Population genetics and evolutionary history of the intertidal brittle star <i>Ophiothrix (Ophiothrix) exigua</i> in the northern China Sea

<i>Ophiothrix (Ophiothrix) exigua</i> is a common brittle star in the northwestern Pacific. As a dominant species, <i>O. exigua</i> inhabiting the intertidal rocky ecosystem are affected by multiple environmental stressors, but molecular insights into their genetic population structure remain poorly studied. In this study, we investigated the population genetics and evolutionary history of six <i>O. exigua</i> populations from the northern China Sea using mitochondrial (<i>COI</i>, <i>NAD4</i>) and nuclear (<i>ITS2</i>, <i>18S</i>) gene markers. High haplotype diversity, low nucleotide diversity, and low rates of gene differentiation among the populations of <i>O. exigua</i> were detected. Pairwise genetic differentiation (Φ_ST) statistics between different localities were negative or low and insignificant, suggesting strong gene flow of this species over the study areas. The phylogenetic analyses showed that the populations exhibited high homogeneity between localities in our study area. Demographic analyses indicated that the populations experienced sustained expansion around 0.2 million years ago. This expansion was likely related to transgressions events in the Yellow Sea during the Pleistocene period. Additional samples of <i>O. exigua</i> from disparate geographical locations, especially the Japan Sea and the Korean Peninsula, will be needed to unravel the population genetic patterns and evolutionary history of this species.