Reproduction of Corals
285
Pocillopora, could be so strong that in the calm zone of flats at some reefs
this coral desappears, remaining abundant only in communities of the
shallow surf zones, where the waves repel fish (Neudecker 1979). Fish which
are scraping periphyton and macrophytes from reef rocks, improve the
recruitment conditions for the coral's planulae (Birkeland 1977; Sutton
1983).
A significant influence on the formation of coral communities exert the
urchins Diadema (Sammarco 1980, 1982) which scrape even the surface of
reef rock and clean the spot for recruitment of planulae. This positive
impact prevails when their number is moderate (1-4 sp. per m- 2 ). But
when the populations of Diadema become denser, they influence
recruitment negatively, because they damage coral spat and the young
colonies of corals (Bak and van Eys 1975). But the latter situation is rare on
reefs. Thus, in general, the influence of urchins on coral recruitment seems
to be positive. Among other biological factors influencing the formation of
coral communities and their structure is the activity of boring benthic fauna,
e.g. boring bivalves and boring sponges, which are sharply specialized with
respect to their preference for living corals to be bored (Kleeman 1980;
Rylaarsdam 1983; Wilkinson 1983b). The influence of microbial diseases of
corals will be discussed below (cf. Sect. 11.3).
7.4 Reproduction of Corals
Just as the other Coelenterate, corals use two ways of reproduction: sexual
(partially including parthenogenesis) and vegetative. During sexual
multiplication, corals produce their larvae planulae which swim in the water
column for some time, and then settle, starting a new colony. In vegetative
propagation pieces of corals, their bailed-out single polyps, or their "buds"
distributed by currents and waves, then recruit again and give birth to new
colonies. The relative importance of these two modes of breeding are
different in different corals, being largely dependent upon their life strategy.
With respect to their sexual breeding, all corals for a long time were
regarded as bisexual and viviparous (Yonge 1973; Stimson 1978). This
concept was the result of observations on several opportunistic species of
corals living in the security of aquaria, which indeed are bisexual and
viviparous, like Pocillopora, Stylophora and Cyphastrea (Atoda 1953;
Harrigan 1972; Rinkevich and Loya 1979a,c). But J. Duerden in 1902 also
described coral species, whose polyps dispersed sperm and eggs instead of
planulae. These corals were Manicina areolata and Favia fragum. Later, the
fruitless attempts to find the hatching of planulae in most corals resulted in
the opinion taking root that their sexual breeding is of secondary importance
in their propagation, even though histological studies invariably
demonstrated the presence of gonads in their polyps. These studies also
285
Pocillopora, could be so strong that in the calm zone of flats at some reefs
this coral desappears, remaining abundant only in communities of the
shallow surf zones, where the waves repel fish (Neudecker 1979). Fish which
are scraping periphyton and macrophytes from reef rocks, improve the
recruitment conditions for the coral's planulae (Birkeland 1977; Sutton
1983).
A significant influence on the formation of coral communities exert the
urchins Diadema (Sammarco 1980, 1982) which scrape even the surface of
reef rock and clean the spot for recruitment of planulae. This positive
impact prevails when their number is moderate (1-4 sp. per m- 2 ). But
when the populations of Diadema become denser, they influence
recruitment negatively, because they damage coral spat and the young
colonies of corals (Bak and van Eys 1975). But the latter situation is rare on
reefs. Thus, in general, the influence of urchins on coral recruitment seems
to be positive. Among other biological factors influencing the formation of
coral communities and their structure is the activity of boring benthic fauna,
e.g. boring bivalves and boring sponges, which are sharply specialized with
respect to their preference for living corals to be bored (Kleeman 1980;
Rylaarsdam 1983; Wilkinson 1983b). The influence of microbial diseases of
corals will be discussed below (cf. Sect. 11.3).
7.4 Reproduction of Corals
Just as the other Coelenterate, corals use two ways of reproduction: sexual
(partially including parthenogenesis) and vegetative. During sexual
multiplication, corals produce their larvae planulae which swim in the water
column for some time, and then settle, starting a new colony. In vegetative
propagation pieces of corals, their bailed-out single polyps, or their "buds"
distributed by currents and waves, then recruit again and give birth to new
colonies. The relative importance of these two modes of breeding are
different in different corals, being largely dependent upon their life strategy.
With respect to their sexual breeding, all corals for a long time were
regarded as bisexual and viviparous (Yonge 1973; Stimson 1978). This
concept was the result of observations on several opportunistic species of
corals living in the security of aquaria, which indeed are bisexual and
viviparous, like Pocillopora, Stylophora and Cyphastrea (Atoda 1953;
Harrigan 1972; Rinkevich and Loya 1979a,c). But J. Duerden in 1902 also
described coral species, whose polyps dispersed sperm and eggs instead of
planulae. These corals were Manicina areolata and Favia fragum. Later, the
fruitless attempts to find the hatching of planulae in most corals resulted in
the opinion taking root that their sexual breeding is of secondary importance
in their propagation, even though histological studies invariably
demonstrated the presence of gonads in their polyps. These studies also