International Journal of Marine Science, 2025, Vol.15, No.5, 233-244 http://www.aquapublisher.com/index.php/ijms 239 6 Case Analysis: The Adaptation Mechanism of Oysters to High-salt Environment 6.1 Phenotype response and physiological changes under high salt stress Salinity is one of the key environmental factors that affect the survival and distribution of marine shellfish. Oysters are broad-salt animals and can adapt to salinity fluctuations through physiological regulation within a certain range. However, when salinity rises or drops sharply beyond the tolerance range, its metabolism and survival will be seriously affected. Under high salt (high osmotic pressure) stress, oysters initiate a series of stress response mechanisms to maintain intracellular osmotic pressure and ionic equilibrium. Oysters can reduce the contact between body fluids and external high saline water bodies by closing the shell cover and reducing water filtering, so as to buffer osmotic shock in the short term (Chen et al., 2021). At the same time, at the molecular level, oysters accumulate organic osmotic protectors (such as free amino acids such as taurine and glycine) and regulate inorganic ions discharge to balance the intra- and extracellular osmotic pressure gradients. In addition, high-salt environments are often accompanied by an increase in oxidative stress, and the oyster body will increase the activity of antioxidant enzymes (such as SOD, catalase) and the expression of protective molecules such as heat shock proteins to prevent cell damage. In terms of energy metabolism, high salt can increase the basal metabolic rate of oysters (She et al., 2022) to cover the energy cost of osmotic regulation, but excessive energy consumption may lead to growth stagnation or even death. 6.2 The relationship between genome structural variation and osmotic regulation genes Genomic structural mutation plays a key role in the genetic basis of oysters adapting to a high-salt environment. Especially for genes related to osmotic pressure regulation, their copy number variation and regulatory region variation will directly affect the salt tolerance of oysters. Ao Li et al.'s study on the Omi oyster genome found that the SLC gene family of this species showed significant expansion, with a total of hundreds of members, significantly more than the Pacific oysters. These genes encode multiple ion and solute transporters and are believed to play an important role in both low-salt and high-salt environments. Population genetic analysis showed that copy numbers of some genes in the SLCgene family differed between oyster populations in different salinity habitats, suggesting that natural selection prefers structural variant copies that favor extreme salinity tolerance (Figure 3). In addition to the SLC family, other genes related to osmotic regulation may also enhance adaptability through structural variation (Qiu et al., 2024). Some studies have compared the two related species of oysters that Pacific oysters, which are more sensitive to salinity, may have a condition that the regulatory efficiency of taurine synthesis pathway is not as good as that of oysters. It is speculated that there are variants in the promoter of the key enzyme gene involved. In molecular breeding practice, people have also begun to pay attention to variation in osmotic regulatory genes (Wang and Mai, 2025). For example, through genome-wide selection and association analysis, some genetic markers related to salinity tolerance were screened, including several CNV sites. 6.3 Case implications: the key role of structural variation in ecological adaptation The case of oyster adaptation to high-salt environments highlights the importance of genomic structural variation. (1) Structural variation enriches the sources of adaptive genetic variation: Compared with the gradient effect produced by point mutations, the genomic rearrangement or replication of large fragments can change the performance of multiple genes at the same time, allowing organisms to achieve significant improvement in adaptability in extreme environments. Oyster amplification of SLC and HSP genes, etc., belong to this category, and a multigene synergistic anti-response effect is generated through a genomic event (Li et al., 2021; Zhang et al., 2022). (2) Structural variations often become targets of natural selection: environmental pressures will select favorable structural variations to carry individuals to proliferate in the population. From the comparison of Omi oysters and Pacific oysters, it can be inferred that populations living in low-salt estuary environments accumulate more osmotic regulatory gene amplification variants, which allow them to survive in low-salt or even freshwater environments, whereas Pacific oysters lack these amplifications and are limited in growth under low-salt. (3) The relationship between structural variation and phenotype can be used to guide breeding and protection: The genetic mechanism of high-salt adaptation tells us that certain beneficial structural variations can be used as molecular markers to assist in the breeding of high-salt-resistant oyster varieties. In terms of environmental protection,
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