International Journal of Aquaculture, 2024, Vol.14, No.4, 184-194 http://www.aquapublisher.com/index.php/ija 188 5 Molecular Evolution of Key Aquatic Adaptations 5.1 Evolution of respiratory systems 5.1.1 Gills and lung evolution The evolution of respiratory systems in aquatic life forms has been a critical adaptation for survival in diverse environments. In vertebrates, the transition from terrestrial to aquatic life necessitated significant modifications in respiratory structures. For instance, cetaceans exhibit a series of evolutionary changes in morphology and physiology, including the development of specialized lungs and the reduction of olfactory structures, which are less useful underwater (Yang et al., 2019). Similarly, the blue-spotted mudskipper, an amphibious fish, has evolved hemoglobin with high oxygen affinity, allowing it to switch between aquatic and aerial respiration effectively (Xu et al., 2018; Storz et al., 2019). In invertebrates, such as the Onchidiidae family, adaptations include the development of gills for underwater respiration and lung sacs for breathing in wetland environments. 5.1.2 Adaptations to hypoxia Aquatic organisms have developed various strategies to cope with hypoxic conditions. Amazonian fishes, particularly the genus Astronotus, exhibit remarkable hypoxia tolerance through metabolic adjustments such as increased glycolytic metabolism and anaerobic respiration (Braz-Mota and Almeida-Val, 2021). These adaptations enable them to survive in environments with fluctuating oxygen levels. Similarly, the blue-spotted mudskipper has evolved hemoglobin with properties that support efficient oxygen transport under hypoxic conditions, facilitating its facultative air-breathing capability (Storz et al., 2019). 5.1.3 Cutaneous respiration Cutaneous respiration, or breathing through the skin, is another adaptation observed in some aquatic and semi-aquatic species. Anolis lizards, for example, have developed a unique underwater rebreathing mechanism that allows them to respire using air bubbles trapped against their skin, enhancing their ability to remain submerged for extended periods (Boccia et al., 2021). This adaptation is facilitated by their hydrophobic skin, which supports the formation of a thin air layer, or plastron, aiding in gas exchange. 5.2 Development of sensory systems 5.2.1 Evolution of vision in water The evolution of vision in aquatic environments has involved significant changes in sensory systems. Marine mammals, for instance, have undergone convergent evolution in sensory genes, resulting in adaptations that enhance their ability to navigate and hunt in underwater environments 24. These adaptations include modifications in genes associated with vision, allowing for improved light detection and image processing in the aquatic medium. 5.2.2 Lateral line system The lateral line system is a mechanosensory system that allows aquatic animals to detect water movements and vibrations. This system is particularly well-developed in fish and some amphibians, providing them with crucial information about their surroundings, such as the presence of predators or prey. The evolution of this system has been a key adaptation for survival in aquatic habitats. 5.2.3 Electroreception Electroreception is the ability to detect electric fields generated by other organisms. This sensory adaptation is found in various aquatic species, including some fish and amphibians. It allows them to locate prey, navigate, and communicate in environments where visibility is limited. The genetic basis of electroreception involves specific ion channels and receptors that have evolved to detect electric signals in the water (Houssaye and Fish, 2016). 5.3 Diversification of reproductive strategies 5.3.1 Sexual vs. asexual reproduction Aquatic life forms exhibit a wide range of reproductive strategies, including both sexual and asexual reproduction. Sexual reproduction, which involves the combination of genetic material from two parents, is common in many
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