Taxonomy & naming
Mastacembelus greshoffi was described by the Belgian-British ichthyologist George Albert Boulenger in 1901 from material collected in the Congo basin. The Catalog of Fishes (Eschmeyer, the authority for valid names) places it in the genus Mastacembelus, the type genus of the family Mastacembelidae, within the order Synbranchiformes. This placement is the essential context for the fish: although it is long and serpentine, it is an eel only by shape, not by relationship. The true eels belong to the entirely separate order Anguilliformes; the spiny eels converged on an elongate, burrowing body independently.
The species has had a convoluted generic history. African spiny eels were frequently split into separate genera — Aethiomastacembelus and Caecomastacembelus were both applied to what is now Mastacembelus greshoffi — but both of these have been reduced to synonymy and the fish is currently placed unambiguously in Mastacembelus. GBIF (species 2351776) and the broader literature now recognise the single valid combination. The genus Mastacembelus is large and ranges across both tropical Africa and tropical Asia; the African members include a notable species flock in Lake Tanganyika, though M. greshoffi itself is a Congo basin river and lake species rather than a rift-lake endemic.
Morphology
Mastacembelus greshoffi is a moderately large spiny eel, recorded to a maximum of 13 in standard length (FishBase). The body is greatly elongated and laterally compressed toward the tail, and the skin is scaleless to the eye — covered in tiny embedded scales beneath a thick protective mucus coat. The snout is drawn into a fleshy, trunk-like proboscis with tubular nostrils, the diagnostic probe of a benthic predator that hunts by scent and touch rather than by sight.
Along the dorsal surface runs a row of short isolated spines — the defining character of the spiny eels — ahead of the soft dorsal fin, which is continuous with the caudal and anal fins around the tapered tail. Coloration is typical of the Congo basin Mastacembelus: a muted brownish to olive-grey ground with darker mottling that offers camouflage against the rocky substrates it favours. Sexual dimorphism is subtle; ripe females tend to be fuller-bodied than males of the same length, but the sexes cannot reliably be distinguished in non-breeding fish.
Habitat
Mastacembelus greshoffi is recorded from the lower Congo River, Pool Malebo (the broad lake-like expansion of the river at Kinshasa and Brazzaville), the middle Congo, the Lukenie basin and Lake Tumba in the Democratic Republic of Congo. It is a demersal species: FishBase notes an affinity for rocky bottoms, rapids and the interface between rivers and lakes — habitats quite different from the soft, silty substrates favoured by many of the larger Asian spiny eels.
The Congo River system is characterised by high discharge and variable current speed. Rocky rapid zones — including the famous cataracts downstream of Pool Malebo — are oxygen-rich and support a distinct ichthyofauna adapted to clinging to and sheltering among boulders. Water temperatures in the Congo basin lowlands run broadly in the warm-tropical range, and water chemistry tends toward soft and moderately acidic conditions typical of large West African river systems, though conditions vary considerably along the river's enormous length. The species is listed as freshwater-only and demersal.
Feeding
Mastacembelus greshoffi is a carnivore. Like all spiny eels it uses its sensitive, probing snout to locate prey among the substrate — invertebrates, insect larvae, worms and similar benthic organisms are the staples of the diet in the wild. Its preference for rocky rapids and lake margins suggests it forages among crevices and boulder faces as well as in soft sediment.
In the aquarium the species accepts the full range of meaty foods used for spiny eels: bloodworm, tubifex, brine shrimp and similar live and frozen invertebrates are all taken. Like most members of the family it is shy and largely nocturnal, feeding most actively after the lights go out; offering food in the evening, after the tank is dim, helps newly imported fish settle and begin feeding. It is a deliberate, unhurried hunter and should not be kept with fast, boisterous fish that will consume food before it can be located.
Mating
As a spiny eel, Mastacembelus greshoffi is an egg-scatterer that gives no parental care — it is neither catadromous nor a bubble-nest builder, two modes that belong to other groups of "aquarium eels" entirely. Spawning in wild Congo basin Mastacembelus is almost certainly triggered by the seasonal flood pulse and associated rises in water temperature and turbidity, which cue many Congo fishes to breed. A courting pair would likely entwine and scatter adhesive eggs among vegetation, roots or fine substrate debris.
Specific field observations of breeding behaviour in M. greshoffi are absent from the literature, and the species has not been widely studied for reproductive biology. What is known of its spawning comes by inference from the general Mastacembelidae pattern and from scattered hobbyist experience rather than from dedicated field or laboratory study.
Breeding
Mastacembelus greshoffi is rarely if ever bred intentionally in the home aquarium. Like most spiny eels it is a challenging species to spawn in captivity: the fish require a spacious, mature set-up with ample hiding places and a soft or mixed substrate, a compatible pair that is difficult to sex reliably, and conditioning with live and frozen foods over an extended period. Spawning, when it occurs, is thought to follow a simulated wet-season trigger — a substantial water change with slightly cooler, softer water — after which a pair scatter adhesive eggs among fine plants or roots with no subsequent parental involvement.
Because captive breeding is so uncommon and no published account specific to this species is available, virtually all fish in the trade are wild-caught from the Congo basin. Given that the species is currently assessed as Least Concern with a stable wild population, this wild-capture supply is not considered unsustainable at present, but any future trade expansion would warrant closer monitoring.
In the aquarium
Mastacembelus greshoffi is an infrequently seen Congo basin spiny eel that appeals to hobbyists interested in West/Central African biotope aquaria or the more obscure members of the family. At a maximum of around 13 in it is a manageable size compared to the giant Asian species, but it still requires a spacious tank — a minimum footprint of around 47 in length — furnished with a soft sand base or mixed sand-and-gravel, numerous cave-like retreats (ceramic pipes, stacked rock, bogwood), and subdued lighting that allows the fish to emerge during the day without feeling exposed.
Like all spiny eels, M. greshoffi is scaleless and sensitive to copper-based medications and to poor water quality; a mature, well-filtered system is essential. It is also a determined escape artist: the family is notorious for climbing or squeezing through any gap, and a tightly sealed, weighted lid is non-negotiable. It can be kept with similarly sized, peaceful fish that cannot be swallowed, but it will take small tankmates at night. Water parameters should reflect its Congo origin: warm (73–82 °F), soft to moderately hard, and near neutral pH (6.0–7.5), with good oxygenation and flow.
Conservation
Mastacembelus greshoffi is assessed as Least Concern on the IUCN Red List (2022 assessment). The species has a wide distribution across the Congo basin — one of the world's largest and most biologically diverse river systems — and no evidence of significant population decline has been identified. The assessment notes a stable population and no imminent threat at the time of evaluation.
The Congo basin faces broad-scale pressures from artisanal and industrial fishing, habitat modification, deforestation in the watershed and, increasingly, proposed large-scale hydropower development. The Grand Inga dam project in particular, if constructed at the lower Congo rapids — precisely the rocky, high-gradient habitat M. greshoffi occupies — would drastically alter flow regimes and river morphology in this ecologically distinctive zone. The extraordinary endemic fish fauna of the lower Congo, including many narrow-range species, has been highlighted as among the most conservation-sensitive in freshwater ichthyology. While M. greshoffi itself is currently secure, the long-term integrity of its rocky rapids habitat is not guaranteed.