Taxonomy & naming
Hemigrammus serrazul was described in 2026 by Katiane M. Ferreira, Andressa C. Ribeiro and Fernando R. Carvalho in Neotropical Ichthyology, from specimens collected during field expeditions to the headwaters of the upper río Paraguai basin in the SESC Serra Azul region, Rosário Oeste municipality, Mato Grosso State, Brazil. The holotype, CPUFMT 8654, is 1.44 in standard length, collected from an unnamed stream tributary of the rio Cuiabazinho at 919 ft elevation. Eschmeyer's Catalog of Fishes lists Hemigrammus serrazul Ferreira, Ribeiro & Carvalho, 2026 as the valid name, placed by the Catalog in Acestrorhamphidae, subfamily Pristellinae; this atlas follows the traditional and still widely used placement of the genus in Characidae.
The describing authors note that Hemigrammus, as currently understood, is not a monophyletic genus — a conclusion reinforced by recent molecular and morphological phylogenies — and continues to be diagnosed by a set of non-exclusive traits inherited from early 20th-century classifications, since no updated synapomorphy-based diagnosis has yet been proposed for the group. Hemigrammus serrazul is the sixth species of the genus recorded from the río Paraguai basin, alongside H. flava, H. lunatus, H. machadoi, H. tridens and H. ulreyi, and is placed by the authors within Géry's informal 'H. gracilis group' of unmarked (spotless) Hemigrammus species, which also includes H. erythrozonus and H. mimus.
Morphology
Hemigrammus serrazul reaches at least 1.49 in standard length among the type series (holotype 1.44 in SL). It is distinguished from most congeners by a uniformly coloured flank lacking both a humeral spot and a caudal-peduncle spot — marked only by a slight, narrow longitudinal stripe — combined with premaxillary teeth in two rows (outer row with three to five tricuspid teeth, inner row with five tetra- to pentacuspid teeth), 12–15 pored lateral-line scales (most commonly 15), and 21–25 branched anal-fin rays (most commonly 22). A further diagnostic feature is the presence of small bony hooks at the tips of the lepidotrichia of the second to fourth branched pelvic-fin rays in mature males, hooks that are notably absent from the anal fin.
In preserved specimens the ground colour is light beige, with the snout and head dorsum light brown and a scattering of melanophores across the upper opercle and infraorbitals; the flanks lack humeral or caudal-peduncle spots entirely, showing only the faint longitudinal stripe. In life, body colour is sexually dichromatic — yellowish to reddish in males, silver in females — with a silvery-yellow midlateral stripe and a dark upper iris in both sexes; the caudal, anal and dorsal fins are reddish in males and largely hyaline in females.
Habitat
Hemigrammus serrazul is known from the rio Cuiabazinho, a headwater tributary of the upper rio Cuiabá basin, and three of its own tributaries, all within the boundaries of the SESC Serra Azul Park in Rosário Oeste municipality, Mato Grosso State, Brazil. The site with the highest and most consistent abundance of the species is a first-order stream, a direct left-bank tributary of the rio Cuiabazinho, described as shallow (5.9–19.7 in), with lentic (still) water, a muddy substrate and abundant macrophytes. The Serra Azul is part of the escarped Serrana Province of Mato Grosso, a relief that serves as a watershed divide separating the rio Cuiabá, río Paraguai and río Arinos basins.
Feeding
Stomach-content analysis of six specimens found ants and aquatic insect larvae, indicating that Hemigrammus serrazul is primarily insectivorous, taking both terrestrial prey falling into the water and aquatic invertebrates from its shallow, macrophyte-rich habitat.
Mating
No specific courtship or spawning behaviour was documented for Hemigrammus serrazul in the description. Sexual dimorphism is present: males larger than 1.15 in SL bear small bony hooks along the second to fourth branched pelvic-fin rays (absent in the anal fin and absent in females), and the anal-fin distal margin is straight in males but slightly concave (falcate) in females. As a genus-level generality, Hemigrammus species typically pair briefly and scatter adhesive eggs among fine vegetation with no parental care, but this has not been documented specifically for H. serrazul.
Breeding
No captive-breeding data exist for Hemigrammus serrazul, which is newly described and not known to be present in the aquarium hobby. Based on the reproductive biology documented for other small Hemigrammus of similar body plan and habitat — an egg-scattering mode among vegetation with no parental care, following the genus pattern — a similar approach would likely apply, but nothing has been confirmed by direct observation for this species.
In the aquarium
Hemigrammus serrazul is not known to be present in the aquarium trade; it was only recently described from surveys within a protected park. Its natural habitat — a shallow, still, macrophyte-rich headwater stream with a muddy bottom — suggests it would suit a well-planted, gently filtered aquarium kept as a shoal, in line with general tetra husbandry practice, but no aquarium-specific care information has been documented for this species.
Conservation
Hemigrammus serrazul has not been formally assessed by the IUCN Red List; its status is Not Evaluated (NE). The describing authors, however, explicitly recommend a status of Least Concern, noting that the species has been found at four nearby localities within the SESC Serra Azul Park, a 5,700-hectare area of relatively well-preserved Cerrado, where it is abundant and no imminent threats were identified. The authors highlight the broader biogeographical significance of the Serra Azul escarpment — a zone of neotectonic uplift favouring the isolation of small-bodied fish lineages — and note that the SESC Serra Azul Park plays a central role in protecting the region's biodiversity on the upper río Paraguai plateau.