Reproduction of Corals
293
Available lots of hard substrate for planulae to settle on are restricted,
because the large reef areas are covered by soft sediments, and settling on
these most larvae die (Highsmith et al. 1980). On the hard bottom biotopes
recruits face high competition from other sessile benthos, and of algae as
well (Highsmith 1980; Benayahu and Loya 1981; Rosen 1981). The mortality
rate of a spat of corals alone is 60-90% during 3-12 months, and it is
highest in the second to fourth month of its life (Loya 1976; Sammarco 1983;
cf. Fig. 7.15). The mortality of planulae and eggs of gorgonaceans was
estimated to be close to 90% (Theodor 1967). The main cause of mortality
of spat is the grazing by fish, urchins and other benthovores feeding on
periphytonic turfs and so scraping the solid surfaces (Sammarco 1980, 1982;
Rylaarsdam 1983). These grazers exert a diverse impact upon the survival of
young corals. Weak grazing results in denser overgrowth on the substrate,
which inhibits recruitment, while by intensive grazing the recruits could be
damaged. Thus optimal conditions for recruitment of corals and for the
survival of their spat seem to be created by moderate grazing, which was
proved experimentally (Sato 1985; Sammarco 1985). Often the planulae
settle in the holes made in reef-rock surface by the urchins Echinometra.
This help them later to escape the scraping grazers (Birkeland and Randall
1981).
As mentioned above, in many corals the larger part of recruited colonies
is formed via vegetative breeding (Connell 1973; Highsmith 1982). Among
the modes of vegetative propagation of corals, the following could be
mentioned: (1) the fragmentation of colonies and their rooting in reef
biotopes; (2) the bailing out of polyps from corallites of adult colony and
their subsequent settling; (3) the production of new colonies in solitary
corals by fission of their corallites or by formation of new colonies by them
through budding; (4) the shedding of separate corallites functioning like
buds; (5) the fission of a colony with subsequent separation of the newly
formed ones; and (6) the separation of pieces of branches by "autotomy".
The propagation by fragmentation is widespread among the ramose
scleractinians and among the gorgonaceans, though it happens also in
massive corals (Crassle 1973; Tunnicliffe 1982; Bothwell 1983b, 1984). Main
factors causing the fragmentations are storms, especially hurricanes
(Highsmith et al. 1980; Woodley et al. 1981), as well as the activity of boring
sponges and clams (Tunnicliffe 1978). Probably the harboring of boring
animals by living corals is a kind of adaptive reaction enhancing their
multiplication and thus improving their chances in the competition for
substrate (Highsmith 1982). Otherwise the ramose corals should develop
defence mechanisms against borers, what actually has not happened. They
are extremely numerous in most of the common corals, especially in
acroporids (cf. Sect. 5.1.4). Some corals produce fragments by fission of the
colony through separate sections at its base (Highsmith 1980). It happens
when a part of the colony has at its base been ruined by siltation or by
borers and the whole colony collapses. Then its parts attach themselves to
293
Available lots of hard substrate for planulae to settle on are restricted,
because the large reef areas are covered by soft sediments, and settling on
these most larvae die (Highsmith et al. 1980). On the hard bottom biotopes
recruits face high competition from other sessile benthos, and of algae as
well (Highsmith 1980; Benayahu and Loya 1981; Rosen 1981). The mortality
rate of a spat of corals alone is 60-90% during 3-12 months, and it is
highest in the second to fourth month of its life (Loya 1976; Sammarco 1983;
cf. Fig. 7.15). The mortality of planulae and eggs of gorgonaceans was
estimated to be close to 90% (Theodor 1967). The main cause of mortality
of spat is the grazing by fish, urchins and other benthovores feeding on
periphytonic turfs and so scraping the solid surfaces (Sammarco 1980, 1982;
Rylaarsdam 1983). These grazers exert a diverse impact upon the survival of
young corals. Weak grazing results in denser overgrowth on the substrate,
which inhibits recruitment, while by intensive grazing the recruits could be
damaged. Thus optimal conditions for recruitment of corals and for the
survival of their spat seem to be created by moderate grazing, which was
proved experimentally (Sato 1985; Sammarco 1985). Often the planulae
settle in the holes made in reef-rock surface by the urchins Echinometra.
This help them later to escape the scraping grazers (Birkeland and Randall
1981).
As mentioned above, in many corals the larger part of recruited colonies
is formed via vegetative breeding (Connell 1973; Highsmith 1982). Among
the modes of vegetative propagation of corals, the following could be
mentioned: (1) the fragmentation of colonies and their rooting in reef
biotopes; (2) the bailing out of polyps from corallites of adult colony and
their subsequent settling; (3) the production of new colonies in solitary
corals by fission of their corallites or by formation of new colonies by them
through budding; (4) the shedding of separate corallites functioning like
buds; (5) the fission of a colony with subsequent separation of the newly
formed ones; and (6) the separation of pieces of branches by "autotomy".
The propagation by fragmentation is widespread among the ramose
scleractinians and among the gorgonaceans, though it happens also in
massive corals (Crassle 1973; Tunnicliffe 1982; Bothwell 1983b, 1984). Main
factors causing the fragmentations are storms, especially hurricanes
(Highsmith et al. 1980; Woodley et al. 1981), as well as the activity of boring
sponges and clams (Tunnicliffe 1978). Probably the harboring of boring
animals by living corals is a kind of adaptive reaction enhancing their
multiplication and thus improving their chances in the competition for
substrate (Highsmith 1982). Otherwise the ramose corals should develop
defence mechanisms against borers, what actually has not happened. They
are extremely numerous in most of the common corals, especially in
acroporids (cf. Sect. 5.1.4). Some corals produce fragments by fission of the
colony through separate sections at its base (Highsmith 1980). It happens
when a part of the colony has at its base been ruined by siltation or by
borers and the whole colony collapses. Then its parts attach themselves to