International Journal of Aquaculture, 2024, Vol.14, No.3, 139-153 http://www.aquapublisher.com/index.php/ija 145 section presents detailed case studies on adaptation in marine and freshwater species, followed by a comparative analysis. Marine species exhibit a range of adaptive strategies to survive in their dynamic and often harsh environments. One notable example is the adaptation of marine diatoms, such as Thalassiosira pseudonana, to varying temperatures. Research has shown that these diatoms can rapidly evolve thermal tolerance, demonstrating significant divergence in temperature response traits over 350 generations. This rapid thermal adaptation involves trade-offs that affect their physiological and ecological performance, highlighting the ability of microorganisms to adjust swiftly to climate change (O'Donnell et al., 2018). Another significant case study is the adaptation of marine mammals, such as cetaceans. Genomic analyses have revealed that cetaceans have undergone extensive genetic changes to adapt to their aquatic lifestyle. These adaptations include modifications in genes related to hypoxia tolerance, lipid metabolism, and sensory functions. The transition from land to water in these species involved positive selection on numerous genes, allowing them to thrive in the marine environment (Tsagkogeorga et al., 2015). Marine species also adapt to changing salinity levels. For example, the Gulf killifish (Fundulus grandis) has developed resistance to extreme salinity through genomic changes that enhance osmoregulation. This adaptation is crucial for survival in highly saline environments, demonstrating the importance of genetic evolution in coping with environmental stressors (Oziolor et al., 2019). 6.2 Adaptation in freshwater species Freshwater species face different adaptive challenges compared to their marine counterparts. The Tibetan Schizothoracinae fish provide a compelling example of adaptation to high-altitude environments. These fish have undergone genomic changes that facilitate survival in low oxygen and cold water conditions. The genetic basis of their adaptation includes genes involved in energy metabolism, transport, and immune response, reflecting the complex evolutionary processes that enable them to thrive in harsh aquatic environments (Tong et al., 2017). Another example is the freshwater prawn genus Macrobrachium. Comparative transcriptomic studies on different species within this genus have identified genes associated with osmoregulation and stress response, which are crucial for adapting to varying salinity levels. These adaptations allow the prawns to maintain homeostasis and survive in diverse freshwater habitats (Rahi et al., 2019) (Figure 2). The adaptation of fireflies to freshwater environments also provides insights into molecular mechanisms of adaptation. In the species Aquatica leii, significant genetic changes have been identified that enhance metabolic efficiency and morphological adaptations, enabling these fireflies to thrive in freshwater conditions (Zhang et al., 2020). Figure 2 presents heatmaps showing the differential expression patterns of transcripts at 0‰ and 15‰ salinities for three different Macrobrachium species. (a) M. australiense: This heatmap shows the expression patterns of 876 transcripts under two salinity conditions. In the heatmap, red indicates upregulated expression, green indicates downregulated expression, and black indicates no significant change. Numbers 1, 2, and 3 represent three biological replicate samples. (b) M. tolmerum: This heatmap displays the expression changes of 861 transcripts at 0‰ and 15‰ salinities. Similar to (a), red and green represent upregulated and downregulated expression, respectively, while black indicates no significant change. The heatmap also includes three biological replicate samples. (c) M. novaehollandiae: This heatmap shows the expression patterns of 925 transcripts under different salinity conditions. Red indicates upregulation, green indicates downregulation, and black indicates no change, with three biological replicate samples labeled as 1, 2, and 3. By comparing the differential expression patterns of these three Macrobrachium species under varying salinity conditions, significant differences in transcript expression can be observed. Each species exhibits specific expression responses to different salinities, reflecting their varying adaptations to environmental salinity changes. These heatmaps provide crucial information for researchers to understand the gene expression regulatory mechanisms of different Macrobrachiumspecies under
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