Taxonomy & naming
Triportheus guentheri was described by the American ichthyologist Samuel Garman in 1890, and the species epithet honours Albert Günther, the German-born British ichthyologist whose nineteenth-century work at the British Museum catalogued much of the era's Neotropical fish material. It sits in the genus Triportheus, within the family Triportheidae — the keeled, deep-bodied "hatchet-tetras" that were long treated as a subfamily or tribe of Characidae before being split out as their own family, a change reflected in modern authorities such as the Catalog of Fishes (Eschmeyer, California Academy of Sciences); older literature and some aggregators still list the species under Characidae, and that older placement should be read as historical rather than current.
Triportheus is a moderately large genus of similar-looking silver characins distributed across South America, and identification within the group typically rests on fin-ray counts, scale counts and the shape of the anal fin rather than colour pattern, since most species share the same silvery, laterally compressed body plan. Triportheus guentheri is distinguished largely by its restricted range: unlike widespread congeners found across the Amazon, Orinoco and Magdalena basins, it is effectively endemic to the São Francisco River drainage of eastern Brazil.
Morphology
Like other members of its genus, Triportheus guentheri has a laterally compressed, silvery body with a pronounced ventral keel running from the pelvic region back toward the anal fin — the hallmark "hatchet-tetra" build that lets these fish make quick, darting movements near the surface. FishBase records a maximum size of 7.5 in standard length (7.5 in), and specimens examined in dietary and population studies from the São Francisco reservoirs ranged from 2.5–7.5 in SL, indicating that most individuals in the wild population run well under the maximum.
Colouration follows the typical Triportheus pattern: bright silver flanks with a faint dark mid-lateral stripe or sheen, a pale to translucent belly along the keel, and clear to lightly dusky fins. No dramatic breeding colours or markings are documented for this species in the available literature.
Habitat
Triportheus guentheri is endemic to the São Francisco River system in Minas Gerais and neighbouring states of eastern Brazil, one of South America's major river basins but one that drains independently of the Amazon and Paraná systems. Within that drainage it has been documented in the river channel itself and, notably, in several of the large hydroelectric reservoirs impounded along its course — Três Marias, Sobradinho and Itaparica — where it maintains abundant populations well adapted to lentic (still-water) reservoir conditions as well as riverine habitat.
Specific temperature and pH data for this species were not captured in available sources; as a São Francisco basin fish it experiences a warm subtropical-to-tropical climate typical of eastern Brazil, but keepers and researchers should treat precise water-chemistry figures as data sparse rather than assume Amazonian blackwater norms.
Feeding
Dietary studies of Triportheus guentheri from the Itaparica Reservoir on the São Francisco River found aquatic insects to be the predominant prey item across all months sampled, consistent with an omnivorous, insect-forward diet typical of the genus. Plant material and other invertebrates round out the diet, placing the species solidly among the opportunistic omnivores rather than the plant-grazing detritivores or the predatory characins found elsewhere in this family grouping.
In the wild this diet is gathered from near the surface and midwater, where the species' keeled, hatchet-like body shape and shoaling habit suit rapid foraging on drifting insects and terrestrial insect fall — a feeding style shared broadly across Triportheus.
Mating
No courtship behaviour, pairing description or sexual dimorphism has been documented for Triportheus guentheri in the sources reviewed. Data sparse: this is a poorly studied, regionally endemic species outside of fisheries-biology sampling, and no aquarium or field account of spawning behaviour was found.
Breeding
No spawning mode, parental care pattern or hobby breeding reports were found for Triportheus guentheri. Like most Triportheus, it is presumed to be an open-water or scatter-spawner typical of small characins, but this is an inference from the wider genus rather than a documented fact for this species, and it is not reported as bred in aquaria. Data sparse — treat any breeding claim for this species with caution until a primary source is available.
In the aquarium
Triportheus guentheri is not an established aquarium species; no hobby care literature, import records or trade reports were found in the sources reviewed for this account. Its close relatives in Triportheus are occasionally seen as fast, silvery shoaling characins for large, well-covered tanks — they are strong jumpers and need generous swimming space — but hobbyist experience specific to T. guentheri is essentially absent, and this species should be treated as effectively unavailable in the trade rather than assumed to share care parameters with better-known congeners.
Given its modest adult size (under 7.5 in) and reservoir/river habitat, a keeper who did encounter this species would want a spacious, tightly covered tank with open swimming room and other peaceable, similarly sized shoaling fish, following general Triportheus husbandry as a starting point rather than species-specific guidance, since none currently exists.
Conservation
The IUCN Red List assesses Triportheus guentheri as Least Concern, reflecting a stable population within its São Francisco basin range and its documented abundance in the river's major reservoirs. As an endemic restricted to a single, heavily engineered river system — one reshaped by a chain of large hydroelectric dams — the species' fortunes are tied to the health of that basin, but current evidence does not indicate a decline or elevated threat level.
The São Francisco is a heavily dammed and regulated river, and while Triportheus guentheri appears to tolerate reservoir conditions well enough to sustain fishery-relevant population sizes (studies sampling thousands of individuals for weight-length analysis attest to its local abundance), broader pressures on the basin — flow regulation, water quality and habitat change — remain long-term watch items for a species with nowhere else to go.