Taxonomy & naming
Mastacembelus robertsi was described by Vreven and Teugels in 1996 from specimens collected in the lower and middle Zaïre (Congo) River basin, where it was placed in the African spiny-eel genus Aethiomastacembelus as A. robertsi. The authors noted close morphological affinities with Aethiomastacembelus congicus and A. marchei, distinguishing the new species by its reduced eye size and distinctive coloration. In a 2004 taxonomic revision of the African Mastacembelidae, Aethiomastacembelus was placed in synonymy with Mastacembelus — the type genus of the family — and the species has been known as Mastacembelus robertsi ever since. The Catalog of Fishes (Eschmeyer, CAS) is the authority for this valid name.
Like all spiny eels, M. robertsi is not a true eel. The true eels — order Anguilliformes — include the catadromous river eels of the genus Anguilla and the various marine groups; they are not closely related to the Mastacembelidae. The spiny eels belong to the order Synbranchiformes, more closely allied to the swamp eels (Synbranchidae) than to the Anguilliformes. Their eel-like shape is a case of evolutionary convergence on a burrowing, elongate body plan. No common name is in established use for M. robertsi in the aquarium trade or the scientific literature.
Morphology
Mastacembelus robertsi is a moderately sized spiny eel, reaching a maximum standard length of about 13.5 in (males and unsexed specimens; FishBase). The body is greatly elongate and round in cross-section anteriorly, tapering toward a continuous caudal region where the dorsal, caudal and anal fins merge. Like all mastacembelids it is essentially scaleless in appearance — the skin is covered in tiny embedded scales and a coating of mucus rather than the visible plate-like scales of most fishes. The snout ends in a fleshy, down-turned rostral organ (the proboscis) with tubular nostrils used to probe the substrate for food. A row of short, isolated spines runs along the dorsal midline ahead of the soft dorsal fin — the diagnostic "spiny" character of the family.
The coloration described from the original specimens from the Congo Rapids region is notably different from the lowland species with which M. robertsi is most closely compared: reduced eye size is a key distinguishing character, consistent with a lifestyle in fast-water, rocky habitat where vision may be less critical than in open-water environments. Sexes are externally similar; ripe females are typically fuller-bodied.
Habitat
Mastacembelus robertsi is native to the lower Congo River and Pool Malebo (the large pool immediately upstream of the Congo Rapids in the Democratic Republic of Congo), and to the lower Kotto and Bomu rivers of the Ubangui drainage in the Central African Republic. FishBase records that specimens have been collected mostly from rapids — fast-flowing, turbulent, well-oxygenated reaches rather than the sluggish lowland channels and floodplains preferred by many aquarium-trade spiny eels.
Water parameters recorded for the species range from 76–84 °F, pH 7.0–7.5, and dissolved oxygen 8.0 mg/l — warm, well-oxygenated, near-neutral fresh water. The Congo River around Pool Malebo and the Upper Congo Rapids is famous for its extreme biodiversity and high endemism, particularly among cichlids and other fishes adapted to the powerful currents and rocky substrates of the rapids zones. The elevated dissolved-oxygen reading and the rapids collection data together suggest M. robertsi is more current-adapted than many of its congeners, and less suited to the still, soft-bottomed environments common in the aquarium hobby.
Feeding
Mastacembelus robertsi is a nocturnal carnivore in the wild. Like other spiny eels it uses the fleshy, probing rostral organ to search the substrate and rock crevices for benthic invertebrates — insect larvae, worms and small crustaceans — after dark, relying more on chemical and tactile senses than on vision. The reduced eye size noted as a diagnostic character may reinforce this interpretation of a primarily substrate-focused, low-light forager.
Specific dietary data for M. robertsi in the wild are sparse, as befits a rarely collected rapids-dwelling species. In captivity, assuming similar preferences to other Mastacembelus of comparable size, it would be expected to take live and frozen bloodworms, earthworms, tubifex and similar invertebrate foods offered in the evening or after lights-out. Feeding after dark reduces competition stress and encourages shy newly-imported specimens to accept food.
