In situ shifts of predominance between autotrophic and heterotrophic feeding in the reef-building coral Mussismilia hispida: an approach using fatty acid trophic markers

Many species of reef-building corals are mixo-trophic, relying on both photoautotrophy performed by their dinoflagellate symbionts and heterotrophy from consumption of zooplankton. Autotrophy and heterotrophy supply corals with specific omega 3 fatty acids, which can be used as trophic markers and record the contribution of each feeding strategy. This study investigated whether the reef-building coral Mussismilia hispida, endemic to Brazil, is able to shift between predominantly autotrophic and predominantly heterotrophic by monitoring the concentration of fatty acids in the host tissue. We then examined whether shifts are related to changes in temperature and wind stress. For that purpose, M. hispida colonies were monitored for a year with monthly tissue sampling. Symbiont concentration was determined and lipid extraction performed. Four fatty acids were quantitatively analyzed by gas chromatography with flame ionization detector: the autotrophy markers: stearidonic acid (SDA), docosapentaenoic acid (DPA), docosahexaenoic acid (DHA), and a heterotrophy marker: cis-gondoic acid (CGA). Three preliminary experiments confirmed the specificity of SDA, DPA and CGA, but not of DHA. Shifts of predominance occurred multiple times during the year and were associated with minimal temperatures and wind stress. Colonies underwent mild bleaching during summer months, which they seemed to compensate with heterotrophic feeding. Our major findings include the validation of three FATM and a trophic index for coral reef ecology studies and also describing the in situ occurrences of shifts between feeding modes, while highlighting the role of temperature and meteorological events.