These authors consider the Synbranchiformes to be part of a monophyletic group called Smegmamorpha, also containing Mugilimorpha, Atherinomorpha, Gasterosteiformes, and Elassomatidae.

[3] The gills are poorly developed, and their openings are usually single, small, confluent across the breast, and restricted to the lower half of the body.

The three families each have a somewhat different distribution: The Synbranchidae are found in Mexico, Central and South America, West Africa (Liberia), Asia, Hawaii, and the Indo-Australian Archipelago.

[3] The Chaudhuriidae are found in northeastern India through Thailand to Korea (including parts of Malaysia and Borneo).

When found in pools, they typically are associated with leaf litter and mats of fine tree roots along the banks.

Swamp eels are capable of overland excursions, and some can live out of water for extended periods of time.

Four species are found exclusively in caves: Rakthamichthys eapeni and R. roseni from India, Ophisternon candidum from Australia, and O. infernale from Mexico.

The fish have numerous sharp dorsal spines and belly thorns which, together with its sliminess make them extremely difficult to handle.