International Journal of Aquaculture, 2024, Vol.14, No.3, 139-153 http://www.aquapublisher.com/index.php/ija 143 For instance, in the marine diatoms, temperature acclimation involves changes in gene expression regulated by epigenetic mechanisms. These changes help diatoms adjust their metabolic processes to cope with different thermal environments (Liang et al., 2019). Epigenetic modifications also contribute to the adaptation of fish to salinity changes, where specific genes involved in ion transport and osmoregulation are differentially methylated in response to salinity stress (Xu et al., 2015). 5 Environmental Factors Influencing Adaptation 5.1 Temperature and climate change Environmental factors play a significant role in shaping the adaptive mechanisms of aquatic species. These factors include temperature and climate change, salinity and water chemistry, and habitat complexity and availability. This section explores how these factors influence adaptation in aquatic organisms. Temperature is a critical environmental factor that influences the physiology, behavior, and distribution of aquatic species. Climate change, characterized by global warming, has significant impacts on aquatic ecosystems, altering the temperature regimes of water bodies and affecting the species that inhabit them. Aquatic species must adapt to these changes to survive and maintain their ecological roles. For instance, a study on marine diatoms (Thalassiosira pseudonana) demonstrated rapid thermal adaptation over 350 generations, showing significant divergence in temperature response traits. This adaptation highlights the ability of microorganisms to evolve rapidly in response to temperature changes, which is crucial for their survival in warming oceans (O’Donnell et al., 2018). Similarly, bivalve mollusks like Unio tumidus exhibit adaptive reactions to temperature changes, modulating their metabolic processes to cope with increased water temperatures (Krasyuk and Khudiyash, 2021). Climate change also interacts with other environmental stressors, compounding their effects on aquatic species. For example, the combination of increased temperatures and pollution can lead to novel stress conditions that require complex adaptive responses from aquatic organisms (Niinemets et al., 2017). 5.2 Salinity and water chemistry Salinity and water chemistry are crucial factors affecting the distribution and adaptation of aquatic species. Changes in salinity levels can result from natural processes or human activities, such as the discharge of industrial effluents and the use of road salts. Aquatic species must adapt to varying salinity levels to maintain osmotic balance and other physiological functions. Research on the Gulf killifish (Fundulus grandis) has shown that populations in polluted habitats have rapidly evolved resistance to high levels of salinity. This adaptation involves genomic changes related to osmoregulation and ion transport, enabling the fish to survive in highly saline environments (Oziolor et al., 2019) (Figure 2). Similarly, studies on freshwater macroinvertebrates, such as the flatworm Dugesia gonocephala, have identified loci associated with adaptation to high copper concentrations from ore mining, indicating a genetic basis for local adaptation to specific water chemistry conditions (Weigand et al., 2018). Epigenetic mechanisms also play a role in salinity adaptation. For instance, changes in DNA methylation patterns have been observed in fish adapting to different salinity levels, influencing the expression of genes involved in ion transport and stress response (Xu et al., 2015). Figure 1 illustrates the variation in sensitivity to pollution among different populations of F. grandis in Galveston Bay, USA. The main content is as follows: (A) Pollution gradient map: Different colors represent pollution levels, ranging from low (blue) to high (black). Populations are classified based on their pollution resistance into four categories: highly resistant (black), intermediate-high resistance (red), intermediate-low resistance (yellow), and sensitive (blue). (B) Cardiac deformities in embryos: This shows the degree of heart deformities in different populations when exposed to PCB126. The results indicate that populations from highly polluted areas have stronger resistance to PCB126, while those from less polluted areas are more sensitive. (C) EC50 and pollution correlation: This shows the positive correlation between the sensitivity of populations to PCB126 (EC50) and the
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