Heterotrophic Feeding of Corals
339
photosynthetically produced by their zooxanthellae. A kind of referee
opinion on the problem autotrophy - heterotrophy in corals was expressed
by Goreau et al. (1971) and then developed by Porter (1976). It said that
among the variety of corals there are taxa with either the heterotrophic or
the autotrophic feeding dominating in them. The preference for a type of
feeding in coral species was supposed to be dependent upon their ecological
adaptations and life strategy, reflected in their morphology, and first of all
upon the size of their polyps. Corals with large polyps and long tentacles
were supposed to be feeding more heterotrophically, and their dependence
upon their zooxanthellae supposedly minimal. Porter's hypothesis seemed to
be attractive. Whether correct or not, it appeared to explain the great
diversity in the forms of colonies and the sizes of polyps among scleractinian
corals, with a view to the background of their trophic specialization, which
provides for the sharing of energy sources and for decreasing competition
between them. But despite its attractiveness this theory still needed direct
approval in experiments.
By the end of the 1970s it became clear that all hermatypic scleractinian
corals, independently of the morphology of their colonies or the size of their
polyps, make use with high efficiency of their mechanisms for predatory
feeding, filter-feeding, "osmotic" feeding with DOM as well as of the
autotrophic products translocated by their symbionts (Lewis and Price 1975;
Lewis 1977b; Sorokin 1978b, 1981c; Falkovski et al. 1984). The next step
needed was the evaluation of the shares of the above-mentioned resources
in the energy balance of the different coral species. In our attempt to solve
this problem we have studied feeding and metabolism of 23 common corals
of the GBR at Heron Island Research Station of the University of
Queensland. The results did not prove the theory of Porter regarding the
subdivision of scleractinian corals into "mostly autothrophic" and "mostly
heterotrophic", and they disproved as well the proposals about their trophic
specialization with respect to their ability to use heterotrophic sources for
feeding. Instead, an amazing uniformity of their parameters for feeding and
metabolism has been established, which is independent of their taxonomic
position, and the morphology of their colonies or of the size of their polyps
(cf. Table 9.14).
During early studies of corals' nutrition, the investigators used the
method of visual observation by binocular microscope of how polyps seized
their food objects (small crustaceans or pieces of fish meat, stained with
neutral dye) (Pratt 1906; Vaughan 1919; Boshma 1925b; Yonge 1930a,b).
To record the predatory feeding of corals in situ, stroboscopic photography
of the polyps at night was employed. The number of polyps with captured
prey was then estimated in this way (Johannes and Tepley 1974). To study
the coral's predation under conditions close to in situ conditions, individual
colonies of corals were covered with plastic bags and the food (planktonic
crustaceans or labeled organic matter) was then injected into the latter
(Lasker 1981; Coffroth 1984). To attract prey, Porter (1974b, 1978)
339
photosynthetically produced by their zooxanthellae. A kind of referee
opinion on the problem autotrophy - heterotrophy in corals was expressed
by Goreau et al. (1971) and then developed by Porter (1976). It said that
among the variety of corals there are taxa with either the heterotrophic or
the autotrophic feeding dominating in them. The preference for a type of
feeding in coral species was supposed to be dependent upon their ecological
adaptations and life strategy, reflected in their morphology, and first of all
upon the size of their polyps. Corals with large polyps and long tentacles
were supposed to be feeding more heterotrophically, and their dependence
upon their zooxanthellae supposedly minimal. Porter's hypothesis seemed to
be attractive. Whether correct or not, it appeared to explain the great
diversity in the forms of colonies and the sizes of polyps among scleractinian
corals, with a view to the background of their trophic specialization, which
provides for the sharing of energy sources and for decreasing competition
between them. But despite its attractiveness this theory still needed direct
approval in experiments.
By the end of the 1970s it became clear that all hermatypic scleractinian
corals, independently of the morphology of their colonies or the size of their
polyps, make use with high efficiency of their mechanisms for predatory
feeding, filter-feeding, "osmotic" feeding with DOM as well as of the
autotrophic products translocated by their symbionts (Lewis and Price 1975;
Lewis 1977b; Sorokin 1978b, 1981c; Falkovski et al. 1984). The next step
needed was the evaluation of the shares of the above-mentioned resources
in the energy balance of the different coral species. In our attempt to solve
this problem we have studied feeding and metabolism of 23 common corals
of the GBR at Heron Island Research Station of the University of
Queensland. The results did not prove the theory of Porter regarding the
subdivision of scleractinian corals into "mostly autothrophic" and "mostly
heterotrophic", and they disproved as well the proposals about their trophic
specialization with respect to their ability to use heterotrophic sources for
feeding. Instead, an amazing uniformity of their parameters for feeding and
metabolism has been established, which is independent of their taxonomic
position, and the morphology of their colonies or of the size of their polyps
(cf. Table 9.14).
During early studies of corals' nutrition, the investigators used the
method of visual observation by binocular microscope of how polyps seized
their food objects (small crustaceans or pieces of fish meat, stained with
neutral dye) (Pratt 1906; Vaughan 1919; Boshma 1925b; Yonge 1930a,b).
To record the predatory feeding of corals in situ, stroboscopic photography
of the polyps at night was employed. The number of polyps with captured
prey was then estimated in this way (Johannes and Tepley 1974). To study
the coral's predation under conditions close to in situ conditions, individual
colonies of corals were covered with plastic bags and the food (planktonic
crustaceans or labeled organic matter) was then injected into the latter
(Lasker 1981; Coffroth 1984). To attract prey, Porter (1974b, 1978)