Genetic connectivity and regional differentiation of Calanus sinicus in Korean coastal wates with a focus on Yeosu Bay

Calanus sinicus , an indicator species of the Yellow Sea cold bottom water, is generally abundant in Korean coastal waters, except in areas where summer environmental conditions are unfavorble. However, this species is notably dominant in Yeosu Bay (YB) along the south-central coast of Korea. This study aimed to provide a comprehensive analysis of the population genetic diversity, genetic structure, and haplotype network of C. sinicus , with a particular focus on investigating the origins of the YB population. To determine whether the C. sinicus YB population is indigenous or introduced, we collected samples from five regions: YB and four surrounding regions of the Korean peninsula (the eastern, southern, and western seas of Korea and the Yellow Sea). We amplified two genetic markers, the ribosomal internal transcribed spacer 1 ( ITS1 , n = 155) and cytochrome c oxidase subunit 1 ( COX1 , n = 117). To assess demographic patterns across the regions, Tajima’s D and Fu and Li’s Fs values were employed. YB population showed relatively low genetic diversity ( π = 0.0034) and positive Tajima’s D (1.07) and Fu and Li’s Fs (1.13) values, indicating possible population contraction or balancing selection. YB also showed the greatest differentiation with the Ulleung Basin with N st and F st values of 0.03 respectively. Only COX1 detected genetic distance based on the N st and F st values, highlighting its effectiveness in resolving regional genetic variation. However, haplotype minimum spanning networks for COX1 showed no significant genetic isolation among the study regions, indicating substantial gene flow among C. sinicus populations. The strong genetic connectivity among C. sinicus populations suggests the presence of an oceanographic mechanism that facilitates the seasonal entry into YB during the summer. • Calanus sinicus , an indicator species of the Yellow Sea cold bottom water, was observed in Yeosu Bay during the summer, and its source of the influx was identified. • The mitochondrial COX1 gene has been proven to be a more effective marker for detecting genetic variation within C. sinicus , compared to the nuclear ITS1 gene. • The Tajima’s D and Fu and Li’s Fs results based on COX1 gene indicate a declining population of C . sinicus in Yeosu Bay. • Also, C . sinicus population in Yeosu Bay has the highest genetic similarity with populations in the Yellow Sea, which aligns with the regional ocean current patterns.