Taxonomy & naming
Phenacogaster tegatus was originally described by Carl H. Eigenmann in 1911 under the name Vesicatrus tegatus, with a type locality at the Rio Jauru in the upper Rio Paraguay basin, Brazil. The type specimen is catalogued as FMNH 54573. The genus Phenacogaster was later recognised as the appropriate placement, and Britski et al. (1999) formally reclassified the species there; Eschmeyer's Catalog of Fishes (CAS) records the current valid combination as Phenacogaster tegatus (Eigenmann, 1911), with the parenthetical authority reflecting the generic transfer.
Phenacogaster is a genus of small characids in the family Characidae distributed across tropical South America. The genus is characterised partly by the presence of a well-developed adipose fin and by the spot or stripe pattern that varies across species. P. tegatus occupies the upper Paraguay drainage and has been confirmed from Argentina, Brazil, and Paraguay, placing it in one of the continent's major river systems rather than in an isolated or restricted range.
Morphology
At a maximum of 1.5 in standard length, Phenacogaster tegatus is a miniature characid with the slender, laterally compressed body form typical of the genus. It carries the standard Characidae complement including an adipose fin, a moderately forked tail, and a small, terminal mouth suited to picking off invertebrates.
The species' most arresting feature is its lateral pattern. Two conspicuous dark spots are positioned along the body — one roughly at mid-flank and a second near the caudal peduncle — in addition to the reflective eye. From the side this array of eye plus two spots reads visually as three eyes per side, the basis for the common name 'six-eyed tetra.' Aquarium Glaser notes that this replication of an eye-spot pattern likely functions as a predator deterrent, deflecting attacks away from the actual head.
Habitat
Phenacogaster tegatus inhabits the upper Paraguay River basin, with the type locality in the Rio Jauru of Mato Grosso, Brazil. Its confirmed range spans Brazil, Paraguay, and Argentina. The upper Paraguay system flows through both the Pantanal wetland complex and adjacent upland tributaries, offering a range of microhabitats from seasonally flooded plains to clear forest streams.
FishBase reports the species in tropical freshwaters at temperatures of 73–81 °F, with Seriously Fish listing a preferred pH of 6.0–7.5 and hardness of approximately 36–215 ppm (roughly dH 2–12). These parameters reflect the variability of the Paraguay basin, where soft, slightly acidic Cerrado streams mix with harder, more productive waters in the lowland floodplain. Phenacogaster tegatus appears to be most at home in well-vegetated, slower-moving stretches where fine-leaved plants and submerged structure provide cover and hunting ground.
Feeding
Phenacogaster tegatus is a micropredator, feeding primarily on small invertebrates and zooplankton in the wild. Seriously Fish characterises the diet as tiny aquatic invertebrates taken from the water column and from vegetation, consistent with the foraging strategy of similarly sized characids that pick off insect larvae, copepods, and small worms at close range.
In the aquarium the species is undemanding and adapts readily to prepared foods. Seriously Fish recommends a diet of high-quality dried foods supplemented with small live or frozen items such as Artemia nauplii and grindal worms. Offering varied foods — micro-pellets or crushed flake alongside occasional live prey — keeps the fish in good condition and encourages natural active behaviour in the school.
Mating
Like the great majority of small characids, Phenacogaster tegatus is an egg-scattering, open spawner with no parental care. Breeding behaviour in the wild is not well documented for this species, but the reproductive pattern typical of the genus involves groups of fish coming together to scatter adhesive eggs among fine-leaved aquatic vegetation, where the eggs attach by short filaments until hatching.
Males likely pursue females through the vegetation during spawning events, and spawning is probably triggered by seasonal water changes or conditioning on live foods in captivity. There is no recorded pair-bonding, and both sexes disperse after spawning is complete.
Breeding
Phenacogaster tegatus has not been widely bred in captivity and detailed breeding accounts for this species specifically are sparse in the published hobby literature. By analogy with other small, egg-scattering Characidae of similar size and habitat, a conditioning pair or small group can be expected to spawn over fine-leaved plants — Java moss, hornwort, or spawning mops — at temperatures in the mid-to-upper range of the species' tolerance (around 79–81 °F).
Eggs are small and adhesive, and adults will consume them if not separated. A well-planted tank or the removal of spawning mops to a separate rearing container gives eggs the best chance. Hatching typically occurs within 24–48 hours at warm temperatures for small characids of this type, and fry require very fine first foods such as infusoria or commercial liquid fry food before graduating to freshly hatched Artemia nauplii. Growth is rapid relative to adult size.
In the aquarium
Phenacogaster tegatus is a peaceful, active schooling fish best kept in groups of at least six to eight individuals, which encourages natural shoaling behaviour and reduces individual shyness. Seriously Fish recommends a minimum footprint of 60 × 12 in for a group, and a well-planted setup with open swimming space suits the species well. Dense vegetation along the sides and back, with a dark substrate and subdued lighting, shows off the spot pattern and keeps the fish settled.
Water chemistry should sit within the species' soft-to-moderately-hard range — pH 6.0–7.5, hardness 36–215 ppm, temperature 73–81 °F. Good filtration and regular partial water changes matter more than chasing precise parameters. Its small adult size makes P. tegatus a natural candidate for nano aquaria and for community tanks with other diminutive, peaceful species. It will not compete well with boisterous or fin-nipping tankmates. The species is offered only on a wholesale basis and may require some searching to source.
Conservation
Phenacogaster tegatus is assessed as Least Concern (LC) on the IUCN Red List, with the most recent assessment dated 4 March 2021. Its distribution across the upper Paraguay basin — spanning Brazil, Paraguay, and Argentina — gives it a reasonably broad range, and no major targeted collection pressure or acute habitat threat specific to the species has been identified.
The upper Paraguay basin and the Pantanal face broader pressures including agricultural expansion, river regulation, and changes to the flood pulse that structures much of the wetland's biological productivity. These regional pressures bear watching for small-bodied characids that depend on vegetated floodplain habitats, even if P. tegatus as a species is not currently considered at risk.