Reduced genetic diversity and restricted gene flow of broadcast-spawning coral Galaxea fascicularis in the South China Sea reveals potential degradation under environmental change.

Under the dual effects of climate change and anthropogenic activities, coral reefs in the South China Sea (SCS) are at serious risk of degradation. Galaxea fascicularis is a widely distributed species in the SCS, and the study of its genetics, survival, and adaptability is conducive to further understanding the future characteristics of coral reefs in the SCS. In this study, 146 G. fascicularis samples were selected from 9 survey stations across 12 latitudes in the SCS, and 8 pairs of microsatellite markers were used to characterize their genetic diversity and structure. The results showed moderate genetic diversity index values (A = 3.444-4.147, H = 0.634-0.782, H = 0.367-0.586). The AMOVA results and pairwise F values showed a moderate level of genetic differentiation (Φ = 0.119, P < 0.05) among G. fascicularis populations in the SCS, whereas its genetic structure showed high genetic differentiation (F = 0.062-0.225) among relatively high-latitude populations (n = 3) and low genetic differentiation (F = 0.012-0.064) in low-latitude populations (n = 6). The living environment of relatively high-latitude populations is disturbed by high-intensity human activities, leading to the specialization of local populations. Mantel test results showed a significant positive correlation between genetic differentiation among G. fascicularis populations and sea surface temperature (SST) variance (R = 0.4885; Mantel test, p = 0.010 < 0.05) in addition to geographical distance (R = 0.1134; Mantel, test p = 0.040 < 0.05), indicating that SST and geographical isolation were primary factors affecting the genetic structure of this species in the SCS. The lower genetic diversity and limited gene flow of G. fascicularis indicate limited genetic adaptation, and corresponding vulnerability may be more pronounced under future environmental changes. These findings provide a theoretical basis for the conservation and restoration of coral reefs in the SCS.