Taxonomy & naming
Sternarchorhynchus mendesi was described by de Santana & Vari in 2010 as part of a major revision of the genus Sternarchorhynchus, which the same authors showed to be the most speciose genus in the family Apteronotidae, with 32 recognised species. It belongs to the order Gymnotiformes, the New-World electric knifefishes — bony fish defined by an electric organ and a long ribbon-like anal fin used for propulsion, and only distantly related to the superficially similar Asian featherback and clown knifefishes (Chitala, Notopterus), which belong to a completely different order and are not knifefish relatives at all.
Within the Apteronotidae, the ghost knifefishes, Sternarchorhynchus is set apart by its long, tubular, downturned snout — the source of the genus name and of the "tube-snouted" common name applied to several of its species, including this one. The species epithet honours George Nilson Mendes, a Brazilian ichthyologist and oceanographer. The Catalog of Fishes is the authority for the valid name.
Morphology
Sternarchorhynchus mendesi is a small, elongated ghost knifefish reaching a maximum recorded total length of about 7.5 in, modest even by the standards of the genus. As in other Apteronotidae the body is compressed and blade-like, without a dorsal or tail fin, with locomotion carried almost entirely by a long anal fin run as a travelling wave along the underside — the mechanism that lets these fish glide forwards or backwards with equal ease. The most distinctive feature is the elongated, tubular snout characteristic of Sternarchorhynchus, an adaptation the genus's describers linked to a specialised grasp-and-suction feeding action on small prey worked out of tight spaces.
Like its relatives it is scaleless, and it carries an electric organ running much of the length of the body along with sensory pores across the skin that read the field the organ produces. In Apteronotidae generally that organ is built from modified nerve tissue rather than muscle, producing a fast, continuous wave-type discharge rather than the intermittent pulses used by some other gymnotiform families; the discharge type has not been specifically documented for S. mendesi, but the family-level pattern is well established. Detailed sexual dimorphism has not been described for this species.
Habitat
The species is known only from the Río San Juan and Río Guanipa basins, part of the Golfo de Paria drainage in northeastern Venezuela, with its type locality at Morichal Largo on the Río Guanipa, about 3 mi south of Aguasay. It is recorded from only eight specimens, a reflection of how narrow and poorly sampled its range is rather than necessarily of true rarity in the field.
Species of Sternarchorhynchus as a group are associated with deep river channels and with rheophilic, swift-flowing rapids habitats, in contrast to the still backwaters and flooded forest favoured by some other gymnotiforms. As a benthopelagic fish it is presumed to occupy the water column near the bottom of these flowing, tannin-tinted Neotropical rivers, in darkness where its electric sense — not its eyes — does the work of finding its way and its food.
Feeding
Direct diet data for Sternarchorhynchus mendesi has not been published; it is presumed, on the basis of the genus, to feed on small invertebrates worked out of crevices, sediment and root tangles using electrolocation to find prey it cannot see. The genus's elongated, tubular snout has been linked by its describers to a specialised grasp-suction feeding mechanism, allowing these fish to draw small prey items out of narrow spaces in deep or fast-flowing channels rather than foraging in the open.
As with other ghost knifefishes, feeding activity in the family is predominantly nocturnal, driven by the fish's electrosensory system rather than vision. No aquarium feeding record exists for this species; any care would have to extrapolate cautiously from better-known Apteronotidae such as the black ghost knifefish.
Mating
No courtship or spawning behaviour has been documented for Sternarchorhynchus mendesi. Across the ghost knifefishes generally, the wave-type electric organ discharge doubles as a social signal encoding a fish's identity, sex and reproductive state, and in well-studied relatives such as the brown ghost knifefish, males and females shift their discharge frequencies toward one another during courtship. Whether S. mendesi shows comparable electric signalling has not been studied; it is inferred from the family pattern rather than observed directly in this species.
Breeding
Breeding biology for this species is essentially undocumented — data sparse across every stage of reproduction, from spawning site to egg development to any parental involvement. Apteronotidae as a family are egg-layers that scatter eggs among roots, plants or other cover, with no substantial parental care recorded in the group, and captive breeding in the family overall is rare, mostly confined to farmed relatives raised under hormone induction rather than by hobbyists. Given that S. mendesi is known from only eight museum specimens and has no established presence in the aquarium trade, there is no basis for describing a breeding protocol; the honest position is that its reproduction in the wild, let alone in captivity, remains unstudied.
In the aquarium
Sternarchorhynchus mendesi is rare in the aquarium trade, documented mainly through occasional import records rather than as an established hobby fish, and any specimens offered for sale are wild-caught from its very narrow Venezuelan range. As with other Apteronotidae it is scaleless, which calls for careful attention to water quality and to medication — copper-based treatments should be avoided and other medications dosed conservatively — and as a presumed nocturnal, electrolocating predator it would need subdued lighting, secure cover such as pipes or root tangles, and tankmates too large to be eaten.
Given how little is known of its biology and how few specimens exist even in museum collections, this is not a species to recommend for the general hobby; anyone encountering it should treat it as a specialist, narrow-range fish and prioritise not adding further collection pressure to a population already assessed as endangered.
Conservation
Sternarchorhynchus mendesi is assessed by the IUCN as Endangered, under criteria B1ab(iii)+2ab(iii) — reflecting a restricted range and an inferred ongoing decline in the extent and quality of its habitat. That assessment rests on a genuinely narrow distribution: the species is known only from the Río San Juan and Río Guanipa basins of the Golfo de Paria drainage in northeastern Venezuela, and from just eight collected specimens.
A single-basin endemic known from so few records is inherently vulnerable to habitat change within that small range — the general pressures on Neotropical river systems include deforestation, water pollution and hydrological alteration, though pressures specific to this species' basin are not detailed in available sources. Its conservation case is a reminder that many gymnotiforms outside the handful of trade staples are narrow-range fish about which very little is published, making every specimen and every record disproportionately valuable to their assessment.