International Journal of Aquaculture, 2025, Vol.15, No.5, 255-265 http://www.aquapublisher.com/index.php/ija 261 bony fish live in freshwater environments, indicating that many lineages have successfully completed the ocean-to-freshwater leap. Taking bone fish as an example, in the late Paleozoic and early Mesozoic period, some primitive bone fish such as the ancient cod and the radial fin fish began to enter the estuary and rivers to adapt to the salinity environment with a large change. By the Cretaceous and Cenozoic, different groups independently invaded freshwater many times, resulting in a rich freshwater fish phase. The transition of fish from ocean to freshwater environment requires overcoming a range of physiological challenges. The ion concentration in freshwater is much lower than that in seawater, and fish must evolve effective gill and kidney regulation mechanisms to prevent excessive salt loss in the body and avoid excessive water entry (Corush, 2019). Ocean-freshwater transitions are often accompanied by species radiation, as geographical barriers in different water systems can lead to rapid species formation. A large lake cichlids in Africa and a river catfish in South America are typical examples: after the ancestors entered the inland from the ocean, they faced the new environment and new ecological niche, and experienced amazing adaptive radiation, resulting in a large number of endemic species. The freshwater environment is relatively closed and the isolation between each basin is high, which makes the fish lineages in different water bodies evolve and diversify independently. At the same time, freshwater environments are more susceptible to geological and climate change (such as drying lakes and diverting rivers), which will drive the isolation and differentiation of local populations and ultimately form new species (Brito et al., 2022). 6.2 Diversification of body shape, breeding methods and foraging strategies Adaptive radiation of fish is reflected in multiple morphological and life history characteristics, among which the evolution of body shape, innovation in reproductive methods and differentiation of foraging strategies are the three most significant aspects. Diversified body size: The body length of fish spans six orders of magnitude, from damselfly less than 1 cm to whale sharks over 10 meters, with a weight difference of more than one million times. In early vertebrate evolution, small fish survived and gained dominance after extinction events in the late Devonian period, a phenomenon known as the "Lilliput effect". Although most fish are oviparous by in vitro fertilization, many special reproductive patterns have evolved, including oviparous and viviparous. There are more than 50 types of ovoviparous species recorded in bony fish. Ovoviparousy refers to the hatching of fertilized eggs in the mother's body, but the embryos mainly rely on yolk to provide nutrition. This is a transitional model between oviparousy and viviparousy. Evolution of ovoviparalysis can increase the survival rate of offspring, especially when predation pressure is high or the environment is unstable, protecting embryos in the mother is an effective strategy. Foraging strategy differentiation: The food properties of fish cover almost all possible types, including filter feeding of plankton, feeding of bottom mud microorganisms, herbivorous, insectivorous, etc. The study found that the fish morphology is highly matched with its food properties. For example, carnivorous fish usually have large forward-protruding mouths, sharp teeth, and short digestive tracts; while filter-feeding fish have wide mouths, fine gill rakes, and long digestive tracts (Corn et al., 2022). Even within the same lake or coral reef, different fishes avoid competition through “trait differentiation” due to food resources separation (McCord et al., 2021). 6.3 Case analysis: rapid radiation of Cichlids in great lakes in Africa The Great Rift Valley Lake in eastern Africa has nurtured one of the fastest and most eye-catching adaptive radiations in the history of biological evolution, the explosive diversity of cichlid fish. Take Lake Malawi as an example. The lake covers an area of about 30 000 square kilometers and was formed about 2 million years ago. However, in such a "short" geological time, about 700 species of cichlids have appeared in Lake Malawi. They evolved from a common ancestor, but differentiated into completely different ecological types: some feed on algae and evolved into algae-scraped lips and teeth; some prey on small fish, large in size and high-speed swimming ability; some specialize in molluscs, whose teeth are blunt and thick, can crush screw shells. Such extensive niche differentiation is largely attributed to the unique jaw structure of the cichlid. Cichlids have a highly malleable pharyngeal jaw that can adapt quickly to different foods. This "dual jaw system" allows cichlids to evolve multiple feeding functions in a short period of time, providing a morphological basis for radiation (Svardal et al., 2019).
RkJQdWJsaXNoZXIy MjQ4ODYzNA==