South American Lungfish

The South American Lungfish (Lepidosiren paradoxa) is the sole living member of the family Lepidosirenidae and one of only six lungfish species alive today. It occurs in slow-moving swamps, floodplains, and marshes across the Amazon and Paraguay River basins of tropical South America. Unlike most fish, it depends heavily on a single lung for respiration and must surface regularly to breathe air, even drowning if kept fully submerged without access to the surface. During seasonal droughts it burrows into mud and enters a dormant state called aestivation, encased in a mucus cocoon until water returns, an adaptation shared with its African relatives.

The South American Lungfish has a highly distinctive, eel-like form:

• Long, cylindrical, snake-like body tapering evenly toward the tail
• Dark grey to nearly black coloring, sometimes with faint mottling
• Tiny, deeply embedded scales giving a smooth appearance
• Thin, thread-like paired fins lacking the fan shape typical of most fish
• Small eyes and a blunt, rounded head

Its unmistakable air-breathing behavior, repeatedly rising to gulp at the surface, combined with its whip-like fins, separates it from any true eel or eel-like fish sharing its habitat.

This lungfish inhabits still or slow-flowing freshwater swamps, marshes, and seasonally flooded areas throughout the Amazon and Paraguay-Paraná river basins of tropical South America. It favors muddy-bottomed, densely vegetated, oxygen-poor waters where its air-breathing lung provides a major advantage over gill-only fish. During the dry season, when its habitat can shrink or dry out completely, it burrows into mud and forms a protective mucus cocoon, remaining dormant for months until rains return and refill its wetland home. This tolerance for extreme seasonal habitat change is central to its ecology.

South American Lungfish must surface periodically to gulp air through their single lung, as their gills alone cannot meet their oxygen needs, especially in stagnant swamp water. They are generally slow-moving, bottom-associated fish, using their reduced, thread-like fins mainly for balance rather than active swimming. During the dry season they aestivate, burrowing into mud and slowing their metabolism dramatically until conditions improve. Uniquely among lungfish, males of this species develop temporary vascularized filaments on their pelvic fins during the breeding season, thought to supply extra oxygen to eggs guarded in burrow nests.