Factors Controlling Coral Community Structure
283
and some of the opportunistic corals that live everywhere on the reef and
can also survive at this low level of illumination, such as Psammocora,
Pocyllopora, Seriatppora, Lobophyllia, and Alveopora (Dinesen 1982;
Kuhlmann 1983; Fricke and Schuhmacher 1983; Titlyanov et a\. 1981, 1988c;
Titlyanov 1987).
The main biological factors which control community structure and
development of sessile benthic fauna at the level of interspecific
relationships of competition for hard substrate and grazing (Daine 1966).
Because of the high density of reef benthic communities these competition
relationships involve extreme strain (Connell 1973; Jackson 1977, 1989). A
significant influence on their structure exerts competition between the corals
themselves (Sheppard 1980). Corals display usually the following kinds of
competition: overgrowth and the shadowing of each other (Shinn 1966;
Potts 1976; Bothwell 1983b), excretion of toxic metabolites inhibiting the
competing species (Sammarco et a\. 1983), and the interspecific aggression
of corals against their competitors (Glynn et a\. 1972; Lang 1973; Dustan
1975; Richardson et a\. 1979; Sheppard 1979; Wellington 1980; Bothwell
1984). In the first case, the quickly growing ramose scleractinians or
octocorals overgrow the slowly growing massive corals, shadow them and
suppress their growth. The use of toxic metabolites is widespread in the
octocorals. Most of them produce different kinds of toxic compounds and
excrete them into the surrounding water. Using this chemical weapon and
the high growth rate they often replace the scleractinian corals, thus exerting
an influence upon their community structure. For example, the scleraxonian
gorgonacean Erythropodium effectively replace many scleractinian corals
and even zoantharians (Karlson 1980). The toxic metabolites excreted by
a\cyonacean Sinularia inhibited in the experiments the growth of
scleractinian corals and even caused their mortality (Sammarco et a\. 1983).
The interspecific aggression is a common feature of scleractinian corals
which have polyps, well armed with cnidoci\. Their main organs of
aggression are their tantacles and mesenterial filaments equipped with
nematocysts. In the process of growth the coral aggressor approaches the
coral-competitor and kills its polyps with the aid of nematocysts, extending
polyps and tentacles to the enemy. In this way the coral-aggressor stops the
growth of the coral competitor at his side (Logan 1984). The critical distance
for such an attack varies in different species of corals. In acroporids and in
pocilloporids, which have small polyps, this distance was 0.5-1 cm, while in
corals with polyps of an average size, it was 1-3 cm (Sheppard 1981a).
The most aggressive corals usually possess special, long tentacles -
sweepers, which are functionally destined to kill the polyps of coral
competitors. They may kill them even at a distance of 3-4 cm. Sweeper
tentacles are possessed by most of the aggressive corals with medium or
large polyps, like Diploria strigosa, Montastrea annularis, Scolymia lacera,
and Galaxea fascicularis (Lang 1973; Richardson et a\. 1979; Wellington
1980; Chornesky 1983). These tentacles are with a powerful cnidocil, which
283
and some of the opportunistic corals that live everywhere on the reef and
can also survive at this low level of illumination, such as Psammocora,
Pocyllopora, Seriatppora, Lobophyllia, and Alveopora (Dinesen 1982;
Kuhlmann 1983; Fricke and Schuhmacher 1983; Titlyanov et a\. 1981, 1988c;
Titlyanov 1987).
The main biological factors which control community structure and
development of sessile benthic fauna at the level of interspecific
relationships of competition for hard substrate and grazing (Daine 1966).
Because of the high density of reef benthic communities these competition
relationships involve extreme strain (Connell 1973; Jackson 1977, 1989). A
significant influence on their structure exerts competition between the corals
themselves (Sheppard 1980). Corals display usually the following kinds of
competition: overgrowth and the shadowing of each other (Shinn 1966;
Potts 1976; Bothwell 1983b), excretion of toxic metabolites inhibiting the
competing species (Sammarco et a\. 1983), and the interspecific aggression
of corals against their competitors (Glynn et a\. 1972; Lang 1973; Dustan
1975; Richardson et a\. 1979; Sheppard 1979; Wellington 1980; Bothwell
1984). In the first case, the quickly growing ramose scleractinians or
octocorals overgrow the slowly growing massive corals, shadow them and
suppress their growth. The use of toxic metabolites is widespread in the
octocorals. Most of them produce different kinds of toxic compounds and
excrete them into the surrounding water. Using this chemical weapon and
the high growth rate they often replace the scleractinian corals, thus exerting
an influence upon their community structure. For example, the scleraxonian
gorgonacean Erythropodium effectively replace many scleractinian corals
and even zoantharians (Karlson 1980). The toxic metabolites excreted by
a\cyonacean Sinularia inhibited in the experiments the growth of
scleractinian corals and even caused their mortality (Sammarco et a\. 1983).
The interspecific aggression is a common feature of scleractinian corals
which have polyps, well armed with cnidoci\. Their main organs of
aggression are their tantacles and mesenterial filaments equipped with
nematocysts. In the process of growth the coral aggressor approaches the
coral-competitor and kills its polyps with the aid of nematocysts, extending
polyps and tentacles to the enemy. In this way the coral-aggressor stops the
growth of the coral competitor at his side (Logan 1984). The critical distance
for such an attack varies in different species of corals. In acroporids and in
pocilloporids, which have small polyps, this distance was 0.5-1 cm, while in
corals with polyps of an average size, it was 1-3 cm (Sheppard 1981a).
The most aggressive corals usually possess special, long tentacles -
sweepers, which are functionally destined to kill the polyps of coral
competitors. They may kill them even at a distance of 3-4 cm. Sweeper
tentacles are possessed by most of the aggressive corals with medium or
large polyps, like Diploria strigosa, Montastrea annularis, Scolymia lacera,
and Galaxea fascicularis (Lang 1973; Richardson et a\. 1979; Wellington
1980; Chornesky 1983). These tentacles are with a powerful cnidocil, which