Taxonomy & naming
Hemigrammus diagonicus was described by Mendonça and Wosiacki in 2011 from specimens collected in the lower Amazon River floodplain near Juruti, in the state of Pará, Brazil. The description was published in Copeia 2011(2):211–215. The type locality is the Juruti floodplain, a seasonally inundated system in the lower Amazon drainage. Eschmeyer's Catalog of Fishes is the authority for the valid name, which remains in Hemigrammus as originally placed.
Hemigrammus is a large, ecologically diverse genus of small Neotropical characids found across much of tropical South America. Within the genus, H. diagonicus is most closely compared to H. silimoni, from which it is differentiated by the shape of the caudal-fin blotch and by morphometric ratios. The species' placement in Characidae follows current Catalog of Fishes usage; tetras of this genus are grouped within the broad family rather than in any of the more narrowly circumscribed characid subfamilies commonly referenced in older literature.
Morphology
Hemigrammus diagonicus is a small, slender tetra with a maximum recorded standard length of 1 in. Like other members of the genus it has a compressed, moderately elongated body with the transparent-to-silvery colouration typical of small Amazonian characids. The most distinctive external feature is the caudal-fin blotch: a diagonal, bar-shaped dark marking confined to the dorsal lobe of the caudal fin, contrasting with the otherwise unpigmented or lightly pigmented fin rays. This diagonal orientation distinguishes the species from H. silimoni, which bears a differently shaped caudal marking.
Beyond the caudal blotch, available data on external pigmentation are sparse for this recently described species. The body is likely silvery to translucent with the lateral stripe and humeral spot arrangement characteristic of many Hemigrammus species, but detailed colour descriptions from preserved or live specimens beyond the type series are not well documented in the accessible literature. Morphometric data used in the original description to separate it from congeners include standard body proportions and fin-ray counts, though specifics were not captured in the sources available.
Habitat
Hemigrammus diagonicus is known from the lower Amazon basin of Brazil, with confirmed records from the Jari River drainage and the Amazonas main channel, concentrated in a relatively narrow latitudinal band roughly between 2°S and 3°S and 55°W to 57°W. The type locality is the floodplain near Juruti, Pará — a seasonally flooded lowland that alternates between shallow lake, backwater, and connected channel conditions with the rise and fall of the Amazon's flood pulse.
The species occupies floodplain habitats and has been documented in floating-meadow communities — the dense mats of aquatic macrophytes (particularly grasses such as Paspalum repens) that colonise the margins and shallows of Amazonian floodplain lakes. These vegetated, shallow environments are characteristic nursery and feeding grounds for small characids. Sympatric species in these floating-meadow communities include other small characids, cichlids such as Acaronia nassa, and further Hemigrammus congeners. Water conditions in lower Amazon floodplains are variable and seasonally dynamic: temperatures are tropical and warm, pH and dissolved-oxygen levels fluctuate with the flood cycle, and small fish adapted to these habitats tolerate considerable variation in water chemistry.
Feeding
No detailed dietary study of Hemigrammus diagonicus is available in the published literature, but the species' ecology and its placement in Hemigrammus allow reasonable inference. Small Amazonian tetras of this genus are generally insectivorous to omnivorous microforagers, taking aquatic invertebrates, small crustaceans, zooplankton, and surface-fallen insects, with some plant material and algae supplementing the diet opportunistically.
The floating-meadow habitats where H. diagonicus has been recorded are productive environments rich in invertebrate prey — chironomid larvae, copepods, cladocerans, and other small invertebrates thrive among the macrophyte roots and stems. The species' trophic level is recorded as 3.2 in FishBase, consistent with a predominantly invertivorous diet with some omnivorous supplementation. In captivity, fish of this size and ecology should readily accept small live and frozen invertebrate foods, dried micro-foods, and quality micro-pellets.
Mating
No specific observations of mating behaviour in Hemigrammus diagonicus have been published. As a member of Hemigrammus and by analogy with other small Amazonian characids, the species is expected to be an egg-scattering open spawner with no parental care. Typical mating behaviour in small Hemigrammus involves brief chasing and parallel-swimming sequences between males and females, culminating in the simultaneous release of eggs and sperm among fine-leaved aquatic plants or other submerged substrate.
In floodplain habitats, spawning in small characids is often associated with the rising flood pulse, when expanding shallow waters provide abundant cover and food for larvae. Adhesive or semi-adhesive eggs are scattered among vegetation, and the adults take no further role in their development. Sexual dimorphism, if present, is likely slight and consistent with the genus pattern of females being plumper when gravid.
Breeding
Hemigrammus diagonicus has not been established in the ornamental fish trade and no captive breeding accounts are documented. Given its small size and floodplain ecology, captive breeding would likely follow the standard approach for small egg-scattering Hemigrammus: a well-conditioned pair or small group spawning over fine-leaved plants or spawning mops in soft, slightly acidic to neutral water, with eggs hatching after a few days at tropical temperatures and adults removed after spawning to prevent egg predation.
The species' restricted known distribution and limited documentation in the hobby mean that practical husbandry data are essentially absent. Any keeper working with this species would be largely pioneering the captive care approach, drawing on protocols developed for closely related and better-studied congeners such as H. bleheri or H. erythrozonus.
In the aquarium
Hemigrammus diagonicus is not an established aquarium species and is not known to be available in the ornamental fish trade. It is a small, recently described tetra from a remote Amazonian floodplain, and there are no hobbyist care accounts to draw on. Were it to become available, its small size (under 1 in SL), floodplain origin, and genus-level ecology suggest it would be suited to a well-planted, quiet nano or species aquarium with soft to moderately hard, neutral to slightly acidic water maintained at tropical temperatures in the 75–82 °F range.
As a schooling characid it would benefit from being kept in a group rather than individually or in pairs. The floating-meadow habitats it occupies in the wild suggest a preference for vegetated, lower-flow environments with diffuse lighting. Tankmates, if any, should be small and non-aggressive. Its rarity in the trade means that any captive specimens would most likely be wild-caught, with all the acclimatisation care that entails for a floodplain specialist.
Conservation
Hemigrammus diagonicus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 07 November 2018. The listing reflects that no major threats to the species have been documented and that floodplain habitat in its lower Amazon range, while subject to ongoing regional pressures, remains broadly intact within the species' known range. The lower Amazon basin, including the Juruti and Jari drainages, does experience pressures from agricultural expansion, bauxite mining (Juruti is a significant mining locality), and water-use changes, but these have not been documented as driving population-level declines in this species specifically.
The species has a limited known distribution confined to a stretch of the lower Amazon floodplain, which in principle increases vulnerability to habitat change compared to species with broader ranges. However, the highly productive and structurally complex nature of Amazonian floodplain systems, and the species' likely tolerance of variable water conditions as a floodplain specialist, are factors consistent with the Least Concern assessment. It is not known to be collected for the ornamental trade in significant numbers.