Taxonomy & naming
Mastacembelus loennbergii was described by George Albert Boulenger in 1898. The species is recorded in Eschmeyer's Catalog of Fishes (California Academy of Sciences) — the authority for valid names — under the genus Mastacembelus, the type genus of the spiny-eel family Mastacembelidae within the order Synbranchiformes. It is not a true eel and shares no close relationship with the catadromous eels of the order Anguilliformes; its eel-like body is a case of convergent evolution on a burrowing, elongate way of life.
African members of Mastacembelus were long distributed across several segregate genera, including Aethiomastacembelus and Caecomastacembelus. These were synonymised with Mastacembelus around 2004–2005 following comprehensive phylogenetic review, and M. loennbergii — previously sometimes referred to as Aethiomastacembelus loennbergii — now sits unambiguously in the resurrected broad Mastacembelus. GBIF records the species under Mastacembelus loennbergii Boulenger, 1898, confirming the synonymisation. The species epithet honours the Swedish zoologist Einar Lönnberg (1865–1942), a prolific describer of African vertebrates.
Morphology
Mastacembelus loennbergii is a small to medium-sized spiny eel, reaching a reported maximum of around 10.5 in total length. The body is greatly elongate, laterally compressed toward the caudal region, and superficially scaleless, covered instead in tiny embedded scales overlaid by a mucous coat. Like all spiny eels the snout is produced into a fleshy, mobile proboscis tipped with tubular nostrils, an adaptation for probing soft substrates in search of invertebrate prey. A series of short, isolated dorsal spines runs ahead of the continuous soft dorsal fin, which merges with the caudal and anal fins around the tapering tail — the "spiny" feature that distinguishes the Mastacembelidae from true eels at a glance.
FishBase records 101 vertebrae for this species, consistent with the elongate vertebral column typical of the family. Detailed colour descriptions from the original type material and subsequent museum specimens are sparse in the accessible literature, but West African Mastacembelus are generally brown to olive-grey with variable banding or mottling that provides camouflage against sandy and gravelly riverbeds. Sexual dimorphism is not well documented; ripe females of other spiny eels become noticeably fuller-bodied than males, and this is likely the case here as well.
Habitat
The species is native to the freshwater river systems of West and Central Africa, recorded from the Chad, Benue and Niger River basins. These are large drainage systems spanning a broad swathe of sub-Saharan Africa, from the Lake Chad catchment south through Nigeria and Cameroon into the Niger delta. Within them, spiny eels typically inhabit the slower reaches of rivers and streams, foraging over sandy or muddy substrates into which they burrow during daylight hours.
FishBase records water parameters for the species of pH 7.4–8.4, hardness 7–30 dH, and temperature 73–81 °F — placing it in moderately alkaline, moderately hard freshwater. This is consistent with the chemistry of many West African river systems downstream of calcareous geology, and somewhat harder and more alkaline than the soft, acidic conditions often assumed for tropical African fishes. The Lake Tanganyika Mastacembelid Diversity Study (PubMed Central, 2010) contextualises the diversity of African mastacembelids and the varied water chemistries they occupy across the continent; M. loennbergii's West African river habitat is distinct from the rift-lake alkaline hard water that the Tanganyika endemic spiny eels occupy.
Feeding
Mastacembelus loennbergii is a demersal, nocturnal carnivore in the manner of all spiny eels. The fleshy, probing snout is the primary sensory organ for locating prey buried in or moving across the substrate — insect larvae, worms and small crustaceans make up the core diet in the wild. It is a benthic ambush and search predator rather than an open-water hunter, and the elongate body allows it to probe into narrow crevices and plant roots where invertebrates shelter.
In the aquarium, meaty benthic foods — earthworms, bloodworm, blackworm, and defrosted invertebrate mixes — presented after lights-out are appropriate. Like other spiny eels it may be slow to begin feeding in a new tank, particularly under bright light or when competing with faster mid-water species. Once settled it often accepts sinking carnivore pellets alongside live and frozen foods.
Mating
Mastacembelus loennbergii is a spiny eel and, as such, is an egg-scattering substrate spawner in fresh water — not a catadromous migrant like the true eels, and not a bubble-nest builder like the swamp eels. Spawning in the Mastacembelidae is generally believed to be triggered by seasonal cues associated with the onset of rains and rising water levels across the river basin, when adults move onto seasonally flooded margins rich in submerged vegetation and leaf litter.
Specific field observations for M. loennbergii are not documented in the accessible literature. By analogy with other African and Asian Mastacembelus, a courting pair is likely to entwine and scatter adhesive eggs among fine plants, roots or debris, with neither individual guarding the spawn or tending the eggs. There is no nest construction and no pair bond beyond the spawning event itself.
Breeding
Mastacembelus loennbergii has not been reported as captively bred in the hobbyist literature, and detailed aquarium spawning accounts are absent from the accessible sources. This is not unusual: the majority of African spiny eels are wild-caught imports for which home-aquarium breeding remains rare or undocumented, owing to their modest profile in the trade relative to the spectacular Asian fire eel (M. erythrotaenia) and tire-track eel (M. favus).
Where captive breeding of related African and Asian Mastacembelus has been achieved, it has typically followed a conditioning period of high-quality live and frozen foods and a simulated wet-season trigger — large water changes with slightly cooler water, raising temperature gradually afterward, and providing dense fine-leaved plants or spawning mops as egg deposition sites. The adhesive eggs hatch within a few days; the larvae are very small and require infusoria-grade first foods before graduating to newly hatched brine shrimp nauplii. No parental care of eggs or fry is provided.
In the aquarium
Mastacembelus loennbergii is an infrequently encountered West African spiny eel in the aquarium trade, but its husbandry requirements follow the same template as the family: a tight-fitting, fully sealed lid is the first and most critical requirement, as spiny eels are accomplished escape artists that will exploit the smallest gap, breathe briefly at the surface, and perish on the floor. A soft sand substrate deep enough to permit burrowing, caves and sheltered retreats, and dim or well-shaded conditions suit the species' nocturnal, secretive habits.
At a maximum of around 10 in it is among the smaller Mastacembelus, and a well-filtered tank of around 35.5–47 in length can comfortably house an adult provided the water chemistry is maintained in the appropriate moderately alkaline, moderately hard range (pH 7.4–8.4, dH 7–30, temperature 73–81 °F). Like all spiny eels it is scaleless and sensitive to copper-based medications and to dissolved waste, so a mature, stable system and copper-free treatments are essential. It can be kept with peaceful to semi-aggressive African cichlids or robust riverine fish too large to be predated, but should not share a tank with very small fishes or invertebrates.
Conservation
Mastacembelus loennbergii is assessed as Least Concern on the IUCN Red List, with a 2019 assessment. It occupies a large native range spanning the Chad, Benue and Niger River basins — major West and Central African catchments with broadly distributed fish faunas — and there is no current evidence of population-level decline sufficient to warrant a threatened listing.
The West African river systems it inhabits face ongoing pressures from agricultural runoff, damming for irrigation and hydropower, artisanal fishing, and expanding human settlements, but the species' relatively wide distribution buffers it against localised impacts. Data on population trends and ecology remain sparse, and a better understanding of its precise distribution, habitat requirements and tolerance of modified environments would strengthen future assessments. Like all scaleless fishes it is intrinsically vulnerable to water quality degradation, and maintaining catchment-level water quality in the Niger and Chad basins is important for the broad community of rheophilic freshwater fishes of which M. loennbergii forms a part.