Coral atoll communities are of special interest to marine biologists since they flourish in tropical waters characterized by low productivity (cf. Stoddart 1969). One of the aims of research in coral reef biology is to discern and describe quantitatively the adaptations that permit reef communities to exploit a marginally suitable habitat. Vital to this aim is an understanding of the nutrition of corals themselves since these lime-secreting cnidarians are the dominant life forms of many reefs and one of the principal reef builders. The general topic of coral nutrition has been reviewed recently (Muscatine 1973, Taylor 1973a) and serves as a convenient point of departure. The present discussion considers the potential nutritional capabilities of corals as mutualistic symbioses and the extent to which specific organismic and ecological manifestations of these capabilities have been described and quantified.

Riffkorallen können autotroph leben

Reef Corals: Mutualistic Symbioses Adapted to Nutrient-Poor Environments

Previous studies have attempted to quantify the nutritional importance of zooxanthellae to the respiratory requirements of their host, from measurements of the 24 h photosynthesis to respiration ratio. This preliminary study explores an alternative approach by trying to account for the energy fixed in photosynthesis in terms of expenditure, storage and loss. The energy budget so derived for Pocillopora eydouxi suggests that about 51% of the photosynthetically fixed energy is used in respiration, 0.9% in growth and 48% is unaccounted for and presumably lost from the colony. By partitioning the energy budget, it is found that 24% of the respiration and 11% of the energy retained in growth is attributable to the zooxanthellae. It is postulated that nutritionally, a coral is analogous to an aphid in being supplied with a high carbon diet and in being obliged to excrete the excess, probably in the form of mucus.

The role of zooxanthellae in the nutritional energy requirements of Pocillopora eydouxi

Lethal and sublethal effects of dredging on reef corals

The feeding behaviour of 35 species of Atlantic reef corals was examined in the laboratory and in the field. Observations were made during the day and at night, using freshly hatched brine shrimp nauplii and finely ground, filtered fresh fish as food sources. Three feeding strategies were observed: Group I–feeding by tentacle capture only; Group II–feeding by entanglement with a mucus net or mucus filaments; Group III–feeding by a combination of tentacle capture and mucus filament entanglement. Group I included corals of the families Poritidae and Pocilloporidae which were normally expanded during both day and night. Group II included corals of the family Agaricidae which were normally expanded at night and contracted during the day. Group III included corals of the other families examined which, with the exception of Dendrogyra cylindrus, were normally expanded only at night.



Feeding responses were elicited by both chemical and tactile stimuli. A preparatory feeding posture was assumed in response to chemical stimuli and consisted of horizontal positioning of the tentacles, elevation of the oral disk to form a cone-like mouth, a wide mouth opening and secretion of mucus by the epidermis of the oral disk. Following the assumption of the preparatory feeding posture, food capture and ingestive movements were elicited by tactile stimuli. However, food capture and ingestive movements were also elicited by chemical stimuli alone in those species which were normally contracted during the day.



While expanded corals captured food with their tentacles or with mucus filaments, contracted corals were able to feed by capturing fine particulate matter with mucus filaments only and thus acted as suspension feeders. By a combination of feeding strategies, reef corals were able to feed both day and night and a wide range of potential food ranging from fine particulate matter to large zooplankton was available to them.

Riffkorallen können autotroph leben

Citations

Reef Corals: Mutualistic Symbioses Adapted to Nutrient-Poor Environments

The role of zooxanthellae in the nutritional energy requirements of Pocillopora eydouxi

Lethal and sublethal effects of dredging on reef corals

Feeding mechanisms and feeding strategies of Atlantic reef corals

4. – nutrition of corals

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Reef Corals: Mutualistic Symbioses Adapted to Nutrient-Poor Environments

The Physiology and Biochemistry of Zooxanthellae Symbiotic with Marine Coelenterates. III. The Effect of Homogenates of Host Tissues on the Excretion of Photosynthetic Products in vitro by Zooxanthellae from Two Marine Coelenterates