Ophisternon is a genus of swamp eels in family Synbranchidae - eel-shaped fish that are not true eels (order Anguilliformes) but belong to the separate order Synbranchiformes, lacking paired pectoral and pelvic fins and breathing air via a modified, highly vascularized pharyngeal chamber. The genus has an unusually broad, scattered distribution for a freshwater fish group, and the atlas's countries reflect that: Australia, Mexico, Papua New Guinea, Belize, Costa Rica, Cuba, Guatemala, and Guinea-Bissau, spanning the Americas, Africa, and Oceania. Atlas species run 12–39.5 in, long and cylindrical, built to burrow into mud and survive in swamps, floodplains, and groundwater habitats that dry out or lose oxygen seasonally.
Swamp eels are more a subject for specialist and biospeleology interest than the mainstream aquarium trade, and Ophisternon carries some of its most striking members on that front: O. candidum (blind cave eel) and O. infernale (blind cave swamp eel) are troglobitic, living in groundwater and cave systems, unpigmented and eyeless as adaptations to permanent darkness. That cave-dwelling life history shows directly in conservation status - three of the six atlas species are assessed Endangered, a heavy toll reflecting how vulnerable isolated cave and groundwater aquifers are to pollution and water-table changes, against three Least Concern species from more open swamp and floodplain habitats.