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No, they die. The liquids that circulate in the body of saltwater fish have approximately the same proportion of salts as seawater. “But if the fish is placed in fresh water, this concentration will be higher in its body than in the environment”, says biologist Naércio Aquino Menezes, from USP’s Zoology Museum. As bodies tend to always be in balance, the fish will end up absorbing more water by osmosis and will not be able to eliminate it, due to its poorly developed kidney. It will swell and may even explode. The opposite happens with a freshwater fish placed in the sea. The concentration of salts in his organism will be much lower than that of the environment and he will lose fluid until he becomes dehydrated. But there are species that endure this change of environment for a longer time – such as the mullet, which lives in lakes where the salinity varies greatly.