Taxonomy & naming
Marcusenius desertus was described in 2016 by Bernd Kramer, Michael van der Bank and Michael Wink in the African Journal of Aquatic Science, based on specimens from the lower Cunene River near its mouth at the Skeleton Coast. It belongs to the family Mormyridae — the elephantnose fishes, Africa's weakly electric mormyrid radiation — within the order Osteoglossiformes, the bonytongues: among the oldest surviving lineages of bony fish, named for a toothed bone on the tongue that bites against the roof of the mouth, and represented today by a scattered, deep-time radiation found across four continents (South America, Africa, Southeast Asia and Australasia). Mormyridae is exclusively African, and its weak electric sense evolved entirely independently of — and is not related to — the electric knifefishes of South America (Gymnotiformes); the two lineages arrived at similar electrosensory systems by convergence, on opposite sides of the Atlantic.
The genus Marcusenius, erected by the American zoologist Theodore Gill in 1862, holds several dozen species scattered across West, Central, East and southern African river systems. Marcusenius desertus was split out from its close relative M. multisquamatus, an escarpment-dwelling congener found further upstream in the same drainage, on the strength of a distinctly shorter snout and smaller eye plus molecular evidence: analysis of the mitochondrial cytochrome b gene confirmed the two populations are reproductively isolated, and the new species carries its own unique electric organ discharge (EOD) signature. As with most mormyrids, close relatives can look near-identical to the eye, and reliable field identification within the genus generally rests on the EOD waveform rather than appearance alone.
Morphology
Like other Marcusenius, M. desertus has the genus's typical short, blunt snout and terminal mouth — a compact head shape distinct from the elongated, trunk-like chin appendage (the Schnauzenorgan) that characterises relatives such as Gnathonemus. The original description separates it from its sympatric congener M. multisquamatus chiefly by a distinctly shorter snout and a smaller eye, alongside the genetic and electrophysiological differences noted above. As in other mormyrids the body is laterally compressed, with a single soft dorsal fin set well back opposite a long-based anal fin, and a deeply forked tail on a narrow peduncle.
Near the base of the tail sits the electric organ, modified muscle tissue that fires a weak, pulse-type discharge unique in waveform to the species — used both to electrolocate prey and obstacles in dark water and to signal identity to other mormyrids. Mormyrids as a family are noted for having, size for size, some of the largest brains and highest brain-to-body ratios of any fish, with an enlarged cerebellum dedicated to processing the constant stream of electrosensory data the EOD returns. FishBase records a maximum size of about 8.1 in for M. desertus, measured there as standard length; no separate total-length or aquarium-specimen size has been documented for this virtually untraded species.
Habitat
Marcusenius desertus is known from the lower Cunene River, the river that forms much of the Angola–Namibia border and empties into the Atlantic Ocean through Skeleton Coast National Park — one of the most arid coastal deserts on Earth. The type locality sits near the river mouth itself, a stretch of harsh desert-margin freshwater quite unlike the warm, forested interior rivers most Mormyridae inhabit. Its close relative M. multisquamatus occupies the same drainage but further upstream on the escarpment, so the two congeners are geographically separated within the Cunene system even though they are sympatric across the basin as a whole.
FishBase records the species from tropical freshwater with a temperature around 68–72 °F at the type locality — a notably cool range for a mormyrid, reflecting the desert-margin, Benguela-influenced coastal climate of the lower Cunene rather than the steady warmth of the Congo or Niger interior. Precise pH and other water-chemistry values have not been documented for this species; given the general behaviour of the family, dim, tannin-influenced water is a reasonable expectation, but that has not been directly measured here.
Feeding
No species-specific feeding study exists for Marcusenius desertus, but as in other mormyrids it almost certainly forages along the riverbed for small invertebrates — aquatic insect larvae, worms and tiny crustaceans — locating them by electrolocation rather than sight in the dim water of the lower Cunene. The electric organ discharge used for navigation and communication is thought to double as a prey-detection tool in exactly this kind of low-visibility habitat.
General care literature for the genus recommends live foods — mosquito larvae, bloodworm, Daphnia and other small crustaceans — as the staple diet for any Marcusenius kept in captivity, offered mainly after dark to match the family's nocturnal foraging habit. No aquarium-specific feeding record exists for M. desertus itself, given how rarely, if ever, the species has reached the hobby.
Mating
As in other mormyrids, the species-specific electric organ discharge is presumed to play a central role in mate recognition and courtship in Marcusenius desertus, letting individuals identify suitable partners of their own kind in water too dim or turbid for visual cues to work reliably. Beyond that broad family pattern, no courtship behaviour has been directly documented for this species.
Nothing has been recorded about seasonal timing of reproduction in M. desertus specifically. Mormyrids in general are thought to be seasonal, flood-cued spawners, but the Cunene's desert-margin lower reaches are a very different hydrological setting from the rainforest floodplains where that pattern is best documented, so it would be speculative to assume the same trigger applies here without direct study.
Breeding
Breeding in Marcusenius desertus is essentially undocumented, which is true of mormyrids generally: captive breeding is rarely if ever achieved for the family, and reproductive biology in the wild remains poorly understood even for well-known species. No aquarium spawning of M. desertus has been recorded.
What little is known of mormyrid reproduction more broadly suggests some species scatter eggs among flooded marginal vegetation during seasonal high water, with a degree of egg-guarding suspected in a few better-studied elephantnoses — but these details have not been confirmed for M. desertus, and given the species was only described in 2016 and appears to be essentially unknown in captivity, the honest position is that its breeding mode, egg description and any parental care remain unrecorded.
In the aquarium
Marcusenius desertus has no established presence in the aquarium trade — it is a narrow-range species described only in 2016 from a harsh desert-margin river mouth, and no species-specific husbandry has been documented. Anyone encountering a fish offered under this name should treat general Marcusenius/Mormyridae care as an untested starting point rather than as species-verified guidance.
That general care is worth stating plainly: a tank of at least 39.4 in, kept dimly lit with plenty of cover (roots, driftwood, caves), a soft, fine sand or gravel substrate the fish can probe without injuring its sensory organs, and stable, clean water — mormyrids are notoriously sensitive to poor water quality and to many medications, so any treatment should be researched as mormyrid-appropriate and dosed conservatively. Live and frozen invertebrate foods (mosquito larvae, bloodworm, Daphnia, small crustaceans) are the expected diet. Mormyrids are generally not kept solitary but can also be intolerant of their own species in close confinement, so a spacious setup with visual breaks matters if more than one is housed together.
Conservation
The IUCN Red List has not evaluated Marcusenius desertus — it carries no formal conservation status. This is unsurprising for a species described as recently as 2016 and known only from a narrow stretch of the lower Cunene River near the Skeleton Coast.
A Not Evaluated rating is not a clean bill of health; it simply means no assessment has yet been made. Given its apparently narrow range in one of the harshest, most arid river systems in Africa, the general pressures facing desert-margin rivers — water abstraction, altered flow regimes and any future development along the Angola–Namibia border — are the kind of concerns that could matter for a range-restricted species like this one, even without data yet available to confirm or rule that out.