjacent areas defined more or less closely by contours of
depth (Fig. 18).
Numerous faunistic studies of sublittoral areas have given rise to
the concept of biocoenoses, communities or associations, but whether
or not such assemblages are held together by biological as well as
physical factors (see Jones, 1950), it is generally agreed that they exist,
and that they can usually be characterized by a set of species. P6rBs
and Picard (1958) noted that certain kinds of sea-bed are rich in
ascidians, and P&Bs (1967) included ascidian species amongst the
animals characteristic of a number of biocoenoses in the Mediterranean.
Of four biocoenoses recognized by Bellan et al. (1961) in an area off
Corsica one was marked by the abundance of a species of Diazonu, and
another was divisible into two facies typified respectively by Polyclinum
aurantium Milne Edwards and by various compound ascidians together
with Microcosmus sulcatw. Parenzan ( 1 959) included eight ascidian
species amongst the 47 animals characterizing one facies in the Gulf of
Naples, and a Dendrodoa grossularia community has been recognized at
Roscoff (Cabioch, 1961).
2. Salinity
Ascidians in general are animals of rather high salinity water, although a number of species can withstand varying degrecs of dilution.
Osmotic regulation takes place, but difficulties imposed by the large
body surfaces, and by the absence of excretory tubules (Barrington,
1965) may restrict penetration into very diluted water. Ciona intestinalis
is amongst the species with a wide salinity tolerance, and Dybern (1967)
noted that it occurs in areas of the Black Sea having a salinity of under
20%, and also at Suez where the salinity reaches 40-41%,. The lower
limit appears to be about 1 I%,, both for the adult and the developmental
stages, and Dybern's experimental results help to explain the distribution which he recorded around southern Scandinavia.
In a similar way the local distribution of Ascidiella scabra (Muller)
in the Skagerak and Kattegat is determined by a requirement for
average salinities above 24%,, although the adult can withstand
occasional reductions to 15%, (Dybern, 1969b).
The prevailing salinity was also found to be the most important
physical factor determining the presence or absence of a number of
ascidian species in different parts of two marine ponds in Norway
(Dybern, 1969a) (Table 111). Of these species Molgula munhttensis
R. € I .
MILIAR
Local differences in the distribution of related species, but on a somewhat larger scale, were shown by a survey of the sea bed off Banyuls
(C. Monniot, 1965b), where Microcosmus sabatieri and 1 1 1 . vulgaris were
dominant in adjacent areas defined more or less closely by contours of
depth (Fig. 18).
Numerous faunistic studies of sublittoral areas have given rise to
the concept of biocoenoses, communities or associations, but whether
or not such assemblages are held together by biological as well as
physical factors (see Jones, 1950), it is generally agreed that they exist,
and that they can usually be characterized by a set of species. P6rBs
and Picard (1958) noted that certain kinds of sea-bed are rich in
ascidians, and P&Bs (1967) included ascidian species amongst the
animals characteristic of a number of biocoenoses in the Mediterranean.
Of four biocoenoses recognized by Bellan et al. (1961) in an area off
Corsica one was marked by the abundance of a species of Diazonu, and
another was divisible into two facies typified respectively by Polyclinum
aurantium Milne Edwards and by various compound ascidians together
with Microcosmus sulcatw. Parenzan ( 1 959) included eight ascidian
species amongst the 47 animals characterizing one facies in the Gulf of
Naples, and a Dendrodoa grossularia community has been recognized at
Roscoff (Cabioch, 1961).
2. Salinity
Ascidians in general are animals of rather high salinity water, although a number of species can withstand varying degrecs of dilution.
Osmotic regulation takes place, but difficulties imposed by the large
body surfaces, and by the absence of excretory tubules (Barrington,
1965) may restrict penetration into very diluted water. Ciona intestinalis
is amongst the species with a wide salinity tolerance, and Dybern (1967)
noted that it occurs in areas of the Black Sea having a salinity of under
20%, and also at Suez where the salinity reaches 40-41%,. The lower
limit appears to be about 1 I%,, both for the adult and the developmental
stages, and Dybern's experimental results help to explain the distribution which he recorded around southern Scandinavia.
In a similar way the local distribution of Ascidiella scabra (Muller)
in the Skagerak and Kattegat is determined by a requirement for
average salinities above 24%,, although the adult can withstand
occasional reductions to 15%, (Dybern, 1969b).
The prevailing salinity was also found to be the most important
physical factor determining the presence or absence of a number of
ascidian species in different parts of two marine ponds in Norway
(Dybern, 1969a) (Table 111). Of these species Molgula munhttensis