# Population genetics and evolutionary history of the intertidal brittle star Ophiothrix (Ophiothrix) exigua in the northern China Sea

## TL;DR

This study explores the genetic diversity and evolutionary history of the brittle star Ophiothrix exigua in the northern China Sea, finding high genetic diversity and low differentiation among populations.

## Contribution

The study provides new insights into the population genetics and Pleistocene demographic history of Ophiothrix exigua in the northwestern Pacific.

## Key findings

- High haplotype diversity and low nucleotide diversity were observed among Ophiothrix exigua populations.
- Low genetic differentiation and strong gene flow were detected between populations in the study area.
- Demographic expansion around 0.2 million years ago was likely linked to Pleistocene transgressions in the Yellow Sea.

## Abstract

Ophiothrix (Ophiothrix) exigua is a common brittle star in the northwestern Pacific. As a dominant species, O. exigua 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 O. exigua populations from the northern China Sea using mitochondrial (COI, NAD4) and nuclear (ITS2, 18S) gene markers. High haplotype diversity, low nucleotide diversity, and low rates of gene differentiation among the populations of O. exigua were detected. Pairwise genetic differentiation (ΦST) 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 O. exigua 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.

This study investigated the genetic diversity, population structure, and demographic history of the brittle star Ophiothrix (Ophiothrix) exigua, a common intertidal species in the northwestern Pacific Ocean, using mitochondrial and nuclear gene markers. The results revealed high genetic diversity but remarkably low genetic differentiation among populations in the Yellow and Bohai Seas, suggesting widespread gene flow and the absence of cryptic speciation within the study area. Demographic analyses indicated that the populations experienced expansion around 0.2 million years ago, likely related to sea level changes during the Pleistocene glaciations.