Universality and diversity of osmoregulation applying urea retention in vertebrates

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Body fluid homeostasis, i.e. volume and composition of body fluids, is a dynamic physiological phenomenon in vertebrates. The amphibious fish, Andamia tetradactylus, and the crab-eating frog, Fejerbarya cancribora, inhabit in rocky shores and estuary areas, respectively. In this study, hydromineral hormones and components in their body fluid, and gene expressions of urea synthetic enzymes and urea transporter were examined. During exposure to air and seawater environments, synthesis and accumulation of urea were increased in the fish and frogs. Thus, urea accumulation is important in the osmoregulation in both species. Tadpoles of the crab-eating frog may also be depending on this osmoregulation in salt water adaptation.