Taxonomy & naming
Sternarchorhynchus mesensis was described by Campos-da-Paz in 2000 as part of a revision of the genus Sternarchorhynchus, a group of long-snouted ghost knifefishes within the family Apteronotidae, order Gymnotiformes — the New-World electric knifefishes. The Apteronotidae are set apart from other gymnotiforms by an electric organ built from modified nerve cells rather than muscle tissue, giving them a fast, continuous wave-type electric organ discharge (EOD) that they use for electrolocation and for signalling to one another. The species name mesensis references the Serra da Mesa area of the upper Rio Tocantins in Goiás state, Brazil, the region from which the type material was collected.
In the original description, Campos-da-Paz grouped S. mesensis with S. curvirostris and S. roseni in what has been called the curvirostris species-group, distinguished from other Sternarchorhynchus largely by anal- and pectoral-fin colour patterns and by morphometric and meristic (fin-ray and scale count) detail rather than by any obvious external feature an aquarist would notice. As with the rest of the order, these are true bony fish, not related to the superficially similar Asian featherback or clown knifefishes (Chitala, Notopterus), which belong to an entirely different lineage. The Catalog of Fishes is the standing authority for the valid name.
Morphology
Sternarchorhynchus species share the family's blade-like, laterally compressed body plan with no dorsal fin and no true tail fin, propelled instead by a long ribbon-like anal fin running along most of the underside. What distinguishes the genus is its long, tubular, downturned snout — an elongated rostrum that gives the group its common nickname of tube-snouted knifefishes — used to probe into sediment, crevices and root tangles in search of hidden prey. Sternarchorhynchus mesensis reaches a recorded maximum of about 10 in in total length, a modest size compared to the larger ghost knifefishes familiar to the hobby such as the black ghost.
As in other Apteronotidae, the skin is scaleless and the body carries the electric organ along its length together with an array of electroreceptive pores that read the field it produces. Fine detail on colour pattern, fin-ray counts and sexual differences for this species is not covered in the available sources, and should be treated as data sparse rather than assumed from relatives in the genus.
Habitat
The species is known from the Rio Tocantins basin in central Brazil, specifically the São Domingos karst region and the upper Rio Tocantins around the Serra da Mesa area of Goiás state — the type locality. Notably, the habitat records for S. mesensis include both surface river channels and subterranean cave-associated waters within this karst landscape, an unusual habitat association for a gymnotiform and one that underlines how little of its ecology has been documented in detail.
It is described as a freshwater, benthopelagic fish of the tropical zone, consistent with the rest of the genus, which favours flowing, often dim or turbid river water where an electric sense is more useful than eyesight. Specific temperature and pH data for this population are not given in the available sources.
Feeding
No species-specific diet studies are available for Sternarchorhynchus mesensis in the sources consulted. Based on the genus's distinctive long, downturned snout — an adaptation shared across Sternarchorhynchus for probing into fine sediment, leaf litter and root tangles — it is reasonable to infer, as for its relatives, a diet of small benthic invertebrates such as aquatic insect larvae and worms, located primarily by electrolocation in the dark rather than by sight. This inference is based on general Sternarchorhynchus biology rather than a direct account of this species, and should be read as such.
Mating
As in other Apteronotidae, the wave-type electric organ discharge in Sternarchorhynchus is thought to carry information about a fish's identity, sex and readiness to spawn, and courtship in the family generally plays out partly through this electric signalling alongside physical approach in the dark. No courtship or spawning behaviour has been specifically documented for Sternarchorhynchus mesensis in the available sources, and nothing here should be taken as an observed account for this species rather than an inference from the wider family.
Breeding
There is no documented breeding record for Sternarchorhynchus mesensis, in the aquarium or in the wild — unsurprising for a narrow-range, cave-associated species that is not established in the ornamental trade. Ghost knifefishes as a group are egg-layers that scatter eggs among cover such as roots and crevices, with no substantial parental care reported, and captive breeding across the family is rare even for the handful of species that are regularly kept. For this species, the honest summary is simply that breeding is undocumented and poorly understood.
In the aquarium
Sternarchorhynchus mesensis is not a fish of the aquarium trade; the sources available describe a taxonomic and conservation record, not an aquarium history, and it should be assumed unavailable to hobbyists. Where any tube-snouted Sternarchorhynchus does occasionally appear as an incidental import, the general precautions for scaleless gymnotiforms apply: dim, sheltered, well-oxygenated water, great care with medications (no copper-based treatments, and reduced doses of anything else), and tankmates too large to be hunted at night. None of this is a specific recommendation for keeping S. mesensis, which is best regarded as a wild, undomesticated species outside the hobby.
Conservation
Sternarchorhynchus mesensis is assessed as Least Concern on the IUCN Red List. It is known from a defined area of the upper Rio Tocantins basin in Goiás, Brazil, including the São Domingos karst system near the Serra da Mesa dam — a hydroelectric development whose reservoir and altered flow regime are worth noting as a potential local pressure on river- and cave-connected habitats in the region, even though the species is not currently flagged as threatened.
As with many narrow-range Neotropical gymnotiforms, the biggest conservation risk for a species like this is less an acute threat than simple data scarcity: little is published on its population trend, and habitat changes from damming, mining or pollution in a karst river system could affect it well before enough monitoring exists to detect a decline.