The size and pattern varies greatly depending on the exact species of Mastacembelus. The smallest are M. latens and M. simba, which only reach a maximum total length of 7–8 cm (2. 8–3. 1 in). At up to 1 m (3. 3 ft), the largest of both the family and this genus is M. erythrotaenia. M. erythrotaenia, often known as the fire eel, is blackish with an orange-red pattern, and it is a popular aquarium fish. Otherwise species in this genus are typically brownish and often have a spotted, speckled or mottled pattern, either in another brown hue, grayish or yellowish. This pattern is reflected in the common name of another species sometimes kept in aquariums, the zig-zag eel M. armatus (alternatively called the tire track eel, a name otherwise used for M. favus). A few others also occasionally appear in the aquarium trade, and some are considered good food fish and eaten locally. M aviceps, M. brichardi, M. crassus and M. latens are found in dark, deep parts of the Congo River and sometimes shallower among rocks. These four species have reduced eyes and are all pinkish-white in color (non-pigmented), similar to cavefish.
Mastacembelus is a genus of many species of spiny eel fish from the family Mastacembelidae . They are native to Africa (c. 45 species) and Asia (c. 15 species).  Most are found in rivers and associated systems (even in rapids  ), but there are also species in other freshwater habitats and a particularly rich radiation is found in the Lake Tanganyika basin with 15 species (14 endemic ).   A few species can even occur in brackish water .