Estivation

African and South American lungfish (Protopterus and Lepidosiren) burrow into mud as water recedes, secrete a mucus cocoon that dries to a leathery capsule, and enter a metabolic depression lasting up to three years. Their urea nitrogen (normally excreted) accumulates in body fluids during estivation. Breathing is sustained by the pair of simple lungs rather than gills. Upon re-flooding, the cocoon dissolves within minutes. Australian lungfish (Neoceratodus) is amphibious but does not fully estivate. The ability to survive complete desiccation makes lungfish extraordinary physiological models for understanding tetrapod invasion of land.