Mating
No specific mating observations are recorded for Mastacembelus robertsi, and the species is rare in captivity. What is known about reproduction in the genus Mastacembelus applies by extension: spiny eels are egg-scattering spawners that reproduce in fresh water and give no parental care whatsoever. They are not catadromous and do not undertake marine spawning migrations — that is the behaviour of the true eels (Anguilla), not of the Mastacembelidae.
In spiny eels generally, courtship involves a male pursuing a ripe female, with the pair entwining among fine vegetation or roots before eggs are scattered. Spawning in wild populations is typically triggered by seasonal flooding, rising water temperature, or the onset of the rainy season — environmental cues that signal suitable conditions for larval development. In M. robertsi, whose preferred rapids habitat experiences distinct seasonal flow variation in both the Congo and Ubangui basins, similar flood-season cues are probable.
Breeding
Mastacembelus robertsi is essentially unrecorded as a captive breeding subject. It is a rarely exported and rarely kept species, and no published accounts of aquarium or pond spawning exist in the accessible literature. Its breeding mode follows the standard Mastacembelidae pattern: a pair scatters adhesive eggs among vegetation, roots or leaf litter in fresh water; the eggs are left unattended; the adults provide no parental care; and the eggs hatch in a few days, producing tiny larvae that require fine foods.
The species' preference for fast, well-oxygenated water in the wild adds an extra dimension of difficulty for the aquarist attempting breeding: replicating the current speeds, dissolved-oxygen levels and water chemistry of the Congo Rapids system in a home aquarium is considerably more demanding than conditioning a typical lowland spiny eel. Specific breeding data for M. robertsi remain sparse, and any captive spawning would be genuinely novel.
In the aquarium
Mastacembelus robertsi is not a species regularly encountered in the aquarium trade; it is an occasional export from Central Africa rather than a staple of the hobby. The handful of specimens that do reach hobbyists share the standard care requirements of the Mastacembelidae: a tight-fitting, fully sealed lid is non-negotiable, as spiny eels are accomplished escape artists that will squeeze through any gap and will be found dried on the floor. A soft sand substrate deep enough to allow burrowing, shaded retreats such as caves, pipes and dense planting, and stable, clean water are the foundation of any spiny-eel set-up.
The care note specific to M. robertsi is its reported preference for well-oxygenated, slightly current-bearing water at 76–84 °F and pH 7.0–7.5 — closer to neutral than the soft, slightly acid conditions often recommended for Southeast Asian spiny eels. Good filtration with some surface movement or supplemental oxygenation is appropriate. Like all mastacembelids it is scaleless and highly sensitive to copper-based medications and to poor water quality; treatment of any disease should use copper-free alternatives and begin with pristine water. It is a nocturnal predator best kept with tankmates too large to be swallowed, fed in the evening with live or frozen invertebrate foods.
Conservation
Mastacembelus robertsi is assessed as Least Concern on the IUCN Red List (2020.2 assessment). The assessment notes that the species is an endemic of the lower Congo and middle Congo basin — a geographically restricted range — but identifies no major documented threats beyond the generalised pressures on river habitat in the region. The lower Congo River and its rapids system, while among the most biodiverse freshwater habitats on Earth, face ongoing pressure from hydropower development proposals, artisanal and commercial fishing, and pollution from Kinshasa and Brazzaville, the twin capitals flanking Pool Malebo.
The Congo basin's endemic fishes are generally less well studied than those of the East African rift lakes, and data limitations mean that many species' population trends are genuinely unknown. The Least Concern rating for M. robertsi reflects an absence of evidence of decline rather than confirmed stability. As an apparently current-adapted rapids specialist with a restricted distribution, it could be more vulnerable to habitat alteration than its formal status suggests, particularly if large-scale hydropower schemes affecting the Congo Rapids zone were to proceed.