306
Morphology and Ecological Physiology of Corals
Zahl 1966; Taylor 1973b; Mashansky et al. 1980). The function of the
periplast is to protect the zooxanthella cell from the attack by lysosomes of
the host cell (Muscatine et al. 1975). At the same time, the periplast is
permeable for metabolite exchange. Some parts of the periplast have
numerous protrusions and exfoliations of the outer membrane. It was
supposed that through them the translocation of lipoid droplets - products
of photosynthesis - proceeds from the zooxanthellae to the host cell (Patton
and Burris 1983). On the side of the host, the filaments of its cell protoplasm
form a kind of capsule around the cells of the zooxanthellae
(Woscoboynikov et al. 1983). Thus, the existence of the complex periplast in
the case of zooxanthellae decreases the difference between intracellular and
interorganismic symbiosis, because their cells are largely isolated from the
cell of the host, moreover, zooxanthellae preserve the ability to multiply
independently of the host in external environments. But after their
penetration into the host's cells an exchange with the metabolites between
them and the polyp soon starts, which is regulated by the host cells, as well
as by the rate of their multiplication. A remarkable feature of the
morphology of zooxanthellae cells is their extremely large chloroplast, which
occupies half of the total volume of their cells. The lamellar system of the
chloroplasts consists of 10-12 complexes. Each of them contains three
tylacoids. The pyrenoid of the chloroplast is attached to it with the aid of a
stalk (schoenberg and Trench 1980; Voscoboynikov et al. 1983).
The primary "infestation" of polyps with zooxanthellae may occur in
various ways. Numerous taxa of hydroids, scleractinians and octocorals, as
well as their eggs or new-born larvae, contain the zooxanthellae cells (Gohar
1940; Atoda 1951; Smith 1980; Fitt 1984). In planulae, they supply them
with products of their photosynthesis thus fomenting their survival
(Richmond 1981). At the same time, many corals produce eggs and larvae in
which zooxanthellae are originally absent (Atoda 1953; Droop 1963; Kinzie
1974; Fitt 1984; Babcock et al. 1986). Among them some octocorals
(Heteroxenia, Briareum, Pseudopterogorgia) and most of the scleractinian
spawning corals. The larvae hatched from these eggs could be infected by
free-swimming motile zoospores of zooxanthellae, which are present in
water. The "infection" takes place also at the stage where settled larvae
formed a first corallite. The polyp attracts zooxanthellae, which swim into its
mesenterial cavity through its mouth and thus "infect" him (Kinzie 1973).
Zooxanthellae may locate the potential host polyps with the aid of
chemoreception. They move along the gradient of concentration of nitrogen
salts excreted by the polyp. The unmotile zoospores get into the gastral
cavity of their potential polyp hosts, having been previously consumed by
some filtering zooplankter, which are then captured by the polyp as its prey
(Fitt 1984). The zoospores are protected by their thick cell wall from being
digested during this procedure (Schoenberg 1980). The zoospores (or cysts),
when they get into the gastral cavity of the polyp, are phagocytosed there by
mesenterial phagocytes and then transferred by them into the polyp's tissues
Morphology and Ecological Physiology of Corals
Zahl 1966; Taylor 1973b; Mashansky et al. 1980). The function of the
periplast is to protect the zooxanthella cell from the attack by lysosomes of
the host cell (Muscatine et al. 1975). At the same time, the periplast is
permeable for metabolite exchange. Some parts of the periplast have
numerous protrusions and exfoliations of the outer membrane. It was
supposed that through them the translocation of lipoid droplets - products
of photosynthesis - proceeds from the zooxanthellae to the host cell (Patton
and Burris 1983). On the side of the host, the filaments of its cell protoplasm
form a kind of capsule around the cells of the zooxanthellae
(Woscoboynikov et al. 1983). Thus, the existence of the complex periplast in
the case of zooxanthellae decreases the difference between intracellular and
interorganismic symbiosis, because their cells are largely isolated from the
cell of the host, moreover, zooxanthellae preserve the ability to multiply
independently of the host in external environments. But after their
penetration into the host's cells an exchange with the metabolites between
them and the polyp soon starts, which is regulated by the host cells, as well
as by the rate of their multiplication. A remarkable feature of the
morphology of zooxanthellae cells is their extremely large chloroplast, which
occupies half of the total volume of their cells. The lamellar system of the
chloroplasts consists of 10-12 complexes. Each of them contains three
tylacoids. The pyrenoid of the chloroplast is attached to it with the aid of a
stalk (schoenberg and Trench 1980; Voscoboynikov et al. 1983).
The primary "infestation" of polyps with zooxanthellae may occur in
various ways. Numerous taxa of hydroids, scleractinians and octocorals, as
well as their eggs or new-born larvae, contain the zooxanthellae cells (Gohar
1940; Atoda 1951; Smith 1980; Fitt 1984). In planulae, they supply them
with products of their photosynthesis thus fomenting their survival
(Richmond 1981). At the same time, many corals produce eggs and larvae in
which zooxanthellae are originally absent (Atoda 1953; Droop 1963; Kinzie
1974; Fitt 1984; Babcock et al. 1986). Among them some octocorals
(Heteroxenia, Briareum, Pseudopterogorgia) and most of the scleractinian
spawning corals. The larvae hatched from these eggs could be infected by
free-swimming motile zoospores of zooxanthellae, which are present in
water. The "infection" takes place also at the stage where settled larvae
formed a first corallite. The polyp attracts zooxanthellae, which swim into its
mesenterial cavity through its mouth and thus "infect" him (Kinzie 1973).
Zooxanthellae may locate the potential host polyps with the aid of
chemoreception. They move along the gradient of concentration of nitrogen
salts excreted by the polyp. The unmotile zoospores get into the gastral
cavity of their potential polyp hosts, having been previously consumed by
some filtering zooplankter, which are then captured by the polyp as its prey
(Fitt 1984). The zoospores are protected by their thick cell wall from being
digested during this procedure (Schoenberg 1980). The zoospores (or cysts),
when they get into the gastral cavity of the polyp, are phagocytosed there by
mesenterial phagocytes and then transferred by them into the polyp's tissues