Taxonomy & naming
Enteromius clauseni was formally described by Dirk F. E. Thys van den Audenaerde in 1976 under the original combination Barbus clauseni. Thys van den Audenaerde was a prominent Belgian ichthyologist who worked extensively on African cyprinids and cichlids, and his original description was based on material collected from the Yewa River on the Nigeria–Benin border.
The genus Barbus was historically used as a near-universal container for Old World barbs, but large-scale molecular and morphological revisions in the decades after the original description progressively fragmented it. African small barbs — those lacking the rostral cap of the true Barbus — were transferred to Enteromius, a genus now used for a substantial radiation of small cyprinids across sub-Saharan Africa. The current valid combination, Enteromius clauseni (Thys van den Audenaerde, 1976), follows this reassignment; the parenthetical authority marks the original genus as different from the one now used. The Catalog of Fishes (Eschmeyer, CAS) treats Enteromius clauseni as the valid name.
The species epithet clauseni honours the Danish ichthyologist Herluf Stenholt Clausen, who collected the type specimen from the Yewa River and whose field work contributed substantially to knowledge of West African freshwater fishes.
Morphology
Enteromius clauseni is a very small barb, recorded at a maximum of 1 in in standard length. This places it firmly among the miniature members of Enteromius, a genus that spans a wide size range across its African distribution. Beyond its diminutive size, detailed morphological data in the available literature is limited — the species is known from few specimens and has not been the subject of extensive anatomical study.
Like other members of the African small-barb assemblage, it is presumed to have the generalised, fusiform body plan typical of rheophilic stream barbs: a laterally compressed profile suited to life in flowing water, a small mouth adapted to picking up invertebrates and organic matter from the substrate, and the small scales and lateral-line pores characteristic of the group. Specific colouration data from fresh specimens are not documented in the accessible cache.
Habitat
The Yewa River (also written Yewa or Ogun–Yewa) is a mid-sized river system draining parts of southwestern Nigeria and southeastern Benin before discharging into the Bight of Benin near the border town of Badagry. The basin is a lowland West African river system characterised by warm water, variable flow seasonality, and significant human activity in its lower reaches, including agriculture, urban encroachment, and water abstraction.
Enteromius clauseni is recorded from a single location within this system, which is the entire known range of the species. No additional populations have been confirmed elsewhere. The precise microhabitat conditions — substrate type, current velocity, riparian cover, water chemistry — are not detailed in the current literature, but West African lowland barbs of this size typically occupy shallow runs and pool margins with soft substrates and organic detritus, often in areas with some aquatic or marginal vegetation.
Feeding
No detailed dietary study exists for Enteromius clauseni in the available literature, and the species' feeding ecology must be inferred from its body size, habitat, and close relatives. African small barbs of comparable size in lowland West African streams are generally opportunistic omnivores, taking small invertebrates — midge larvae, microcrustaceans, and worms — alongside fine organic detritus, algal films, and plant matter scraped or sucked from the substrate and benthic surfaces.
The small mouth of miniature Enteromius species is consistent with this generalist picking strategy rather than pursuit predation. In the absence of species-specific data, dietary accounts from ecologically similar congeners in comparable West African lowland systems are the best available guide.
Mating
Enteromius clauseni, in common with all barbs, is an egg-scattering, non-guarding spawner. No specific spawning observations have been recorded in the literature for this species, but the reproductive biology follows the uniform barb pattern: courtship consists of a driving male pursuing a gravid female through vegetation or over the substrate, with eggs and milt released in brief, vigorous contact presses as the pair move together.
There is no pair bond, no territory maintained beyond a momentary association during spawning, and no preparation of a nest or spawning site. Colour and activity in males intensify during readiness. Once spawning is complete the fish separate, and the adhesive eggs are left wherever they settle.
Breeding
Barbs are universally egg-scatterers that provide no parental care. Enteromius clauseni scatters small adhesive eggs over fine-leaved plants, roots, or the substrate surface, after which both parents take no further interest in the spawn. Adults readily consume their own eggs and, if given the opportunity, the newly hatched larvae as well.
No captive breeding records for Enteromius clauseni appear in the literature, and the species is not known in the aquarium trade. Breeding behaviour has not been documented under observation. By analogy with closely related West African miniature barbs, a spawning event likely produces a modest clutch of adhesive eggs that hatch within one to two days at ambient West African water temperatures, with larvae becoming free-swimming a few days thereafter.
In the aquarium
Enteromius clauseni is not known in the aquarium hobby and is not available in the ornamental trade. Its extreme rarity — confined to a single location in a river system under significant human pressure — means it has not been collected for fishkeeping, and no captive-maintenance guidelines exist in the hobbyist literature.
Were it ever to become available, the general care framework for miniature West African stream barbs would apply: a warm (75–81 °F), well-oxygenated tank with fine-leaved plants or other cover, a soft substrate, gentle current, and tankmates of comparable size. The species would be unsuitable for the mainstream trade given its conservation status, and any captive stock would be more appropriately handled within a conservation-breeding programme.
Conservation
Enteromius clauseni is assessed as Critically Endangered (CR) on the IUCN Red List, with the assessment dated 10 August 2018. The primary driver of its critical status is its extreme range restriction: the species is known from a single location in the Yewa River, a trans-boundary system shared between Nigeria and Benin. Under IUCN criteria, a species known from only one location is inherently vulnerable to stochastic events — a single pollution incident, a dam, severe drought, or rapid change in land use within the catchment could eliminate the species entirely.
The Yewa River basin faces multiple pressures common to lowland West African river systems: agricultural runoff, urban expansion along its lower course, water extraction for irrigation and domestic use, and the general degradation of riparian habitat. No targeted conservation action is known to be in place for this species specifically. Its status underscores the vulnerability of single-river endemics throughout the West African freshwater fish fauna, many of which remain inadequately surveyed and receive little directed protection.