International Journal of Aquaculture, 2024, Vol.14, No.3, 139-153 http://www.aquapublisher.com/index.php/ija 142 similar genetic changes occurring independently in different lineages to adapt to aquatic environments (Foote et al., 2015). The genomic adaptations in aquatic insects also highlight the role of genetic evolution in long-term adaptation. For example, fireflies that transitioned to freshwater habitats have undergone significant genomic changes that enhance their metabolic efficiency and morphological adaptations (Zhang et al., 2020). Moreover, the adaptation of aquatic plants through genetic evolution has been crucial for their survival and proliferation in diverse aquatic habitats. The genomic analysis of aquatic angiosperms like those in the Alismatales order reveals how whole-genome duplications and gene losses have facilitated their adaptation to both freshwater and marine environments (Chen et al., 2022). 4 Molecular Mechanisms of Adaptation 4.1 Gene regulation and expression Adaptation at the molecular level involves various mechanisms that enable organisms to survive and thrive in changing environments. These mechanisms include gene regulation and expression, genomic changes and mutations, and epigenetic modifications. This section explores each of these mechanisms and their roles in the adaptation of aquatic species. Gene regulation and expression play a crucial role in how organisms respond to environmental changes. Regulatory mechanisms control the timing, location, and amount of gene expression, allowing organisms to adapt to different environmental conditions. For instance, in the freshwater firefly species Aquatica leii, differential gene expression patterns have been observed between aquatic larvae and terrestrial adults. These patterns are primarily associated with metabolic efficiency, energy production, and hypoxia response, which are essential for adapting to freshwater environments (Zhang et al., 2020). The transcriptome sequencing of Ranunculus bungei, an aquatic plant, and its terrestrial relatives revealed that genes involved in water transport and microtubule cytoskeleton organization are differentially expressed in response to aquatic habitats. These genes help the plant adjust its physiology to cope with submerged conditions (Chen et al., 2015). Similarly, the molecular adaptation mechanisms in cetaceans include the regulation of genes involved in body shape changes, osmotic regulation, immune defense, dietary changes, sensory systems, and hypoxic tolerance, reflecting the complex genetic adjustments required for a secondary aquatic life (Yang et al., 2019). 4.2 Genomic changes and mutations Genomic changes and mutations are fundamental to the evolutionary adaptation of species. Mutations introduce genetic variation, which natural selection can act upon. In the case of aquatic adaptations, specific genes often undergo positive selection, leading to advantageous traits that enhance survival in aquatic environments. For example, comparative genomic studies on marine mammals have identified genes related to thermoregulation, such as those involved in the formation of blubber and vascular development, which have undergone unique changes to adapt to marine life (Yuan et al., 2021). The molecular basis of freshwater adaptation in fireflies also includes fast-evolving genes and positively selected genes that contribute to metabolic efficiency and morphological adaptations. These genetic changes are essential for the fireflies to cope with the challenges of freshwater habitats (Zhang et al., 2020). In another example, the freshwater adaptation in prawns of the genus Macrobrachium involves differential gene expression and positive selection in genes related to osmoregulation, hemolymph regulation, and stress response, facilitating their survival in varying salinity conditions (Rahi et al., 2019). 4.3 Epigenetic modifications Epigenetic modifications, such as DNA methylation and histone modifications, play a significant role in regulating gene expression without altering the underlying DNA sequence. These modifications can be influenced by environmental factors and can provide a rapid and reversible means of adaptation. In aquatic species, epigenetic changes can help organisms respond to environmental stresses, such as changes in salinity, temperature, and oxygen availability.
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