4.3 Demographic history
This study utilized the Tajima’s D , Fu’sF S tests, and mismatch analysis to suggest that a population expansion event of the L. spadiceus population may have occurred from 0.025 Mya to 0.010 Mya in the Late Pleistocene.L. spadiceus is mainly distributed less 50 m depth, and spawns in coastal habitats and shallow shorelines. Therefore, the L. spadiceus distribution is closely related to historical sea level fluctuations. When sea level was 120 m lower than the present level during the last glacial maximum of the Pleistocene, the northern South China Sea included Beibu Gulf, which was part of the South China continent, Hainan Island, and Taiwan Island were connected to mainland China. The entire South China Sea was separated from the Indian Ocean to form a semi-closed basin (Wang, 1990). The survival range of marine fish decreased sharply; therefore, the L. spadiceus population may have moved and survived in one or more glacial refuges during this period, such as the semi-closed South China Sea. In the Late Pleistocene, the glaciation began to disappear and L. spadiceusmight have experienced rapid population expansion when favorable conditions occurred.
Many studies have demonstrated a weak genetic differentiation between the geographical populations of surface marine fish that can migrate long distances or swim. This can be attributed to the free dispersal of floating eggs, fish larvae, juveniles, and adults, as well as the absence of significant geographical obstacles in the open ocean environment. Consequently, gene exchange occurs extensively and widely among these marine fish populations (Canfield et al., 2022; Hewitt, 2000; Palumbi, 1994). However, it should be noted that L. spadiceus , being a benthic fish, does not exhibit a long-distance migration behavior according to its life history. Therefore, the observed panmixia among populations may be attributed to their early life habits. The active diffusion of fish larvae and juveniles as well as marine environmental factors, such as ocean circulation and climate change in the Late Pleistocene, have played crucial roles in shaping the systematic geographical pattern and population genetic structure ofL. spadiceus .