International Journal of Marine Science, 2025, Vol.15, No.1, 15-27 http://www.aquapublisher.com/index.php/ijms 18 diffusion and isolation. The retreat during the ice age caused large-scale exposure of the continental shelf, many shallow sea oyster habitats disappeared, and populations were compressed to shelter areas; the sea during the interglacial period expanded the coastline again, providing new colonial space for oysters. Baker et al. (2019)'s study identified a unique haplotype of the 19th-century North Sea oyster through ancient DNA, supporting that the North Sea might have served as a post-ice shelter for independence and nurtured groups that were genetically distinct from other parts of the Atlantic Ocean (Baker et al., 2019). In addition to climate change, geological tectonic events also have an irreversible impact on oyster distribution. From the late Miocene to the Pliocene, the gradual uplift of the isthmus of Central and American finally cut off the ocean connection between the Pacific and the Atlantic Ocean about 3 million years ago, contributing to the long-term isolation of the two oceanic oyster populations (Stange et al., 2018). The subsequent evolutionary differences formed a significant difference between the Atlantic flat oysters and the Pacific Cup oysters today. Group historical dynamic reconstruction shows that many oyster populations experienced obvious bottlenecks and expansions in the late Pleistocene. Monteiro et al. (2024) used the "Continuous Markov Model" (PSMC) analysis to find that the effective size of oysters in the northwest Pacific Ocean generally decreased about 100,000 years ago, and then slowly recovered from the glacial trough. This period corresponds to the harshest stage of the last ice age, which may reflect the adverse effects of the climate environment on oyster survival at that time, as well as the re-growth of populations when the interglacial period arrives. 3.2 The role of ocean currents, temperature-salt gradients and ocean obstacles Physical and ecological factors in the marine environment continue to affect the connectivity and differentiation of oyster populations during the current climate period. Marine circulation and coastal currents are the main carriers driving the diffusion of phytoplankton juveniles. The juvenile stage of oysters can float in water for several days to weeks, and their diffusion distance depends to a large extent on the velocity and direction of the ocean current. For example, the Kuroshima tide system in the northwest Pacific flows from south to north, which helps transport oyster juveniles from Taiwan and South China to East China and southwestern Japan, thereby promoting population connectivity in these regions (Takeuchi et al., 2020). Secondly, environmental factors such as temperature and salinity gradients also affect the survival and settlement of oyster juveniles, thereby shaping the genetic connectivity pattern. Oysters are widely salty and warm species, but extreme conditions will exceed their tolerance range. Taking the Baltic Sea as an example, the surface salinity of this sea area gradually decreases from 20‰ at the entrance to below 5‰ of the inner bay. Although European flat oysters were once distributed in the western Baltic Sea, they were unable to reproduce in the lower salty central and eastern waters (Manuel et al., 2023). Marine topography and ecological obstacles cannot be ignored. In addition to headlands, freshwater plumes in large estuaries, long sandbanks, strong tide straits, etc. may all become obstacles to the spread of oyster juveniles. The Skagrak Strait in Northern Europe connects the North Sea and the Baltic Sea. Its complex hydrology makes oyster juveniles prone to retention or are washed into the low-salt zone where discomfortable survival, forming a genetic boundary of populations. 3.3 The evolution of man-made activities and global oyster distribution pattern Since modern times, the impact of human activities on the distribution of oysters and the genetic structure of population has become increasingly prominent. On the one hand, overfishing and habitat destruction have led to a significant decline in native oyster populations in many areas and even functional extinction. For example, native European oysters in the North Sea and Baltic Sea were almost extinct due to overcapacity and disease in the late 19th century (Pouvreau et al., 2023). On the other hand, artificial introduction and aquaculture have widely broken the original geographical isolation pattern of oysters. Since the end of the 19th century, Pacific oysters have been introduced as breeding varieties to the west coast of North America, Europe, Australia and other places, forming a naturalized population of "self-sustaining" (Lallias et al., 2013). These introduced Pacific oyster populations originated from the same ancestors as those of native Asian populations, and are therefore genetically similar.
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