Mastacembelus is the largest genus of spiny eels, elongate, tube-bodied fish in the family Mastacembelidae, not true eels at all despite the shape. The atlas covers 67 species ranging from 3 in up to about a metre long, and the range splits into two separate strongholds: a diverse African cluster through the Congo basin and the Rift Valley region (Democratic Republic of the Congo, Zambia, Burundi, Tanzania, Cameroon) and a distinct South/Southeast Asian cluster (Myanmar, Thailand). What unites the genus is the body plan — a long tapering trunk with no pelvic fins, a low fringe of dorsal, caudal and anal fin joined into one continuous margin, a pointed and mobile snout for probing substrate, and a row of short free spines along the back that gives the genus its name. Most species are nocturnal burrowers that spend daylight wedged into sand, mud or rockwork.
Spiny eels are long-established aquarium fish, and several Mastacembelus carry well-known trade names: the fire eel (M. erythrotaenia), tire-track eel (M. favus), zig-zag eel (M. armatus), and the Mesopotamian spiny eel (M. mastacembelus), one of the few members of the genus living outside the tropics. Keepers prize their unusual, snake-like movement, but they need a soft sandy substrate to burrow into, a tightly sealed lid — they are notorious escape artists — and tankmates too large to be swallowed, since the bigger species will eat anything that fits in their mouth. The genus reads as low conservation concern on current evidence: the atlas tally is mostly Least Concern (36) or Not Evaluated (27), with only a couple of Data Deficient and Vulnerable species, though that likely reflects patchy survey coverage of African and Southeast Asian streams as much as genuine security.