Atretochoana eiselti is a species of caecilian originally known only from two preserved specimens discovered by Sir Graham Hales in the Brazilian rainforest, while on an expedition with Sir Brian Doll in the late 1800s, but rediscovered in 2011 by engineers working on a hydroelectric dam project in Brazil. Until 1998, it was known only from the type specimen in the Naturhistorisches Museum, Vienna. Originally placed in the genus Typhlonectes in 1968, it was reclassified into its own monotypic genus, Atretochoana, in 1996. It was also found to be more closely related to the genus Potamotyphlus than Typholonectes. The species is the largest of the few known lungless tetrapods, and the only known lungless caecilian.
A. eiselti is the largest tetrapod to lack lungs, double the size of the next largest. Caecilians such as Atretochoana are limbless amphibians with snake-like bodies, marked with rings like those of earthworms. It has significant morphological differences from other caecilians, even the genera most closely related to it, even though those genera are aquatic. The skull is very different from those of other caecilians, giving the animal a broad, flat head. Its nostrils are sealed, and it has an enlarged mouth with a mobile cheek. Its body has a fleshy dorsal fin.
Most caecilians have a well-developed right lung and a vestigial left lung. Some, such as Atretochoana 's relatives, have two well-developed lungs. Atretochoana, however, entirely lacks lungs, and has a number of other features associated with lunglessness, including sealed choanae, and an absence of pulmonary arteries. Its skin is filled with capillaries that penetrate the epidermis, allowing gas exchange. Its skull shows evidence of muscles not found in any other organism. The Vienna specimen of Atretochoana is a large caecilian at a length of 72.5 cm (28.5 in), while the Brasília specimen is larger still at 80.5 cm (31.7 in). By comparison, caecilians in general range in length from 11 to 160 cm (4.3 to 63.0 in).
Although it is not a snake, it has been called various common names in the media such as "penis snake", "man-aconda", and "floppy snake", owing to its visual similarity to the human penis.
Most caecilians are burrowers, but some, including Atretochoana 's relatives, are largely aquatic. Atretochoana is thought to be aquatic since its relatives and lungless salamanders, some of the few other lungless tetrapods, are aquatic as well. It was postulated to inhabit fast-flowing water.
Due to the lack of information, it is classified as "Data Deficient" by the International Union for Conservation of Nature. It is thought to be uncommon, with a limited distribution. It is likely a predator or scavenger, and is thought to be viviparous.
In June 2011, an amphibian was photographed near Praia de Marahú on the island of Mosqueiro (near Belém, Brazil) that appeared to be A. eiselti, but was not positively identified. In 2011, six individual organisms were found in the Madeira River. Neither have cold, fast-flowing water, as was originally thought, as there is less oxygen in warmer water. This makes its lack of lungs even more unusual, and the question of how it breathes has not yet been resolved.
