International Journal of Aquaculture, 2024, Vol.14, No.4, 184-194 http://www.aquapublisher.com/index.php/ija 189 aquatic species and promotes genetic diversity. Asexual reproduction, on the other hand, allows for rapid population growth and is observed in some invertebrates and plants. 5.3.2 Viviparity and oviparity Viviparity (live birth) and oviparity (egg laying) are two reproductive strategies that have evolved in aquatic organisms. Marine mammals, such as cetaceans, exhibit viviparity, giving birth to live young that are well-developed and capable of swimming shortly after birth (Yang et al., 2019). In contrast, many fish and amphibians are oviparous, laying eggs that develop and hatch outside the mother's body. 5.3.3 Parental care Parental care is a significant adaptation that enhances the survival of offspring in aquatic environments. This behavior is observed in various species, including fish, amphibians, and some invertebrates. Parental care strategies range from guarding eggs to providing food and protection for the young (Harrison, 2015). These behaviors increase the chances of offspring survival and contribute to the success of the species. 6 Case Studies in Aquatic Diversification 6.1 Diversification of fish in coral reefs Coral reefs are hotspots of biodiversity, particularly for fish species. The evolutionary history of reef fishes and corals shows a marked congruence, with both groups exhibiting rapid lineage diversification during the Oligocene and Miocene epochs (34-5.3 million years ago) (Bellwood et al., 2017). This period saw the establishment of modern coral reef ecosystems, characterized by high turnover and fast growth, which facilitated the colonization of new habitats and the appearance of new taxa. Despite the rapid increase in biodiversity, functional changes in fishes and corals over the last 5.3 million years have been limited, suggesting that recent diversification may be driven more by ecological opportunities, such as color variation in fish, rather than by significant functional innovations. Functional traits, such as diet, body size, and water column use, have played a fundamental role in the evolution and diversification of reef fish lineages. Independent transitions to planktivory across different reef fish families highlight how ecological opportunities for exploiting different resources can drive speciation and adaptation (Floeter et al., 2108). The generalist feeding strategy has been pivotal during the evolutionary history of reef fishes, acting as a reservoir for future diversity (Gajdzik et al., 2019). 6.2 Evolution of marine mammals Marine mammals have undergone significant evolutionary changes, often driven by climatic factors. For instance, global cooling has been identified as a driver of diversification in marine clades, such as the Anomura crustaceans. Speciation rates in these clades are correlated with cooler global temperatures, suggesting that climate change has historically played a crucial role in shaping marine biodiversity (Davis et al., 2016). This pattern contrasts with freshwater clades, where speciation rates are positively correlated with global warming, indicating that different environmental pressures can lead to diversification in marine and freshwater habitats. 6.3 Speciation in freshwater environments Transitions between marine and freshwater environments have contributed to diversification. Diatoms, for example, have repeatedly colonized and diversified in freshwater habitats. These transitions often trigger rapid morphological or physiological changes and can lead to increased rates of speciation and extinction over longer timescales (Figure 2) (Roberts et al., 2023). The ability of some taxa to adapt to low salinity environments through specific genetic changes further underscores the role of habitat transitions in driving diversification. These transformations are crucial factors driving the diversification of diatom species. Particularly in freshwater environments, diatoms have undergone significant diversification, a process that may be driven by multiple genes and evolutionary mechanisms. Research also indicates discrepancies between gene trees and species trees, which may be due to inconsistencies in gene ordering or estimation errors in the gene trees. These findings suggest that diatoms are capable of adapting to different environments through complex evolutionary mechanisms, thereby
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