Summer autotrophy and winter heterotrophy in the temperate symbiotic coral

Carbon and nitrogen isotope ratios (d13C and d15N) were determined in the zooxanthellae, host tissue, and whole symbiotic association of the symbiotic temperate coral Cladocora caespitosa, as well as in the different components of the coral’s potential food sources (plankton, particulate organic matter in seawater and in the sediment). Data were collected both in winter and summer at three different locations, to assess the extent of auto and heterotrophy in this species and get a better understanding of the functioning of temperate symbioses. There was a marked seasonal difference in the signature of the zooxanthellae and host tissue, highlighting two clear feeding patterns. In summer, d13C signatures of the coral host and the zooxanthellae were similar (217%) and very different from the signature of the food sources (from 221% to 225%), suggesting that corals were relying on autotrophy for the acquisition of carbon. d15N values also suggested that nitrogen was not acquired through feeding. Conversely, in winter, the d13C signature of the host decreased by ca. 223% to 228%, and was more comparable to the signature of the external food sources (between 224% and 225%), suggesting a substantial reliance of C. caespitosa on external food sources during this season. There was also a 3%enrichment between the d15N signatures of the food (4–5%) and the signature of the symbiotic association (7–8%), suggesting that nitrogen was also acquired through feeding. Overall, these results give evidence that C. caespitosa and temperate corals in general derive a large fraction of their energy from heterotrophic feeding in winter.