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.

Reef Corals: Mutualistic Symbioses Adapted to Nutrient-Poor Environments

The dual character of corals, that they are both auto- and heterotrophs, was recognized early in the twentieth Century. It is generally accepted that the symbiotic association between corals and their endosymbiotic algae (called zooxanthellae) is fundamental to the development of coral reefs in oligotrophic tropical oceans because zooxanthellae transfer the major part of their photosynthates to the coral host (autotrophic nutrition). However, numerous studies have confirmed that many species of corals are also active heterotrophs, ingesting organisms ranging from bacteria to mesozooplankton. Heterotrophy accounts for between 0 and 66% of the fixed carbon incorporated into coral skeletons and can meet from 15 to 35% of daily metabolic requirements in healthy corals and up to 100% in bleached corals. Apart from this carbon input, feeding is likely to be important to most scleractinian corals, since nitrogen, phosphorus, and other nutrients that cannot be supplied from photosynthesis by the coral’s symbiotic algae must come from zooplankton capture, particulate matter or dissolved compounds. A recent study showed that during bleaching events some coral species, by increasing their feeding rates, are able to maintain and restore energy reserves.

This review assesses the importance and effects of heterotrophy in tropical scleractinian corals. We first provide background information on the different food sources (from dissolved organic matter to meso- and macrozooplankton). We then consider the nutritional inputs of feeding. Finally, we review feeding effects on the different physiological parameters of corals (tissue composition, photosynthesis and skeletal growth).

Heterotrophy in tropical scleractinian corals.

An equation is derived which rigorously defines the photosynthesis: respiration ratio (P:R) for any alga: invertebrate symbiotic association arid permits the computation of the fractional contribution of translocated algal carbon to the daily respiratory carbon requirement of the host animal. The equation is applied to two species of symbiotic reef corals, using O2 flux data from 24-h continuous measurements in situ. Given certain assumptions, the algae in the shallow-water Hawaiian reef corals Pocillopora damicornis and Fungia scutaria can supply of the order of 63 and 69% of the daily respiratory carbon demand of their respective animal hosts.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1981.26.4.0601

Estimating the daily contribution of carbon from zooxanthellae to coral animal respiration1

The zonation of dominant species and growth forms on coral reefs is one of the most striking patterns found in any natural community (42, 46, 115). Numerous factors may be important in controlling coral distributions and species diversity. These include light, usually correlated with depth; sedimentation; temperature; wave energy; plankton availability; frequency of mortality caused by storms or tidal exposure; and grazing by fish and urchins. Three major studies that correlate variation in physical factors with coral diversity and species composition over depth and horizontal position conclude that the reef environment is extremely heterogeneous and that species composition and diversity are determined not by physical gradients but by microhabitat conditions and complex biotic interactions (19, 88, 89). In spite of the heterogeneity of coral reefs strong evidence suggests that predictable patterns of species diversity exist along a depth gradient. Three separate studies on well-developed reefs have found similar patterns of species diversity in relation to depth. On each reef, diversity (measured by species richness and/or H') was low near the surface and increased to a maximum between 15 and 30 m in depth. (Red Sea 0-30 m (74); Jamaica 0-30 m (58); Jamaica 15-56 m (72); Indian Ocean 0-60 m (108). Below 30 m, diversity decreased gradually at the two sites where deeper surveys were made. Coral reefs have been described recently as being nonequilibrium systems, where competitive exclusion is prevented by frequent disturbances, as pre-

Patterns of Species Diversity on Coral Reefs

Nine of the ten most common sponge species on the fore-reef slope of Davies Reef(Great Barrier Reef) contain symbiotic cyanobacteria. Six of the ten are net primary producers, with three times more oxygen produced by photosynthesis than is consumed during respiration. Light interception is enhanced by morphological flattening, thereby increasing the potential for phototrophic nutrition, a factor crucial in the ecology of most sessile coral reef invertebrates.

https://science.sciencemag.org/content/sci/219/4583/410.full.pdf

Net Primary Productivity in Coral Reef Sponges

Over 15 days of continuous monitoring, the mean nocturnal concentration of drifting net zooplankton in a coral reef community on the Bermuda platform was 0.83 mg m−3. The mean respiration rate of reef-building corals at the study site was calculated to be 6.4 g O2 day−1 m−2 of reef bottom. Using these data we show that the energy requirement of these corals was more than an order of magnitude greater than the energy they could obtain by capturing the sparse zooplankton in the surrounding waters.

The question follows, why, if corals can exist in the absence of energetically significant quantities of zooplankton, do their elaborate anatomic modifications for catching zooplankton persist? Evidence indicates that corals cannot obtain their phosphorus requirements from solution. We therefore suggest that relatively small amounts of zooplankton may be important as a source of phosphorus and perhaps of other essential nutrients for both the coral and its algal symbionts.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.1970.15.4.0579

Nancy T. Kuenzel

Papers

The role of zooplankton in the nutrition of some scleractinian corals1

9 – Analog Computer Models of “The Wolves of Isle Royale”