design: Total genomic DNA was extracted from clam samples using the DNeasy Blood and Tissue Kit (Qiagen, Singapore), following the manufacturers protocol. PCR was done for the amplification of the ITS2 region of Symbiodiniaceae, using the SYM_VAR primer pair (Hume et al. 2018). For each clam individual, a unique 8 base-pair barcode was generated using Barcode Generator 2.8 (Comai and Howell 2012) and conjugated to the 5 end of the primer pair used specifically for that individual. Purification of derived amplicons were conducted with the QIAquick PCR Purification Kit (Qiagen, Singapore), individual sample libraries were normalised to ca. 12.6 ng L-1, and pooled to form a single library of 25 L volume. The pooled libraries was sequenced on the Illumina MiSeq sequencing platform for 300-bp paired-end reads (MiSeq Reagent Kit v.3) at Macrogen (Singapore).