Taxonomy & naming
Arhinolemur gaiero was described in 2020 by Birindelli, Britski & Ramirez as part of the broader revision that split the old, unwieldy genus Leporinus and erected Megaleporinus for its largest-bodied members, family Anostomidae, order Characiformes. The type locality is the Rio de Contas, Bahia, in northeastern Brazil, and the description is based on a holotype measuring 12.5 in standard length — effectively the size ceiling recorded for the species.
The species is distinguished from its congeners, including the much larger and more widespread M. obtusidens of the Paraná basin, by a combination of characters: an irregular dark longitudinal stripe running from the supracleithrum to a midlateral blotch, a partially closed anterior cranial fontanel, three premaxillary and three dentary teeth, 37–38 lateral-line scales and 16 scale rows around the caudal peduncle. The name gaiero references the local Portuguese term "piau-gaiero", used in Bahia for a Megaleporinus that shows a preference for sheltering around submerged tree trunks — a fitting vernacular tag for a fish that, like the rest of its genus, spends much of its time grazing structure rather than open water.
This fish was long listed as Megaleporinus gaiero; Eschmeyer's Catalog of Fishes places the species in Arhinolemur, and the current combination Arhinolemur gaiero is followed here.
Morphology
Arhinolemur gaiero shares the elongated, laterally compressed body and blunt, downturned head typical of headstanders in this genus, built for grazing at an oblique, nose-down angle over rock and wood surfaces. The original description notes a distinctive colour pattern unusual among Anostomidae: an irregular dark stripe running from the shoulder region to a midlateral blotch, layered over three separate dark blotches spaced down the flank, on an otherwise silvery body. In life the fins carry red pigment, a detail specifically recorded in the description and one of the more visually striking field marks for the species.
At a holotype length of 12.5 in standard length — roughly 12.5 in — A. gaiero is a mid-sized member of its genus, notably smaller than the 30 in reported for its close relative M. obtusidens. Diagnostic dental and scale counts (three premaxillary and three dentary teeth, 37–38 lateral-line scales, 16 caudal-peduncle scale rows, a partially closed anterior cranial fontanel) separate it from similar-looking congeners but are not something a hobbyist can readily check outside a museum specimen.
Habitat
The species is currently known only from the Rio de Contas basin in Bahia, northeastern Brazil — a coastal drainage system separate from the Amazon, Paraná and São Francisco basins that hold most of the genus's other species. No specific published water-chemistry data (pH, hardness) exist for A. gaiero itself; the Rio de Contas is a warm, subtropical-to-tropical northeastern Brazilian river system, and its close relative M. obtusidens is documented from medium to large rivers with comparable warm conditions.
Given the lack of species-specific measurements, any temperature or pH figures for A. gaiero should be treated as inferred from its genus and region rather than confirmed for this species — a gap consistent with how recently it was described and how narrow its known range is.
Feeding
Arhinolemur gaiero has not been the subject of a dedicated diet study, but its genus is well characterised: Megaleporinus species are head-down grazers of algae and biofilm (aufwuchs) on submerged rock, wood and vegetation, and the better-studied congener M. obtusidens is documented taking aquatic plants, seeds, grains and incidental invertebrates such as crabs and river snails alongside its plant-based grazing. A. gaiero is presumed to follow the same broadly herbivorous, grazing ecology, though this has not been directly confirmed in the literature.
In practical terms this places the species firmly in the algae-and-plant-grazer camp rather than among the predatory or scale-eating characins in this atlas — a fish built to scrape surfaces clean, supplementing that diet with occasional animal matter rather than the reverse.
Mating
No courtship behaviour has been published for Arhinolemur gaiero. Data are sparse even for the genus as a whole; the better-known M. obtusidens is described as forming distinct pairs that move to weedy, vegetated areas to spawn as part of a broader potamodromous (river-migratory) breeding strategy. Whether A. gaiero shows comparable pairing behaviour in the Rio de Contas is undocumented, and no external sexual dimorphism has been reported for the species.
Breeding
As with most headstanders, Arhinolemur gaiero is presumed to be an egg-scattering species with no parental care, following the pattern documented across Anostomidae generally and specifically in related Leporinus and Megaleporinus species. It has not been reported as bred in the aquarium hobby, and given its status as a narrow-range, recently described endemic, there is no indication it is farmed or propagated commercially either. Any aquarium breeding reports for the genus concern other, longer-established species such as M. obtusidens, not A. gaiero specifically — this species should be treated as effectively unbred in captivity until documented otherwise.
In the aquarium
Arhinolemur gaiero is not an established aquarium fish — it was described in 2020 from a small number of specimens in a single Brazilian river system, and there is no evidence of it appearing in the trade. Anyone encountering material sold under this name should treat the identification with real skepticism, since Megaleporinus and Leporinus species are frequently mislabelled at the point of sale.
Were it ever to appear, the genus's general care template applies: these are active, hardy grazers that need a large, long tank with driftwood and rock surfaces to graze, a herbivore-leaning diet with vegetable matter as the base, and — as with many Leporinus-group headstanders — a watchful eye for fin-nipping and territorial behaviour toward tankmates and conspecifics as they mature. At a documented maximum around 12.5 in, it would need considerably more space than a typical community setup, though far less than the near-metre-long M. obtusidens.
Conservation
Arhinolemur gaiero was assessed as Endangered essentially concurrent with its 2020 description, reflecting its restriction to a single, relatively small coastal river basin — the Rio de Contas in Bahia — rather than the wide, multi-basin distributions typical of better-known Megaleporinus species. Narrow-range endemics restricted to one northeastern Brazilian drainage are especially exposed to damming, water abstraction and habitat degradation, since there is no adjacent population to buffer local decline.
No specific conservation programme or IUCN numeric assessment page could be confirmed in available sources at the time of writing; the Endangered designation traces to the species' original description and its inherently restricted range rather than to a separately documented population trend.