International Journal of Marine Science, 2025, Vol.15, No.1, 15-27 http://www.aquapublisher.com/index.php/ijms 17 brackish environments, including estuarine bays, lagoons, mangrove swamps, and nearshore shallow seas. They typically attach to hard substrates such as rocks, shells, or artificial structures, or to shell debris on soft bottoms, which supports the formation of complex, three-dimensional reef structures (Qurani et al., 2020). Figure 1 (A) Saccostrea sp. clustering above coral at Moreton Island, Queensland; (B) Oysters, coral, hydroids and sponges co-habiting a dock at the Smithsonian’s Bocas Del Toro Marine Station in Panama; (C) Isognomon sp. and coral co-occurring on a reef in the Philippines; (D) Multiple Saccostrea species clustering on aquaculture furniture on a coral reef at Magnetic Island, Queensland (Adopted from Richardson et al., 2022) Oysters exhibit a broad tolerance to variations in salinity and temperature. For example, Crassostrea gigas demonstrates strong adaptability and can thrive and reproduce across a salinity range of 20‰~35‰ and temperaturesrangingfrom10 °Cintemperatezonestoover30 °Cinsubtropicalwaters.Manyoysterspecies release planktonic larvae, with larval durations ranging from one to three weeks, enabling dispersal to nearby areas via ocean currents. However, extreme environmental conditions, such as the low salinity of the Baltic Sea or highly saline and hot conditions in certain tropical areas, may limit oyster distribution and define the boundaries of population ranges. 2.3 Population differentiation phenomenon under biogeographical zoning Oyster populations in different regions around the world often show biogeographic differentiation genetically due to geographical isolation and environmental differences. On the one hand, intercontinental or interocean isolation has led to the evolution of oysters from different oceans into unique species. On the other hand, environmental gradients and physical barriers between different climatic zones and sea areas within the ocean also lead to population-level genetic structures. Studies have used the DNA of museum specimens to reconstruct the population structure of European flat oysters in the 19th century. It was found that there was a unique haplotype group along the North Sea to France at that time, while other groups were limited to narrower areas, which may reflect multiple shelters and regeneration processes after the last ice age. Although modern European flat oyster populations show a certain mixture due to large human transplants, wild populations in local regions (such as the UK and Nordic) still retain obvious genetic differences, suggesting their historically limited connectivity and environmental selection pressures (Monteiro et al., 2024). 3 Historical and Ecological Drivers of Biogeographic Pattern 3.1 Effects of paleoclimatic and geological events on the diffusion of oysters The current distribution and genetic pattern of global oyster populations are deeply affected by repeated fluctuations in the Quaternary paleoclimate and earlier geological events. During the interglacial climate cycle of ice-age, drastic rise and fall of sea levels change the scope of coastal habitats and shapes the history of oysters'
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