Composition and Genetic Diversity of Zooxanthellae Clades in Coral Pocillopora meandrina in a Bleaching Prone Atoll

Background Coral hosts expelling the zooxanthellae from their tissues as a stress response is a major cause of coral loss in coral reefs worldwide. As the zooxanthellae exist in different clades, it is assumed that corals' responses to stress could depend on the composition and diversity of zooxanthellae clades in coral hosts. Knowledge of zooxanthellae characteristics, therefore, is crucial to understanding bleaching stress. Methods and Results We studied zooxanthellae clade characteristics in coral Pocillopora meandrina in a bleaching-prone atoll in the Lakshadweep archipelago. We found the coexistence of multiple clades, B, C, D, and E, in the genus Symbiodinium and clade C in Cladocopium within single hosts. Clade composition revealed the dominance of clade C with no significant difference among all samples across 5 and 10 m depths (R = -0.064, P > 0.9), supporting clade-host specificity. Significant differences among clades were observed from the pairwise distance data (R = 0.477, P = 0.0001; multivariate analysis of pairwise distance data - ANOSIM) and the genetic diversity analyses, where clade C exhibiting highest genetic diversity among Symbiodinium clades (Hn = 25, Hd = 1, π = 0.0133 at 5 m, and Hn = 18, Hd = 0.9, π = 0.0125 at 10 m). Conclusions These findings explain the consistent dominance of clade C among different localities and depth gradients, which could be a suitable clade adapting to different light and temperature conditions. The present study suggests that monitoring changes in clade composition and genetic diversity in coral hosts is crucial to understanding bleaching stress.