When rivers dry up, lungfish burrow into mud and enter suspended animation for years, breathing air and metabolizing their own muscle tissue. They represent the evolutionary link between fish and tetrapods, with both gills and lungs.
Mitch Ames, CC BY-SA 4.0, via Wikimedia Commons
A State of Suspended Animation
When the seasonal floodplains of Zambia dry under the African sun, most fish face certain death. The West African lungfish (Protopterus annectens) has a different strategy. It begins a process of suspended animation, called estivation, that allows it to survive without food or water for three to five years. As the water recedes, the lungfish burrows into the mud. It then secretes immense amounts of mucus, which hardens into a dry, watertight cocoon, leaving only a small opening for its mouth.
Inside this subterranean chamber, the lungfish undergoes a physiological transformation. Its metabolism plummets, and its heart rate can drop from around 25 beats per minute to just two. It relies exclusively on its paired lungs—which are modified swim bladders—to breathe air through the opening in its cocoon. To avoid the toxic buildup of nitrogenous waste, the lungfish converts ammonia into less-toxic urea, which accumulates in its tissues. For energy, it slowly metabolizes its own muscle tissue, a process that can continue for years until the rains return and soften the mud, signaling the time to emerge.
The Fish That Walked
The lungfish is part of the evolutionary history of life on Earth: the transition of vertebrates from water to land. As part of the Sarcopterygii class, or lobe-finned fishes, lungfish are the closest living relatives to tetrapods—the four-limbed vertebrates that include amphibians, reptiles, birds, and mammals. Their fossil record dates back over 410 million years to the Early Devonian period, a time when the first terrestrial ecosystems were forming.
The anatomical features that allow the lungfish to survive drought are the very traits that were foundational for life on land. Their fleshy, limb-like lobed fins contain a well-developed internal skeleton. Studies have shown that African lungfish can use these pelvic fins to lift their bodies and propel themselves along the bottom, a form of underwater "walking." This suggests that the fundamental mechanics of walking evolved in aquatic ancestors long before the first vertebrates set foot on shore. The combination of functional lungs and limb-like fins is a pre-adaptation for a terrestrial existence, showing a significant events in evolutionary history.
💡Fun Facts
The lungfish cocoon is not an inert shell, it's a living tissue packed with immune cells called granulocytes that actively trap bacteria, forming an immunological barrier.
To survive estivation, the lungfish digests its own tail muscle for nutrients, emerging significantly thinner when the rains return.
The genome of the lungfish is one of the largest of any vertebrate.
Upon re-emerging, a lungfish can excrete years' worth of accumulated urea within a few days.
Blue Lagoon National Park, a habitat for lungfish in Zambia's Central Province, is typically open from 06:00 to 18:00 daily.
Admission
Park entry fees apply and vary for citizens, residents, and international visitors.
Accessibility
Access requires a 4x4 vehicle, especially during the wet season (November-April). The terrain consists of unpaved tracks through floodplain and woodland.
