International Journal of Marine Science, 2025, Vol.15, No.3, 144-153 http://www.aquapublisher.com/index.php/ijms 150 indicating that the repeated generation of land bridges is an important driving force for the differentiation of the lineage between islands and continents. However, not all species strictly follow the land bridge isolation model. Molecular evidence reveals that the lineage differentiation of some species earlier than the last land bridge. For example, the major branches of the big-eyed big-eyed big-hide fish differentiated on Hainan Island are estimated to be about 1.05 million to 160,000 years ago, which is earlier than the land bridge in the last ice age (Chen and Jang-Liaw, 2023). Therefore, its geographical genealogy pattern does not match the simple island-land binary caused by the isolation of the Qiongzhou Strait. In addition, for some marine fish, the Qiongzhou Strait is not an absolute biological obstacle. Inshore fish with strong swimming power may also migrate through offshore waters during the existence of the straits, so there is no obvious differentiation of island land (Yan et al., 2021). 6.3 Coupled analysis of marine environment changes and species formation mechanisms Species formation is often the result of the combined action of geographical isolation and environmental selection. In the history of differentiation of offshore fish in Hainan, the profound impact of changes in the marine environment is naturally indispensable. Molecular systems science provides us with clues to explore the coupling relationship between these effects and species formation mechanisms. Sea level lift driven by climate warm and cold cycles not only affects isolation patterns through land bridges, but also changes the scope of coastal habitats (Sun et al., 2013). When the climate warms up and sea levels rise, these differentiated lineages may re-contact, and ecological interactions such as competition, hybridization or substitution occur. Secondly, changes in the current pattern may affect the direction and intensity of gene flow. During the ice and interglacial periods, the circulation patterns and intensity of the South China Sea differed significantly, with different levels of Kuroshio invasion and the status of water exchange with the Beibu Gulf (Hu et al., 2018). This will affect the diffusion path of young slapped fish. The differences in environmental selection pressures in different regions can also promote lineage differentiation, i.e. ecological species formation. Offshore Hainan, the coastal ecosystems are diverse (mangroves, seagrass beds, coral reefs, sand mud base, etc.), and fish experience different selection pressures in different habitats, and ecologically adaptable genetic differentiation may occur over time. Some reef fish adapt to coral habitats with high structural complexity, while nearshore sandy benthic fish evolved into different adaptation characteristics (Figure 2) (Wang et al., 2022). If sea level changes or other factors expand the intertwined distribution areas of the two habitats, the two groups may each embark on different evolutionary paths and eventually accumulate to a point where it is difficult to hybridize. Figure 2 Maps of the study region in mainland China (A) and Hainan Island (B) indicate sites where 21 sampling localities of O. hainanensis were collected (circles) (Adopted from Wang et al., 2022) 7 The Ecological and Protection Significance of Research Results The molecular systems research results collected by this research institute have greatly enriched our understanding of the diversity of fish offshore in Hainan Island. Given the hidden species and misidentified heteronomial species
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