304
Morphology and Ecological Physiology of Corals
habitats have algal symbionts. Among them there are several hundred
species of actinians, corals, zoantharians, hydroids, and sessile jellyfish
(Muscatine 1974). All of them harbor in their tissues the same species of
symbiotic dinoflagellates-the zooxanthellae Symbiodinium microadriaticum,
Freudentahl. They were first found by K. Brandt in the cells of a planktonic
radiolarian at the end of the last century. Initial studies of their functional
role as symbionts of coelenterates were made in 1920-1930 (Boshma 1925a;
Yonge and Nicholls 1931). The culture of this alga in artificial medium was
done successfully first by Kawaguti (1944), who discovered the motile stages
of zooxanthellae, which, being in host tissues, looked like round motile
cells. Later it was shown that the motile form of zooxanthellae was very
similar to the free-living dinoflagellate Gymnodinium (Mc Laughlin and Zahl
1966). They appeared in the culture mostly at midday and their appearance
seemed to be connected with illumination conditions (Lerch and Cook
1984). They have a rather complex cycle of development and their cells
living in the host's tissues represents one of its stages (d. Fig. 8.1). In
particular, it is one of two kinds of their immobile cysts which are capable of
cell division (Mc Laughlin and Zahl 1966; Steele 1975; Schoenberg and
Trench 1980). They are disposed by groups of two-three cells inside the
vacuoles of the ectoderm cells of the polyp which cover its body, tentacles,
oral disc and mesentery (Kawaguti 1964; Vandermeulen et al. 1972; Smith
1979). Some of them are located in the intercellular spaces (Woscoboynikov
et al. 1983). In cultures, among the cells of zooxanthellae their following
cellular forms could be found: (1) the immobile cysts with thick cell walls;
(2) the motile flagellated gymnodinium forms; (3) zoosporangia containing
one large motile zoospora; and (4) zoosporangia containing two or three
nonmotile zoospores. The motile zoospores swim actively and may penetrate
into young polyps of corals and into the tissues of other symbiotic coelenterates (Schoenberg and Trench 1980).
The strains of zooxanthellae, which live in different coelenterate hosts,
are very similar in their taxonomic signs, e.g. the size of cells and their
morphology, the composition of pigments, and their life cycle, thus
belonging to the same species, Symbiodinium microadriaticum. Such a wide
scale of hosts encompassing several hundreds of coelenterate taxa and even
clams (tridacna) with different immunic-genetic characteristics and
containing one single species of the symbiont is itself an outstanding
biological phenomenon (Schoenberg and Trench 1980). The latter authors
showed, moreover, that the one host, for example the actinian Aiptasia,
which had expelled its zooxanthellae, might be infected by the strange strain
of these algae, taken from some other coelenterates (corals) or even from
the tridacna clams. Nevertheless, it seems that the morphologically identical
strains of zooxanthellae, living in taxonomically different hosts differ by
some of their biochemical and physiological features (Kinzie et al. 1977;
Schoenberg 1980; Chang et al. 1983; Colley and Trench 1985). They might
be different even genetically, so that they could be related to different
Morphology and Ecological Physiology of Corals
habitats have algal symbionts. Among them there are several hundred
species of actinians, corals, zoantharians, hydroids, and sessile jellyfish
(Muscatine 1974). All of them harbor in their tissues the same species of
symbiotic dinoflagellates-the zooxanthellae Symbiodinium microadriaticum,
Freudentahl. They were first found by K. Brandt in the cells of a planktonic
radiolarian at the end of the last century. Initial studies of their functional
role as symbionts of coelenterates were made in 1920-1930 (Boshma 1925a;
Yonge and Nicholls 1931). The culture of this alga in artificial medium was
done successfully first by Kawaguti (1944), who discovered the motile stages
of zooxanthellae, which, being in host tissues, looked like round motile
cells. Later it was shown that the motile form of zooxanthellae was very
similar to the free-living dinoflagellate Gymnodinium (Mc Laughlin and Zahl
1966). They appeared in the culture mostly at midday and their appearance
seemed to be connected with illumination conditions (Lerch and Cook
1984). They have a rather complex cycle of development and their cells
living in the host's tissues represents one of its stages (d. Fig. 8.1). In
particular, it is one of two kinds of their immobile cysts which are capable of
cell division (Mc Laughlin and Zahl 1966; Steele 1975; Schoenberg and
Trench 1980). They are disposed by groups of two-three cells inside the
vacuoles of the ectoderm cells of the polyp which cover its body, tentacles,
oral disc and mesentery (Kawaguti 1964; Vandermeulen et al. 1972; Smith
1979). Some of them are located in the intercellular spaces (Woscoboynikov
et al. 1983). In cultures, among the cells of zooxanthellae their following
cellular forms could be found: (1) the immobile cysts with thick cell walls;
(2) the motile flagellated gymnodinium forms; (3) zoosporangia containing
one large motile zoospora; and (4) zoosporangia containing two or three
nonmotile zoospores. The motile zoospores swim actively and may penetrate
into young polyps of corals and into the tissues of other symbiotic coelenterates (Schoenberg and Trench 1980).
The strains of zooxanthellae, which live in different coelenterate hosts,
are very similar in their taxonomic signs, e.g. the size of cells and their
morphology, the composition of pigments, and their life cycle, thus
belonging to the same species, Symbiodinium microadriaticum. Such a wide
scale of hosts encompassing several hundreds of coelenterate taxa and even
clams (tridacna) with different immunic-genetic characteristics and
containing one single species of the symbiont is itself an outstanding
biological phenomenon (Schoenberg and Trench 1980). The latter authors
showed, moreover, that the one host, for example the actinian Aiptasia,
which had expelled its zooxanthellae, might be infected by the strange strain
of these algae, taken from some other coelenterates (corals) or even from
the tridacna clams. Nevertheless, it seems that the morphologically identical
strains of zooxanthellae, living in taxonomically different hosts differ by
some of their biochemical and physiological features (Kinzie et al. 1977;
Schoenberg 1980; Chang et al. 1983; Colley and Trench 1985). They might
be different even genetically, so that they could be related to different