ABSTRACT
Coral energy and nutrient acquisition strategies are complex and sensitive to environmental conditions such as water flow. While high water flow can enhance feeding in hard corals, knowledge about the effects of water flow on the feeding of soft corals, particularly those pulsating, is still limited. In this study, we thus investigated the effects of feeding and water flow on the physiology of the pulsating soft coral Xenia umbellata. We crossed three feeding treatments: (i) no feeding, (ii) particulate organic matter (POM) in the form of phytoplankton and (iii) dissolved organic carbon (DOC) in the form of glucose, with four water volume exchange rates (200, 350, 500 and 650 L h-1) over 15 days. Various ecophysiological parameters were assessed including pulsation rate, growth rate, isotopic and elemental ratios of carbon (C) and nitrogen (N) as well as photo-physiological parameters of the Symbiodiniaceae (cell density, chlorophyll-a and mitotic index). Water flow had no significant effect but feeding had a substantial impact on the physiology of the X. umbellata holobiont. In the absence of food, corals exhibited significantly lower pulsation rates, lower Symbiodiniaceae cell density and lower mitotic indices compared to the fed treatments, yet significantly higher chlorophyll-a per cell and total N content. Differences were also observed between the two feeding treatments, with significantly higher pulsation rates and lower chlorophyll-a per cell in the DOC treatment, but higher C and N content in the POM treatment. Our findings suggest that the X. umbellata holobiont can be viable under different trophic strategies, though favouring mixotrophy. Additionally, the physiology of the X. umbellata may be regulated through its own pulsating behaviour without any positive or negative effects from different water flow. Therefore, this study contributes to our understanding of soft coral ecology, particularly regarding the competitive success and widespread distribution of X. umbellata.
