| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 107 === Most scleractinian corals are mixotrophic and can complement photosynthetically acquired carbon with heterotrophy. However, the abilities to shift of diet during stress period seem to be species-specific and have been hypothesized to offer a competitive advantage for species to be naturally selected. Using carbon and nitrogen stable isotopes on both the coral host and its symbiotic algae, we propose here to characterize isotopic niches of coral species (i.e. a proxy of trophic niches) and scrutinize the causes of intraspecific variability in order to provide some insights on their trophic strategies. We hypothesized a species may exhibit various trophic behaviors with changes in environmental conditions, and that specialist and generalist species could be distinguished. In this study, we targeted five coral species: Isopora palifera (IP), Porites lutea (PL), Psammocora profundacella (PP), Stylophora pistillata (SP), and Tubastrea cf. coccinea (TC). Coral samples were collected from contrasted environmental conditions confounded within the effects of latitude (Northern Taiwan and Green Island), depth (10 m and 40 m), and seasons (warming and cooling). Stable isotopic compositions of host tissues and symbionts were analyzed, and then values of host tissues were adjusted using baseline values from the symbionts in order to be comparable between each species and condition. Colonies were assigned to clusters based on isotopic composition by using an unsupervised learning algorithm (k-means clustering). Difference between “generalists” and “specialists” was defined according to the number of clusters exhibited by coral species. Trophic niche size and position differed among coral species, which illustrate various degree of plasticity. SP and TC had the smallest and largest niche size respectively, while other species resided in between. Environmental conditions could explain most of the variation observed within species, except in PP and IP where the carbon sources and niche positions did not change. Five clusters were distinguished among colonies. PP performed in four clusters, which could indicate that this species tends to be more generalist in term of trophic behaviors. To contrast, SP and TC were more specialized by exhibiting only two clusters. The generalists are hypothesized to have higher environmental tolerance than specialists performing particularly well in a narrow range of environmental conditions. Overall, this study represents a preliminary step toward an integration of trophic plasticity into a definition of coral adaptive strategies, which allow us to better understand the current responses of corals to climate change.
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