Taxonomy & naming
Triportheus rotundatus was described by the Scottish naturalist William Jardine in 1841, and its original combination has shifted over time as the classification of the pectoral-keeled characins was worked out; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the currently accepted combination and synonymy. A neotype (specimen MCP 34863, 4 in SL) was subsequently designated from the rio Purus at lago Amapá in Amazonas, Brazil, fixing the application of the name after the original type material became difficult to trace to a precise locality; the original type locality is cited to the river Padauari, an Amazon tributary near the Guiana–Venezuela border.
The genus Triportheus sits within the characin assemblage historically grouped with the pacus and silver dollars, though it is a slender, surface-dwelling lineage quite distinct in build and habit from either group; some regional literature has placed it in its own family-level grouping (Triportheidae) separate from Characidae proper. The species epithet rotundatus ("rounded") refers to the fish's deep, rounded body profile. In the aquarium trade it circulates as the "Catalina tetra," a vernacular name with no taxonomic weight.
Morphology
Triportheus rotundatus has the unmistakable build of a keeled hatchet-tetra: a deep, laterally compressed body with a pronounced ventral keel running from the throat back through a muscular, bulging pectoral girdle, mirroring — on a smaller scale — the same adaptation that powers the true hatchetfishes' leaps clear of the water. The pectoral fins are notably long, reaching back well past the origin of the pelvic fins, and together with the keel musculature they let the fish generate rapid bursts of thrust to skim and skitter across the surface when startled, rather than swim in the conventional sense.
FishBase records a maximum published length of 10.5 in total length, though this is an outlier figure; specimens documented in aquarium and trade contexts typically run around 7 in standard length, with commonly available fish closer to 6 in. Colouration is a bright, undecorated silver typical of open-water Amazon characins, with the deep body and long, sickle-shaped pectorals the most useful field marks separating it from similarly silver tetras and hemiodontids sharing its range.
Habitat
The species is distributed across the Amazon basin, with confirmed records extending to the rio Nanay in Peru and multiple Amazon tributaries in Brazil, including the rio Purus and the Padauari drainage near the Guiana–Venezuela border. It occupies open, flowing lowland Amazon waters rather than densely vegetated or heavily blackwater habitat specifically, consistent with a surface-oriented, actively swimming schooling fish.
Available husbandry and trade sources describe water conditions in the broad range typical of lowland Amazon systems — soft to neutral and mildly acidic, with a preferred pH cited around 6.5–7.2 and a temperature range of roughly 72–82 °F. There is no specific documentation in available sources tying the species to true blackwater chemistry, so its natural water should be read as generalist lowland Amazon rather than an extreme, tannin-stained specialisation.
Feeding
Triportheus rotundatus is an omnivore and a surface feeder in the wild, taking fallen fruit and seeds from the flooded forest canopy alongside insects that land on or fall into the water — the same allochthonous food sources exploited by many other Amazon surface- and mid-water characins during the flooded season. This diet is consistent with its keeled, surface-skittering morphology, which is built for exploiting food (and evading predators) right at the water's surface rather than foraging on the bottom.
In the aquarium it takes standard ornamental fish foods readily and is not a fussy feeder; a varied diet of quality flake or small pellet supplemented with occasional live or frozen insect-based foods reflects its natural omnivory well. As a surface-oriented species it should be offered food that reaches the upper water column, where it spends most of its time.
Mating
No courtship behaviour, pairing, or external sexual dimorphism has been documented for Triportheus rotundatus in the sources available for this account. Aquarium husbandry literature describes the species as group-living and, notably, prone to becoming quarrelsome when kept in insufficient numbers — a social dynamic that matters for conditioning a group but that is not itself evidence of a pairing or courtship display. Data on how this species selects mates or times reproduction in the wild is sparse.
Breeding
Breeding of Triportheus rotundatus has not been documented in the aquarium according to the husbandry sources available for this account, and no aquaculture or hobby spawning reports were found. Given the species' close kinship with other open-water, surface-oriented Amazon characins, an egg-scattering strategy typical of the broader group would be a reasonable expectation, but this has not been confirmed for T. rotundatus specifically and should not be stated as documented fact. Prospective keepers should treat this as an essentially unbred aquarium fish and expect wild or farm-collected stock rather than captive-bred offspring.
In the aquarium
Triportheus rotundatus is a genuinely modest-sized characin for this group — typical aquarium specimens run around 6 in, well short of the pacu-scale warnings that apply elsewhere in this family — but it is an active, surface-dwelling schooler that needs horizontal swimming space and a secure lid, since its keeled, skittering escape response can send a startled fish clear out of an open tank. It should be kept in a real group; husbandry sources specifically note that small numbers make the species quarrelsome, while an adequately sized school settles into calmer, more natural shoaling behaviour.
It is described as peaceful overall but territorial towards its own kind, and it does well alongside catfishes, cichlids and silver dollars as tankmates — fish that occupy different water levels or are too robust to be bothered by occasional squabbling. Water should be kept warm (roughly 72–82 °F), soft to neutral and mildly acidic (around pH 6.5–7.2), with strong surface agitation and open swimming room reflecting its origins in flowing, open Amazon water rather than dense cover.
Conservation
Triportheus rotundatus is assessed by the IUCN Red List as Least Concern, consistent with its wide distribution across the Amazon basin and multiple tributary systems in Brazil and Peru. It faces no documented specific conservation pressure, and its presence in the aquarium trade under the "Catalina tetra" name appears to rely on collection from wild Amazon populations rather than any captive-breeding program, though this is not itself flagged as a conservation concern given the species' broad range and stable status.