Taxonomy & naming
Heterocheirodon jacuiensis was formally described by Luiz Roberto Malabarba and Vinicius Abilhoa Bertaco in 1999, with the holotype a male of 1 in standard length (MCP 23670) collected from a lagoon at the left margin of the Rio Jacuí near the bridge of road BR 290 at Cachoeira do Sul, Rio Grande do Sul, Brazil. The paratype is a female of 1.5 in SL from the same locality. The Catalog of Fishes records the valid name as Heterocheirodon jacuiensis Malabarba & Bertaco, 1999.
The genus Heterocheirodon belongs to the subfamily Cheirodontinae, tribe Cheirodontini, within the family Characidae. The original description distinguished H. jacuiensis from its only congener, H. yatai, on the basis of anal-fin ray count and the relative lengths of the anal-fin base and caudal peduncle. These meristic and morphometric characters remain the most reliable diagnostic tools for separating the two species.
Morphology
Heterocheirodon jacuiensis is a small, slender characid reaching approximately 1.5 in standard length, with the female paratype measuring 1.5 in SL slightly exceeding the male holotype of 1 in SL. The body form is typical of cheirodontine tetras: moderately compressed laterally, with a relatively slender caudal peduncle and a pointed snout.
As with most members of the Cheirodontinae, the species presents the compact, silver-bodied profile associated with schooling freshwater tetras of South America. Diagnostic meristic features — particularly anal-fin ray count and the proportional length of the anal-fin base — separate it from H. yatai; detailed colour pattern information is sparse in available records.
Habitat
The species is endemic to the Laguna dos Patos drainage system in Rio Grande do Sul, the southernmost state of Brazil, with the type locality a lagoon adjacent to the Rio Jacuí near Cachoeira do Sul. It inhabits lentic, still-water environments and is described as benthopelagic, occurring over sandy substrate.
Water conditions at the type locality and surrounds are characteristic of the subtropical lowland plains of southern Brazil: temperatures around 77 °F, with a slightly acidic to neutral pH of approximately 6.5. The Laguna dos Patos drainage is a large, low-gradient fluvial system that drains into a shallow coastal lagoon, providing extensive still-water and slow-flowing habitat suited to small schooling characids.
Feeding
Heterocheirodon jacuiensis is an omnivore, consistent with the feeding habits typical of cheirodontine characids. Small aquatic invertebrates, microcrustaceans, and plant or algal material are presumed components of the natural diet based on the ecology of closely related taxa.
In the aquarium, small omnivorous tetras of this group accept a wide variety of prepared foods including fine-grade flake and micro-pellets, supplemented with small live or frozen foods such as Artemia nauplii, Daphnia, and micro-worms to maintain condition.
Mating
No specific mating behaviour has been documented for H. jacuiensis in the literature. Based on the biology of other Cheirodontini, the species is expected to practise open, egg-scattering spawning without pair bond formation or parental investment. Males likely display schooling behaviour and brief courtship activity proximate to fine-leaved vegetation or substrates suitable for egg deposition.
As with many small characids, spawning activity in the wild is probably influenced by seasonal cues — changes in temperature, photoperiod, and water level associated with the rainfall regime of subtropical Rio Grande do Sul.
Breeding
Breeding details specific to H. jacuiensis are not documented in available sources. The tribe Cheirodontini is characterised by non-guarding, egg-scattering spawners that deposit small, adhesive or semi-adhesive eggs among fine-leaved aquatic vegetation or over substrate, with no parental care; the eggs and fry are abandoned immediately after spawning.
In an aquarium setting, conditioning a group with live and frozen foods and providing fine-leaved plants or spawning mops would represent the standard approach for this type of small characid. Eggs would be expected to hatch within a few days at subtropical temperatures, producing small larvae that require infusoria-grade first foods before graduating to newly hatched brine shrimp.
In the aquarium
Heterocheirodon jacuiensis is rarely encountered in the ornamental fish trade and is essentially unknown in the wider hobby. As a small (1.5 in SL), peaceful, schooling omnivore from subtropical southern Brazil, it would in principle be suited to a cool-temperate or unheated subtropical aquarium — Rio Grande do Sul experiences genuine winters, and the species likely tolerates temperatures considerably below the 77 °F reported in summer records.
For the specialist aquarist interested in the cheirodontine characids of the Río de la Plata and Pampean drainages, a species tank replicating the still, sandy-bottomed lagoon habitat of the Laguna dos Patos drainage — sparse fine-leaved vegetation, subdued lighting, and slightly acidic to neutral water — would be the appropriate approach. The fish's scarcity in collections means husbandry experience is limited; data sparse.
Conservation
Heterocheirodon jacuiensis is assessed as Least Concern on the IUCN Red List, with the assessment recorded via FishBase as dating from 2018. The Laguna dos Patos drainage is the largest coastal drainage system in South America and supports extensive freshwater habitat; the relatively broad availability of suitable still-water environments within that system presumably underlies the LC assessment.
The Laguna dos Patos region of Rio Grande do Sul is subject to ongoing land-use change, including agricultural conversion and urban expansion, which can degrade water quality and alter the hydrology of the shallow lagoons and floodplain water bodies the species inhabits. No specific conservation actions are known to target this species.