Taxonomy & naming
Mastacembelus niger was described by the French ichthyologist Henri-Émile Sauvage in 1879. The Catalog of Fishes (Eschmeyer, CAS) records it as a valid species within the genus Mastacembelus, the type genus of the family Mastacembelidae, placed in the order Synbranchiformes. This order also contains the swamp eels (Synbranchidae); the spiny eels and swamp eels are kin to one another and wholly unrelated to the Anguilliformes, the true eels. The sinuous, burrowing body of Mastacembelus niger is convergent with that of marine and catadromous eels, not homologous.
For much of the twentieth century many African Mastacembelidae were assigned to separate genera, including Aethiomastacembelus, which encompassed 19 African spiny-eel species that authors considered morphologically distinct from the Asian Mastacembelus. Vreven (2005) reviewed the subfamily and concluded that no generic-level distinction was justified; Aethiomastacembelus was synonymised with Mastacembelus, and M. niger was transferred accordingly. GBIF and FishBase both follow this arrangement, recording the species as Mastacembelus niger Sauvage, 1879.
Morphology
Mastacembelus niger reaches a maximum recorded standard length of 15 in, making it a modest-sized member of its genus — smaller than the large Asian spiny eels that dominate the aquarium trade. The body is markedly elongate and laterally compressed posteriorly, with the dorsal, caudal and anal fins confluent around the tail tip in the typical spiny-eel fashion. A row of short, isolated dorsal spines runs ahead of the soft dorsal fin.
The skin is scaleless in appearance, though tiny embedded scales are present beneath a heavy mucus coat. The ground colour is dark — brown to near-black, with the epithet niger (Latin: black) capturing this overall tone. The snout is drawn into a short fleshy proboscis tipped with tubular nostrils, used for probing the substrate after prey. Reliable external differences between the sexes have not been reported; ripe females may show greater abdominal fullness, as in related species.
Habitat
Mastacembelus niger is a freshwater demersal species native to lowland rivers of West-Central Africa. Its confirmed range runs from the Mbonge River in Cameroon in the west to the Shiloango River in the DRC, and it occurs within the Congo River basin between the Kinsuka rapids below Pool Malebo and the Wagenia Falls at Kisangani — a stretch of the middle Congo and its tributaries that encompasses diverse lowland river habitats.
Like other West-Central African lowland spiny eels, it inhabits soft-bottomed rivers, streams and marginal pools where it can burrow into sand, mud or leaf litter. It favours warm equatorial fresh water and is most active at night, sheltering by day beneath submerged roots, leaf packs and crevices. It is demersal, spending virtually its entire life at or near the substrate. As with all Mastacembelidae, it lacks scales and is sensitive to deteriorating water conditions.
Feeding
Mastacembelus niger is a nocturnal benthic carnivore. Its fleshy, probing snout is adapted for rooting through soft substrate in low-light conditions, seeking out the small invertebrates — insect larvae, aquatic worms and crustaceans — that make up the bulk of its diet. Small fishes may also be taken opportunistically by larger individuals.
In the aquarium the species should be offered meaty foods after dark: bloodworm, tubifex, earthworm pieces and similar invertebrate foods are well accepted. Feeding at or after lights-out encourages shy or newly imported fish to emerge and eat. Competition with faster, bolder tankmates at feeding time should be avoided, as spiny eels tend to be deliberate and unhurried feeders.
Mating
Mastacembelus niger is a spiny eel and reproduces as substrate egg-scatterers in fresh water — it is not a true eel and does not undergo any marine spawning migration. As in other Mastacembelidae, spawning is believed to be seasonal and triggered by environmental cues associated with the onset of the rainy season, such as rising water levels, lower temperatures from rainfall, and increased dissolved oxygen.
Courting pairs are thought to entwine among vegetation before scattering small adhesive eggs among plant roots, leaf litter or fine-leaved plants. No pair bond is maintained beyond spawning, and neither parent guards or tends the eggs. Field observations of spawning behaviour are limited, owing to the species' nocturnal habits and turbid lowland habitats.
Breeding
Mastacembelus niger has not been established as a regularly bred aquarium fish. Spiny eels in general are rarely spawned in home aquaria, and the Congo-basin species attract less attention from breeders than the larger, more colourful Asian species. The few captive-breeding observations from Mastacembelidae indicate that adhesive eggs are scattered among fine vegetation and hatch within a few days; the larvae are tiny and require very fine first foods.
No parental care is provided — the adults scatter eggs and take no further role. Breeding attempts, where made, typically follow large partial water changes with cooler, softer water to simulate seasonal flooding. Because wild-caught individuals from this range are uncommon in the trade, captive breeding would be ecologically valuable, but the species remains rarely seen, let alone spawned, in the hobby.
In the aquarium
Mastacembelus niger is occasionally available through specialist importers of West-Central African fishes but is not common in the trade. In the aquarium it requires the essentials shared by all spiny eels: a deep bed of fine soft sand it can burrow into, adequate caves and shaded retreats, and a fully sealed, tight-fitting lid. The lid is non-negotiable — spiny eels are accomplished escape artists that can climb glass, squeeze through tiny gaps and breathe at the surface, and an uncovered tank means a dead eel on the floor.
The species is scaleless and sensitive to copper-based medications and to poor water quality; only a mature, stable system suits it. It is nocturnal and may remain invisible during the day, emerging after dark to feed. At 15 in it is manageable in a moderately sized aquarium, and it can be kept with peaceful tankmates that are too large to be swallowed and will not compete for food or fin-nip. Water temperature in the mid-to-upper twenties Celsius suits it, with soft to moderately hard, near-neutral fresh water reflecting its lowland river origins.
Conservation
Mastacembelus niger is assessed as Least Concern on the IUCN Red List (assessment 2022.2), reflecting its occurrence across a sizeable tract of the West-Central African lowland river system — from coastal Cameroon rivers to the middle Congo basin — without evidence of significant range-wide decline.
The Congo basin faces ongoing pressures from artisanal and commercial fishing, forest clearance, agricultural runoff and, in some areas, sand and gravel extraction. Mastacembelus niger is harvested locally for food and enters the aquarium trade in small numbers. Its tolerance of demersal lowland-river conditions across a geographically broad area underpins its current secure status, but as with many African riverine fish, the quality of its habitat data is limited and local declines could go undetected at the assessment scale.