Nutrition of the temperate Australian soft coral Capnella gaboensis - II. The role of zooxanthellae and feeding

We examined the ability of Capnella gaboensis Verseveldt, 1977 (Coelenterata: Octocorallia: Alcyonacea: Nephtheidae) to utilize heterotrophic food sources, and the importance of heterotrophic nutrition and photosynthesis in its diet, by using preserved material and histological sections of field-collected specimens and by means of laboratory experiments in which coral branches were fed with 14C-labelled food of different sizes. The study was conducted from April 1982 to August 1984. C. gaboensis receives nutrition from the photosynthesis of its symbiotic zooxanthellae, Symbiodinium sp., and from heterotrophic sources. Up to 10% of the algal photosynthate was translocated to the animal-host tissues. The contribution of translocated carbon from the zooxanthellae to the daily respiratory carbon requirement of the animal was estimated to be well below 50% in all seasons except in the summer of 1983-1984, indicating that the coral must rely on additional sources of nutrition (i.e., heterotrophy) for most, if not all, of the year. Field (Sydney Harbour: 33°50′S; 151°15′E) and laboratory observations and experiments indicated that this coral probably feeds upon zooplankton, small particulate matter and dissolved organic matter

Nutrition of the temperate Australian soft coral Capnella gaboensis - I. Photosynthesis and carbon fixation

Capnella gaboensis Verseveldt, 1977 (Coelenterata: Octocorallia: Alcyonacea: Nephtheidae) is an abundant soft coral in the temperate waters of south-eastern Australia. From 1981 to 1984, using material collected from Sydney Harbour (33°50′S; 151°15′E), we investigated certain aspects of its apparently obligate association with its symbiotic zooxanthellae, Symbiodinium sp. Numbers of zooxanthellae and chlorophyll content were recorded throughout the first year, then net photosynthesis and respiratory rates of the coral as a function of photo-flux densities, temperature and season were measured in later years. The fractions into which photosynthetically fixed carbon was incorporated were also determined. The zooxanthellae contained a mean of 4.1 μg chlorophyll a 10-6 zooxanthellae. Neither the numbers of zooxanthellae in C. gaboensis nor the chlorophyll a content varied on a seasonal basis. Photon-flux densities in the field ranged from 5 to 120 μE m-2 s-1 over the year. The maximum net oxygen-exchange rate recorded for C. gaboensis was 9.4 μmol O2 mg-1 chlorophyll a h-1 at 871 μE m-2 s-1. The maximum carbon fixation rate obtained was 65.6 μmol C mg-1 chlorophyll a h-1 at 100 μE m-2 s-1. Photosynthesis of C. gaboensis was not light-saturated at 871 μE m-2 s-1; the light compensation point was in the range 50 to 90 μE m-2 s-1 and the optimum temperature was 25°C. Photosynthetic rates were highest in populations sampled in summer. Labelling with 14C showed that photosynthetically fixed carbon was initially incorporated into the aqueous-methanol (low molecular weight) fraction of the coral tissues. From 20 to 100 min after the introduction of the 14C label the rate of incorporation was fairly evenly divided between the aqueous methanol-soluble, the chloroform-soluble (lipid) and the insoluble fractions. In the light, little 14C was released as particulate and/or dissolved organic carbon. Translocation of products of photosynthesis represented up to approximately 10% of the total fixation