This mixed phenomenon in the ZZ population and north-south differentiation of
S. tenuifilis populations might result from two main factors. Firstly, historical glacial action. Some scholars have discussed that the phylogeographic pattern and genetic structure of marine fish in the coastal China’s seas are affected by the Quaternary glacial-interglacial cycle (
Maggs et al., 2008;
Larmuseau et al., 2009). During the glacial period, the sea level of the East China Sea, which is connected to the Bohai Sea and Yellow Sea, dropped by about 150 m from the present, and the sea level of the South China Sea dropped by about 100–120 m, resulting in the sea level in the East China Sea is slightly lower (or in a transitional state). After the glacial period, the sea level rose and the marginal sea was reconnected.
Xu (2014) reported that the northern and southern populations of
S. tenuifilis had different population expansion times during the Pleistocene using mitochondrial control region sequences. Therefore, successive isolation and reconnection of marginal seas in China during the Pleistocene may have hindered gene flow between
S. tenuifilis populations to some extent, promoting the divergence of genetic lineages in
S. tenuifilis populations. Secondly, spatial heterogeneity. Many studies have shown that eurythermal fish living at different latitudes have undergone natural selection with spatial heterogeneity, leading to changes in population structure and heritable phenotypic differentiation, accompanied by optimized adaptations to selection pressures imposed by local environments (
Han et al., 2021;
Tamaki and Honza, 1991).
Setipinna tenuifilis has a wide latitude distribution in the northwestern Pacific Ocean. Differences in water temperature (
Fig. 5a) can create different levels of selection pressure, which can affect traits related to
S. tenuifilis growth and reproduction. It has been reported the southern population of
S. tenuifilis in China’s coast enters the spawning period earlier than the northern population, and the asynchrony of spawning in the northern and southern populations may further limit the genetic exchange between the northern and southern populations (
Zhao et al., 2016). In addition, the high flux of nutrient input from the Changjiang River Estuary to the East China Sea (
Fig. 5b) has led to a high level of primary productivity (
Mu et al., 2020). It may be a common feeding ground for the northern and southern
S. tenuifilis populations, leading to the phenomenon of two genetic backgrounds in ZZ population.