Gene ontology (GO) enrichment analysis
To identify molecular mechanisms involved in the polyp bail-out response, GO enrichment analyses were conducted for up- and downregulated DEGs. As most eriched GO terms in our results are likely involved as general responses of corals to osmotic fluctuations, the discussion of which is beyond the scope of this study, we paid specific attention to biological processes and signaling pathways related to apoptosis, ECM degradation, and cell survival, which participate directly in polyp bail-out. Among upregulated DEGs, GO categories such as apoptotic process , extrinsic apoptotic signaling pathway , negative regulation of cell death , fibroblast growth factor (FGF) receptor signaling pathway , and tumor necrosis factor (TNF)-mediated signaling pathway were significantly enriched at both 12 h and 24 h (p <0.05), while enrichment of I-κB kinase/NF-κB signaling was significant at 12 h but not at 24 h (Table 1). In addition, in our transcriptomic data we found remarkable upregulation of eight matrix metalloproteinase (MMP)-encoding transcripts (>10-fold expression change from 0 h to 24 h), although enrichment of the GO category proteolysiswas insignificant (p = 0.20 at 12 h and 0.14 at 24 h). Among downregulated DEGs, no specific signaling pathway or apoptosis/proteolysis-related GO category showed significant enrichment.