Introduction to South American Lungfish: Lungfish appeared in the Devonian period hundreds of millions of years ago and is an extremely primitive freshwater fish. After the female fish lays eggs in the nest, the male fish will guard the nest intently and bravely protect their young until the young fish hatch. It has the special habit of hibernating. The American lungfish uses its lungs to swallow air. They still have gills when they are young, but they disappear when they mature.