1 INTRODUCTION
In marine species, the phylogeography and genetic differentiation were influenced by historical events, including ocean current systems, vicariance, Pleistocene climatic cycles, and life-history characteristics of organisms (Liu et al., 2007; Ding et al., 2018). It is worth noting that some marine fishes have fragile genetic structure due to their extensive larval and adult dispersal (Ashrafzadeh et al., 2021; Caccavo et al., 2018). Climate oscillations during the Pleistocene greatly altered the environment of marginal seas of the western Pacific, including the South China Sea (SCS). During glacial periods, the SCS formed a semi-enclosed sac-shaped gulf and exposed approximately 0.7 million km2 of continental shelf (Wang & Sun, 1994). Previous molecular studies have shown that many marine fishes with high mobility exhibit low genetic structure in the SCS, such asCirrhimuraena chinensis (Li et al., 2014) and Nuchequula mannusella (Gao et al., 2019). The phylogeographic study of marine fishes alive in the SCS has particular significance for interpreting the consequences of past events, geological configurations, and modern oceanographic aspects in this environment (He et al., 2010).
Lagocephalus spadiceus is a non-toxic Lagocephalus species (Tuney, 2016), belonging to Tetraodontiformes, Tetraodontidae, andLagocephalus . It is a nearshore warm-water demersal fish that inhabits depth between 3 to 200 meters (Tuncer et al., 2008), distributed along the southern coast of Africa in the Indian Ocean, eastward to the Indonesia and the Philippines of the Pacific Ocean, and northward to the coast of China (Liu et al., 2016). In China, it occurs along the coastal areas of the SCS. Almost L. spadiceus is imported from China in Japan (Yamaguchi et al., 2013), but this also makes it an easy target for widespread exploitations. Recently, since the continuous increase in fishing intensity and deteriorating environmental conditions, the wild resources of L. spadiceus have been drastically reduced (Hardy et al., 2014). There are fewer reports on germplasm resource and genetic diversity evaluation of L. spadiceus . To better protect and rationally develop the wild germplasm resources of L. spadiceus , it is imperative to conduct a genetic diversity assessment to establish a theoretical basis for the scientific conservation and sustainable utilization of its genetic resources.
Genetic diversity is a vital component of biodiversity and prerequisite for the continuous adaptation of species or populations to environment change and survival evolution. Species with a higher genetic diversity possess a greater ability to environmental changes (Roldan et al., 2000). Mitochondrial DNA (mtDNA) constitutes a tiny fraction of organismal genome size but has been widely used as a marker of molecular diversity in animals for the past four decades (Galtier et al., 2009). This tool has been widely embraced by population geneticists, following the works of Avise et al (1987) and Moritz et al (1987), among others. Experimentally, mtDNA is present in most cells in high copynumber and is relatively easy, rapid, and inexpensive to sequence (Zink & Barrowclough, 2008). Due to the relationship between the rate and time of evolution, effective information sites are different, and their resolving power is different. Therefore, concatenating COI and Cyt b markers can increase the number of effective genetic sites, resulting in more accurate and effective information compared to single gene analysis (Halasan et al., 2021; Barrientos-Villalobos & Schmitter-Soto, 2019).
Our study examines the phylogeography, population genetic diversity, and demographic history of L. spadiceus using COI and Cytb gene datasets. As of current knowledge, there has not been a documented report on the population genetic of L. spadiceus . This study aims to address the lack of information on L. spadiceus in the region by providing a comprehensive background report. Meanwhile, our study has contributed to a better understanding of the evolutionary process that have influenced the phylogeography of coastal marine fishes in China.