Taxonomy & naming
Brienomyrus adustus was originally described as Marcusenius adustus by the American ichthyologist Henry Weed Fowler in 1936, from a coastal river about 19 mi east of Kribi, Cameroon — the species' type locality. It was later placed in the genus Brienomyrus, within the family Mormyridae and the order Osteoglossiformes, the bonytongues: a scattered, deep-time radiation of fishes found on four continents (South America, Africa, Southeast Asia and Australasia), united not by any shared modern habitat but by that toothed lingual bone, and counted among the most ancient lineages of living teleosts. The Mormyridae are an exclusively African family, every member weakly electric, and entirely convergent with — not related to — the electric knifefishes of South America (Gymnotiformes), which evolved an equivalent electric sense independently on the opposite side of the Atlantic.
The taxonomic status of this name is unsettled. Eschmeyer's Catalog of Fishes (CAS) — the standing authority for valid fish names — currently treats Brienomyrus adustus as a junior synonym of Brienomyrus brachyistius (Gill, 1862), following Lévèque et al. (1991); FishBase carries the same synonymy. That kind of lumping is common in this family: many small mormyrids are all but indistinguishable by eye and are reliably separated only by their species-specific electric organ discharge (EOD), so names built on preserved, silent museum specimens have repeatedly needed revision. This article follows the name as catalogued historically while flagging the synonymy plainly; a reader encountering the fish under either name should treat the two as referring to the same, or a very closely related, population.
The genus name Brienomyrus pairs -myrus, from Greek myros, "eel" — a suffix shared by several other mormyrid genera such as Mormyrus and Pollimyrus, alluding to the elongated body many members share — with a root thought to honour the Belgian zoologist Paul Brien, who studied the reproductive and developmental biology of African mormyrids. The species epithet adustus is Latin for "browned" or "scorched," a plain description of the fish's dark coloration.
Morphology
Like other mormyrids, Brienomyrus adustus has a laterally compressed body, a small, close-set mouth, and a caudal peduncle housing the tail-mounted electric organ that defines the family. FishBase records a modest maximum size of about 3.9 in total length, making it one of the smaller members of a family that ranges from a few centimetres up to the metre-plus Mormyrus and Gnathonemus giants — this is a fish sized for a modestly proportioned aquarium, not a size-trap species.
Detailed species-level description beyond the original type material is thin, and the close resemblance among small West African mormyrids compounds the problem: many are reliably told apart only by their electric organ discharge rather than by fin shape, colour or body proportion, which is part of why this name has drifted into synonymy with B. brachyistius. As in other mormyrids, an outsized cerebellum drives one of the highest brain-to-body ratios recorded in any vertebrate, the physiological cost of running a sensitive electrolocation and communication system built around that tail organ.
Habitat
The type locality for Brienomyrus adustus is a coastal river roughly 19 mi east of Kribi in Cameroon, and FishBase describes its broader range simply as West-central African freshwater — a range that, given the synonymy question noted above, may in practice describe part of the wider distribution of B. brachyistius. Data on the species' precise range beyond the type locality is sparse.
Mormyrids of this region are fish of soft, acidic blackwater rivers and floodplains: muddy, heavily vegetated, sluggish stretches littered with submerged wood and leaf debris rather than open, fast-flowing water. That dim, low-visibility, tannin-stained water is exactly the setting in which an electric sense outperforms eyesight, and it is the habitat this species is presumed to share with its West African relatives.
Feeding
No species-specific feeding study is available for Brienomyrus adustus, but its biology is presumed to follow the mormyrid pattern: a nocturnal, electrolocation-guided forager that sweeps soft substrate for small aquatic invertebrates — worms, insect larvae and similar prey — hidden in mud, sand or leaf litter, using its weak electric organ discharge to detect them in place of the vision that dim, turbid water makes unreliable.
In the aquarium, hobby sources report it takes frozen and prepared foods once settled, consistent with other small mormyrids. Feeding after dark, when the fish is naturally active, gets the best results, and a soft substrate protects the sensitive electroreceptive skin it relies on to forage.
Mating
Courtship and mate recognition in mormyrids are believed to run substantially through the electric organ discharge rather than through visual display — the pulse itself may carry information about species identity, sex and readiness to spawn — but this has not been specifically documented for Brienomyrus adustus. Across the family, wild spawning is thought to be seasonal and triggered by the rains and rising water of the African wet season.
The electrical and acoustic noise of a home aquarium — filters, heaters, pumps and reflective glass — readily interferes with the weak EOD signal mormyrids rely on for this kind of communication, which is one reason natural courtship essentially never plays out in captivity for fish in this family.
Breeding
Brienomyrus adustus is, so far as the available sources record, not bred in the aquarium hobby, and — as with most mormyrids — its reproductive biology in the wild is poorly understood. Breeding is presumed to be seasonal and flood-triggered, and some mormyrids are thought to guard eggs among vegetation, but no such detail has been confirmed for this species.
Aquarists should not expect or attempt to breed this fish. Virtually all mormyrids in the trade, this species included, are wild-caught, and any claim of captive spawning should be treated with real scepticism absent solid documentation.
In the aquarium
Hobby sources selling this species under the trade name "baby whale" describe water in the range of pH 6.3–7.4 at 77–82 °F, consistent with soft, slightly acidic West African river conditions. It wants a dimly lit tank with soft substrate and ample hiding places — roots, wood and plant cover — to feel secure during the day, becoming more active after dark.
Like other mormyrids it is described as peaceful and unsuited to aggressive tankmates, but keepers of this family should stay cautious generally: mormyrids can be intolerant of their own kind in a confined space, and — being scaleless-skinned, electrosensitive fish — they are notably sensitive to poor water quality and to many aquarium medications, copper-based treatments in particular. Water quality should be kept excellent and stable, and any dosing approached carefully. As with all mormyrids, this is not a fish for a brightly lit, noisy community tank; it wants calm equipment and calm company, and breeding in captivity is rarely if ever achieved.
Conservation
The IUCN Red List has not published a species-specific assessment for Brienomyrus adustus, and the research behind this article located no accessible listing; the status here is recorded as Not Evaluated (NE). The taxonomic uncertainty around the name — currently treated by Catalog of Fishes as a junior synonym of Brienomyrus brachyistius — likely contributes to why no independent assessment exists.
There is no evidence in the available sources of a specific conservation concern for this fish or its West-central African coastal river habitat, but data on small, little-studied mormyrids like this one is generally thin. The usual pressures on West African freshwater systems — habitat degradation, sedimentation and water-quality decline — remain the relevant long-term backdrop even absent a formal assessment.