THE BIOLOGY OF ASCTDTANS
81
troublesome mainly because they have to be removed when the
structures are painted.
The settlement and growth of ascidians m;iy also bc a problcin in
fisheries. In oyster culture, for example, they often smother surfaces
intended for the settlement of the molluscan larvae, impede dredging,
or contribute to the silting of oyster beds (Cole, 195G; Waugh, 1957;
Millar, 1961b ; Hancock, 1969). Some degree of control can be achieved
by treating the settlement surfaces with chemicals (Loosanoff, 1960 ;
Waugh, 1957).
Accidental transport is a conscquence of fouling, and has accounted
for the introduction of Styela clava to the south coast of England,
probably from Korean waters (Carlisle, 1954b ; Houghton and Millar,
1960). The species has subsequently multiplied and spread to the extent
that it is now the dominant large ascidian in certain sheltered coastal
waters. It is probable that the widespread occurrence of Ciona intestinulis and Diplosomu listerianum in the harbours of many countries
is likewisc the result of introductions on ships’ hulls.
Certain structural and biological characters help to cxplain why
ascidians are important as fouling organisms. Firstly, because the
branchial cavity is large, the total volume and surface area of an
individual are high in relation to the tissue weight, and an ascidian
therefore occupies a large space or covers a large area. Secondly, being
efficient filter-feeders, they grow fast and can outpace many of their
competitors. Larval behaviour, too, pre-adapts them as fouling
animals, since the larvae initially swim upwards, reaching the surface
layers of water where they encounter floats and ships’ hulls, to which
they attach themselves. Moreover, the short pelagic life of the larva
favours heavy settlenient near the parents, and in harbours there is
generally a stock of adults. Some species, in addition, are resistant to
reduced salinity or to pollution (Huus, 1933; Berner, 1944; SentzBraconnot, 19GG; Dybern, 1967, 196%; Diehl, 1957; Van Name, 1945;
Gunter and Hall, 1963; Dragovich and Kelly, 1964), and these are conditions met in harbours, where fouling on ships is a problem.
B. Food of man, and of commercial fish
In some countries, mainly those of the far East and certain parts of
the Mediterranean, ascidians are eaten by man and are sufficiently
important to merit an entry in the F.A.O. Yearbook of Fishery Statistics
(Food and Agriculture Organization, 1964). Microcosmus sulcatus, and
occasionally Styela plicata and Polycarpa pomuria are taken in the
Mediterranean (Harant, 1951), Halocynthia roretzi in Japan, where it is
cultured in the north of Honsyu (Tokioka, 1953), and Pyura chilensis
81
troublesome mainly because they have to be removed when the
structures are painted.
The settlement and growth of ascidians m;iy also bc a problcin in
fisheries. In oyster culture, for example, they often smother surfaces
intended for the settlement of the molluscan larvae, impede dredging,
or contribute to the silting of oyster beds (Cole, 195G; Waugh, 1957;
Millar, 1961b ; Hancock, 1969). Some degree of control can be achieved
by treating the settlement surfaces with chemicals (Loosanoff, 1960 ;
Waugh, 1957).
Accidental transport is a conscquence of fouling, and has accounted
for the introduction of Styela clava to the south coast of England,
probably from Korean waters (Carlisle, 1954b ; Houghton and Millar,
1960). The species has subsequently multiplied and spread to the extent
that it is now the dominant large ascidian in certain sheltered coastal
waters. It is probable that the widespread occurrence of Ciona intestinulis and Diplosomu listerianum in the harbours of many countries
is likewisc the result of introductions on ships’ hulls.
Certain structural and biological characters help to cxplain why
ascidians are important as fouling organisms. Firstly, because the
branchial cavity is large, the total volume and surface area of an
individual are high in relation to the tissue weight, and an ascidian
therefore occupies a large space or covers a large area. Secondly, being
efficient filter-feeders, they grow fast and can outpace many of their
competitors. Larval behaviour, too, pre-adapts them as fouling
animals, since the larvae initially swim upwards, reaching the surface
layers of water where they encounter floats and ships’ hulls, to which
they attach themselves. Moreover, the short pelagic life of the larva
favours heavy settlenient near the parents, and in harbours there is
generally a stock of adults. Some species, in addition, are resistant to
reduced salinity or to pollution (Huus, 1933; Berner, 1944; SentzBraconnot, 19GG; Dybern, 1967, 196%; Diehl, 1957; Van Name, 1945;
Gunter and Hall, 1963; Dragovich and Kelly, 1964), and these are conditions met in harbours, where fouling on ships is a problem.
B. Food of man, and of commercial fish
In some countries, mainly those of the far East and certain parts of
the Mediterranean, ascidians are eaten by man and are sufficiently
important to merit an entry in the F.A.O. Yearbook of Fishery Statistics
(Food and Agriculture Organization, 1964). Microcosmus sulcatus, and
occasionally Styela plicata and Polycarpa pomuria are taken in the
Mediterranean (Harant, 1951), Halocynthia roretzi in Japan, where it is
cultured in the north of Honsyu (Tokioka, 1953), and Pyura chilensis