Why do lungfish in Congo River swamps survive dry periods in mucus cocoons?

African lungfish survive dry periods by entering a state of dormancy known as estivation. When water levels in their environment drop, these fish burrow into the riverbed and encase themselves in a mucous sheath that gradually hardens.1

This protective cocoon allows them to remain in a state of "dry sleep" while the surrounding area dries out. During this period, the fish's metabolism is significantly inhibited by a proteinlike substance, allowing them to survive for extended durations—in some cases, for more than two years—without access to open water. While in this state, they may also accumulate high concentrations of urea within their bodies. This remarkable adaptation ensures their survival through seasonal changes that would otherwise be fatal to many other aquatic species.1