A rediscovery in the Amazon
Atretochoana eiselti is not a snake, but a caecilian—a mysterious order of limbless amphibians more closely related to frogs and salamanders. For over a century, its existence was known only from two preserved female specimens. The first, the holotype, was collected in South America sometime in the late 1800s and held at the Naturhistorisches Museum in Vienna. It was formally described in 1968 by Edward H. Taylor. A second specimen was identified at the University of Brasília in 1998, though its origin was also unknown. The species remained a scientific enigma, with no known wild population.
This changed in 2011. While draining a section of the Madeira River for the construction of the Santo Antônio Dam, engineers discovered six living Atretochoana. This confirmed the species was not extinct and provided the first information about its natural habitat. Individuals have since been found near the mouth of the Amazon, over 2,500 kilometers away, suggesting a wider, if sparsely populated, distribution.
It has a broad, flat head, It has a broad, flat head, sealed nostrils, and a fleshy dorsal fin. The largest specimen on record measured 80.5 centimeters (31.7 inches) long, making it the largest known caecilian by a significant margin. Its skin is ringed, similar to an earthworm, and typically a blue-grey color.
Life without lungs
Atretochoana eiselti is the largest tetrapod (the group of vertebrates that includes amphibians, reptiles, birds, and mammals) known to science that completely lacks lungs. While some salamanders are also lungless, Atretochoana is more than double the size of the next largest lungless tetrapod. It has no pulmonary arteries, and its choanae, or internal nostrils, are sealed.
The animal survives through cutaneous respiration, a process where gas exchange happens entirely through the skin. Its skin is densely packed with capillaries that penetrate the epidermis, allowing dissolved oxygen from the water to diffuse directly into its blood. This biological strategy requires a large surface area and is common in amphibians, but Atretochoana is an extreme example.
Scientists initially believed it lived in cold and fast-flowing waters to survive. Its discovery in the warmer, slower-moving Madeira River challenged this assumption and deepened the mystery of how it meets its metabolic needs. Its diet is not definitively known but is presumed to consist of small fish, worms, and other aquatic invertebrates, which it likely detects using a powerful sense of smell to compensate for its poor eyesight.