Synbranchus is a genus of swamp eels (family Synbranchidae) widespread across Central and South America - the atlas records it from Brazil, Argentina, Belize, Bolivia, Colombia, Costa Rica, Ecuador, and French Guiana. It has the classic swamp-eel body plan: a scaleless, eel-like form with no paired fins and reduced dorsal and anal fins, and it runs notably large for the family, with atlas species spanning 15.5–59 in.
Like other synbranchids, Synbranchus is a facultative air-breather, gulping air at the surface and exchanging gas across a modified, well-vascularized buccal cavity, which lets it survive in the oxygen-poor water of swamps, flooded grassland, and seasonal pools, and even burrow into damp mud to wait out a dry season. The marbled swamp eel (S. marmoratus) is the most widespread and best-documented species, a nocturnal predator of small fish, crustaceans, and invertebrates found across much of lowland South America; it is eaten as food in parts of its range and only occasionally kept by specialist fishkeepers drawn to its unusual, near-limbless anatomy rather than by the mainstream hobby. Conservation looks reassuring where assessed, matching the brief's tally of 2 Least Concern and 1 Not Evaluated.