Total genomic DNA was extracted from 40 samples using FastDNA Spin Kit (MP Biomedicals, U.S.) according to manufacturer's instructions. The Symbiodiniaceae ITS2 region of rDNA was amplified using primers GTL2-F (5'-GAATTGCAGAACTCCGTG-3') and GTL2-R (5'- GGGATCCATATGCTTAAGTTCAGCGGGT- 3'). Triplicate PCR products were pooled for each sample. Sequencing was performed on the Illumina MiSeq PE311 platform according to the standard protocols by Majorbio Bio-Pharm Technology Co. Ltd. (Shanghai, China).