46
R. H. BfIILAB
ascidians have been cited as possible examples (Grave, 1935 ; Glaser and
Anslow, 1949). Littoral species may be gregarious, but live in such a
complex environment that it would be difficult for an observer to separate the results of gregarious behaviour from those of other phenomena.
In the sublittoral region, however, a much simpler situation exists, and
C. Monniot (19654 believes that the micro-distribution of a number of
species points to gregarious settlement of their larvae. Thus Bolteniopsis prenenti Harant was found in isolated groups of 30-40 individuals. Microcosmus vulgaris Heller showed similar aggregations
separated by apparently suitable areas which were unoccupied, and
Molgula manhattensis, M . occulta Kupffer and Dendrodoa grossularia
(Van Beneden) provided further examples. C. Monniot (1965b) further
states that in the formation of the characteristic “ blocs Microcosmus”
on the sea bed off Banyuls-sur-Mer, the first species to become attached
attracts larvae of its own species and only later those of others. Lambert ( 1968) observed that larvae of Corella willmeriana Herdman tended
to settle near adults, but the settlement of larvae close to adults is not
in itself conclusive evidence of gregarious behaviour, because the adults
may influence larval behaviour in the same way as inanimate objects
projecting from the substratum, by altering the pattern of light and
shade or the flow of water ; gregarious settlement concerns a response of
the larvae to the presence of living adults of the same species. There is
scope in this field for experiments, particularly as the larvae of some
simple ascidims are readily obtained by artificial fertilization.
Another biotic factor is suggested by an observation of Goodbody
( 196 1 b) that panels uscd in experiments on settlement failed to develop
a normal community-of which ascidians formed a prominent partwhen placed close to mature sponge-anemone-ophiuran
communities. Sponges were thought to be responsible, but the way in
which they suppressed other communities is not known.
We have seen that the nature of the sea-bed influences ascidians, but
they themselves presumably have a significant effect on the surface
which they populate if they are sufficiently numerous. Diehl (1957)
noted the deposit of faeces on the surface of the sediment surrounding
Styela coriacea ; and the clarification of turbid water by Ciona (Berner,
1944) involves the removal of suspended organic and inorganic particles
from the water and their deposition on the bottom. A dense population
of Ciona, such as that recorded by Elroi and Komarovsky (1961) may
filter as much as 11 000 l/h over every m2 of substratum occupied,
assuming the filtration rate determined in experiments by Goldberg
et al. (1951). Even in the less dense populations more commonly
occurring in this and other species, these animals must remove much
R. H. BfIILAB
ascidians have been cited as possible examples (Grave, 1935 ; Glaser and
Anslow, 1949). Littoral species may be gregarious, but live in such a
complex environment that it would be difficult for an observer to separate the results of gregarious behaviour from those of other phenomena.
In the sublittoral region, however, a much simpler situation exists, and
C. Monniot (19654 believes that the micro-distribution of a number of
species points to gregarious settlement of their larvae. Thus Bolteniopsis prenenti Harant was found in isolated groups of 30-40 individuals. Microcosmus vulgaris Heller showed similar aggregations
separated by apparently suitable areas which were unoccupied, and
Molgula manhattensis, M . occulta Kupffer and Dendrodoa grossularia
(Van Beneden) provided further examples. C. Monniot (1965b) further
states that in the formation of the characteristic “ blocs Microcosmus”
on the sea bed off Banyuls-sur-Mer, the first species to become attached
attracts larvae of its own species and only later those of others. Lambert ( 1968) observed that larvae of Corella willmeriana Herdman tended
to settle near adults, but the settlement of larvae close to adults is not
in itself conclusive evidence of gregarious behaviour, because the adults
may influence larval behaviour in the same way as inanimate objects
projecting from the substratum, by altering the pattern of light and
shade or the flow of water ; gregarious settlement concerns a response of
the larvae to the presence of living adults of the same species. There is
scope in this field for experiments, particularly as the larvae of some
simple ascidims are readily obtained by artificial fertilization.
Another biotic factor is suggested by an observation of Goodbody
( 196 1 b) that panels uscd in experiments on settlement failed to develop
a normal community-of which ascidians formed a prominent partwhen placed close to mature sponge-anemone-ophiuran
communities. Sponges were thought to be responsible, but the way in
which they suppressed other communities is not known.
We have seen that the nature of the sea-bed influences ascidians, but
they themselves presumably have a significant effect on the surface
which they populate if they are sufficiently numerous. Diehl (1957)
noted the deposit of faeces on the surface of the sediment surrounding
Styela coriacea ; and the clarification of turbid water by Ciona (Berner,
1944) involves the removal of suspended organic and inorganic particles
from the water and their deposition on the bottom. A dense population
of Ciona, such as that recorded by Elroi and Komarovsky (1961) may
filter as much as 11 000 l/h over every m2 of substratum occupied,
assuming the filtration rate determined in experiments by Goldberg
et al. (1951). Even in the less dense populations more commonly
occurring in this and other species, these animals must remove much