Heterotrophy Confers Corals with Resistance but Limits Their Range Expansion: A Case of Marginal Coral Communities

Abstract

Climate change may threaten the survival of corals due to ocean warming and increased occurrence of extreme weather events. Yet, marginal coral communities demonstrate remarkable adaptability, largely due to stress-tolerant species. The mechanisms underlying their resilience to environmental fluctuations remains largely unexplored. Here, we investigated 3 coral species ( Cyphastrea serailia , Dipsastraea speciosa , and Duncanopsammia peltata ) living at the northern edge of their distribution along the South China coast (Dongshan). Combining physiological and stable isotope analysis, we assessed their traits (symbiont characteristics, C:N ratio, and biomass) and trophic flexibility (i.e., shifts in isotopic niche or nutritional mode) in response to environmental fluctuations across seasons (autumn, winter, and late spring). Our results revealed distinct physiological traits among these corals, but consistently showed lower symbiont density, higher biomass, and increased chlorophyll content in winter. All corals highly relied on heterotrophy as a crucial adaptive response (consistently no overlap between host and symbiont isotopic niches), but they differed in the degree of trophic plasticity, with D. speciosa showing the highest variability in heterotrophic contribution (36.3% across seasons), followed by C. serailia (21.8%) and D. peltata (8.6%). Notably, the species-specific differences in trophic plasticity well corresponded to the variation in overall isotopic niche and coral distribution. As such, trophic plasticity may play a role in shaping spatial distribution of corals in the marginal community. Overall, this study enhances our understanding of coral resilience in marginal communities and offer new insights into possible mechanisms driving the distribution of reef-building corals in the changing ocean.