Since life began in the oceans, most life, including freshwater life, has a chemical composition more like the ocean than fresh water. It appears that most freshwater life did not originate in fresh water, but is secondarily adapted, having passed from ocean to land and then back again to fresh water. As improbable as this may seem, the bodily fluids of aquatic animals show a strong similarity to oceans, and indeed, most studies of ion balance in freshwater physiology document the complex regulatory mechanisms by which fish, amphibians and invertebrates attempt to _____________________________________________. It is these sorts of unexpected complexities and apparent contradictions that make ecology so interesting. The idea of a fish in a freshwater lake struggling to accumulate salts inside its body to mimic the ocean reminds one of the other great contradiction of the biosphere: plants are bathed in an atmosphere composed of roughly three-quarters nitrogen, yet their growth is frequently restricted by lack of nitrogen.
* amphibian: 양서류 ** invertebrate: 무척추동물