80
R. 11. MILLAR
about 22 ascidian species listed in a report on marine fouling (Organization for Economic Co-operation and Development, 1966), four were
classed as principal fouling species, and a number of workers have noted
the group as contributing the dominant fouling organisms (Relini,
1964; Stubbings and Houghton, 1964 ; Berner, 1944; Scheer, 1945;
Weiss, 1948; Skerman, 1959; Kawahara, 1962). Often they are not
prominent in the early stages of the development of a sessile community, but become the principal organisms in later stages. Thus
Berner (1944) found that on ships’ hulls at Marseilles there was a
regular succession, from Enteromorpha, through Bryozoa and sessile
annelids to barnacles and Ciona intestinulis. In Kingston Harbour,
Jamaica the primary colonizers were algae and barnacles, but the
ascidians Didemnum conchyliatum (Sluiter) and Ascidia nigra became
and remained dominant for many months (Goodbody, 1963b). The
sequence, and the species involved, naturally vary with the location,
and in California, U.S.A., six stages in fouling were recognized, one of
which was dominated by an almost pure growth of Cionu, which was
subsequently replaced by mussels and barnacles (Scheer, 1945). At
Auckland, New Zealand, the initial settlers were barnacles and hydroids
but these subsequently gave way to oysters and the ascidian Microcosmus kura Brewin (Skerman, 1959). Similarly in Japanese waters,
Styelaplicata became one of the two dominant species in the later stages
of fouling (Kawahara, 1962).
The relative importance of ascidians depends on the season at which
surfaces are exposed to fouling, and ReIini (1964) found that at Genoa,
Italy, they were most abundant on test panels in winter, but varied
considerably in abundance from year to year. This was also the experience of Elroi and Komarovsky (1961) with Ciona at Haifa, Israel,
and of Stubbings and Houghton (1964) during their study of Chichester
Harbour, England, where Diplosoma Zisterianum gradually declined in
numbers over a period of several years.
Some idea of the importance which ascidians may have in the
fouling of vessels is given by the numbers recorded by Elroi and
Komarovsky (1961), who estimated 2 500-10 000 specimens of Cionu
per m2 of exposed plate ; the individuals reach a length of 25 cm, and
the total wet weight on 1 m2 was 140 kg. Under favourable conditions
Ascidiella aspersa may settle a t the rate of 1 800/m2 per day (Millar,
1961b). It is not so much the weight of the animals which creates a
problem, for their under-water weight is not great, but the additional
friction which they cause to a moving ship. This must vary in a complex way with the size, shape and roughness of the body, but has not
been studied. On floats and other stationary objects ascidians are
R. 11. MILLAR
about 22 ascidian species listed in a report on marine fouling (Organization for Economic Co-operation and Development, 1966), four were
classed as principal fouling species, and a number of workers have noted
the group as contributing the dominant fouling organisms (Relini,
1964; Stubbings and Houghton, 1964 ; Berner, 1944; Scheer, 1945;
Weiss, 1948; Skerman, 1959; Kawahara, 1962). Often they are not
prominent in the early stages of the development of a sessile community, but become the principal organisms in later stages. Thus
Berner (1944) found that on ships’ hulls at Marseilles there was a
regular succession, from Enteromorpha, through Bryozoa and sessile
annelids to barnacles and Ciona intestinulis. In Kingston Harbour,
Jamaica the primary colonizers were algae and barnacles, but the
ascidians Didemnum conchyliatum (Sluiter) and Ascidia nigra became
and remained dominant for many months (Goodbody, 1963b). The
sequence, and the species involved, naturally vary with the location,
and in California, U.S.A., six stages in fouling were recognized, one of
which was dominated by an almost pure growth of Cionu, which was
subsequently replaced by mussels and barnacles (Scheer, 1945). At
Auckland, New Zealand, the initial settlers were barnacles and hydroids
but these subsequently gave way to oysters and the ascidian Microcosmus kura Brewin (Skerman, 1959). Similarly in Japanese waters,
Styelaplicata became one of the two dominant species in the later stages
of fouling (Kawahara, 1962).
The relative importance of ascidians depends on the season at which
surfaces are exposed to fouling, and ReIini (1964) found that at Genoa,
Italy, they were most abundant on test panels in winter, but varied
considerably in abundance from year to year. This was also the experience of Elroi and Komarovsky (1961) with Ciona at Haifa, Israel,
and of Stubbings and Houghton (1964) during their study of Chichester
Harbour, England, where Diplosoma Zisterianum gradually declined in
numbers over a period of several years.
Some idea of the importance which ascidians may have in the
fouling of vessels is given by the numbers recorded by Elroi and
Komarovsky (1961), who estimated 2 500-10 000 specimens of Cionu
per m2 of exposed plate ; the individuals reach a length of 25 cm, and
the total wet weight on 1 m2 was 140 kg. Under favourable conditions
Ascidiella aspersa may settle a t the rate of 1 800/m2 per day (Millar,
1961b). It is not so much the weight of the animals which creates a
problem, for their under-water weight is not great, but the additional
friction which they cause to a moving ship. This must vary in a complex way with the size, shape and roughness of the body, but has not
been studied. On floats and other stationary objects ascidians are