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 .