36
R. H. MlLLAR
agency, rapidly become established. Styela clava Herdman was accidentally introduced to the south coast of England from far eastern
waters, and multiplied until it became the dominant ascidian in some
localities (Carlisle, 1954b ; Houghton and Millar, 1960 ; Stubbings and
Houghton, 1964). Small numbers have been found recently on the
French side of the English Channel and the species is expected to spread
further along European coasts (C. Monniot, 1970). An instance ofemore
local introduction is Botryllus schlosseri, first seen within the sluice-dock
at Ostend, Belgium in 1960, having apparently been introduced on
oysters imported from Holland; in the space of a few months it had
reproduced and spread extensively (Polk, 1962).
Even without human intervention the constitution of an ascidian
population is not static and there is obviously e great need for longterm quantitative studies.
C . Factors affecting distribution and abundance
The local distribution of species presents many ecological problems,
and the striking differences in ascidian faunas of apparently similar
habitats (Prenant, 1928) for the most part still await explanation.
1. Substratum
The nature of the substratum is one of the factors determining the
presence or absence of ascidian species, most of which either live
attached to solid objects or are adapted to life on loose deposits. A few,
however, have taken advantage of the fact that the test is a living and
plastic tissue able to respond to differences in the substratum. Thus
some specimens of Nicrocosmus sulcatus are attached by the base of the
body to a solid object, but others develop rhizoids which form a network penetrating the soft deposit and securely anchoring the animal
(Costa, 1960). An almost identical case was recorded by Savilov (1958)
in the unrelated Chelyosoma mucleayanum Broderip and Sowerby.
Responses to differences in the substratum and to other environmental
factors may account for much of the variability so often noted in
ascidians. For example P6rQs (1946) was able to relate the type of
colony in Polyclinum aurantium to the sand-content of the water and to
the space available.
The remarkable plasticity of the body, and especially the test,
enables the adult not only to modify its shape and mode of attachment
but also confers a limited freedom of movement. Carlisle (1961) observed movement over the substratum in several species, in one case
amounting to 8 cm in three months, and according to Lafargue (1968)
Monniot found that colonies of Diplosoma would move away when
R. H. MlLLAR
agency, rapidly become established. Styela clava Herdman was accidentally introduced to the south coast of England from far eastern
waters, and multiplied until it became the dominant ascidian in some
localities (Carlisle, 1954b ; Houghton and Millar, 1960 ; Stubbings and
Houghton, 1964). Small numbers have been found recently on the
French side of the English Channel and the species is expected to spread
further along European coasts (C. Monniot, 1970). An instance ofemore
local introduction is Botryllus schlosseri, first seen within the sluice-dock
at Ostend, Belgium in 1960, having apparently been introduced on
oysters imported from Holland; in the space of a few months it had
reproduced and spread extensively (Polk, 1962).
Even without human intervention the constitution of an ascidian
population is not static and there is obviously e great need for longterm quantitative studies.
C . Factors affecting distribution and abundance
The local distribution of species presents many ecological problems,
and the striking differences in ascidian faunas of apparently similar
habitats (Prenant, 1928) for the most part still await explanation.
1. Substratum
The nature of the substratum is one of the factors determining the
presence or absence of ascidian species, most of which either live
attached to solid objects or are adapted to life on loose deposits. A few,
however, have taken advantage of the fact that the test is a living and
plastic tissue able to respond to differences in the substratum. Thus
some specimens of Nicrocosmus sulcatus are attached by the base of the
body to a solid object, but others develop rhizoids which form a network penetrating the soft deposit and securely anchoring the animal
(Costa, 1960). An almost identical case was recorded by Savilov (1958)
in the unrelated Chelyosoma mucleayanum Broderip and Sowerby.
Responses to differences in the substratum and to other environmental
factors may account for much of the variability so often noted in
ascidians. For example P6rQs (1946) was able to relate the type of
colony in Polyclinum aurantium to the sand-content of the water and to
the space available.
The remarkable plasticity of the body, and especially the test,
enables the adult not only to modify its shape and mode of attachment
but also confers a limited freedom of movement. Carlisle (1961) observed movement over the substratum in several species, in one case
amounting to 8 cm in three months, and according to Lafargue (1968)
Monniot found that colonies of Diplosoma would move away when