Taxonomy & naming
Hisonotus heterogaster was described by Carvalho and Reis in 2011 as part of their comprehensive revision of Hisonotus from the laguna dos Patos system of southern Brazil (Neotropical Ichthyology 9(1):1–48). The type locality is an unnamed stream (arroio sem nome) in the rio Jacuí drainage, Rio Grande do Sul, Brazil (paratype material: LBP 3335), at approximately 29–30°S, 52–53°W. The Catalog of Fishes (Eschmeyer, CAS; species ID 71783) recognises the valid combination Hisonotus heterogaster Carvalho & Reis, 2011.
The species belongs to the subfamily Hypoptopomatinae within Loricariidae. It was diagnosed within the 2011 revision on the basis of its distinctive abdominal plate arrangement — specifically, the absence of a median plate series, which produces the 'different belly' that the species epithet describes.
Morphology
Hisonotus heterogaster reaches a maximum recorded length of 1.5 in standard length (SL), making it slightly larger than H. carreiro and H. chromodontus within the same subfamily. The body is depressed and armoured with overlapping bony plates in the typical Hypoptopomatinae manner, with a ventral sucking mouth adapted for benthic grazing.
The key diagnostic feature distinguishing H. heterogaster from its congeners in the laguna dos Patos system is the arrangement of abdominal plates: the median plate series is absent, producing a distinctly different ventral plate mosaic — the character that gave rise to the name heterogaster. Beyond this, detailed meristic and morphometric data are established in the original description by Carvalho and Reis (2011). No captive observations or photographic documentation from aquarium maintenance are available.
Habitat
The species is known from tributaries of the rio Jacuí drainage, which forms part of the laguna dos Patos hydrological system in Rio Grande do Sul, Brazil. Type material was collected from an unnamed stream at approximately 29–30°S, 52–53°W, in the upland interior of Rio Grande do Sul.
The rio Jacuí is the main river of the laguna dos Patos watershed and drains much of the central interior of Rio Grande do Sul before emptying into the Guaíba estuary. Its upper tributaries run through subtropical highland terrain. Data on water chemistry at the specific collection sites have not been published, and the IUCN's Data Deficient assessment for this species reflects limited distributional and population information even at a basic level.
Feeding
No direct dietary data exist for Hisonotus heterogaster. By analogy with other Hypoptopomatinae and the morphology of its sucking mouth and fine teeth, the species is presumed to graze aufwuchs — the biofilm of algae, diatoms, bacteria, and organic detritus — from hard substrate surfaces in its native streams. This inference is consistent with the feeding ecology established across the subfamily.
Mating
No mating or courtship behaviour has been documented for Hisonotus heterogaster. The species has no captive history, and field observations of reproductive behaviour are absent from the published record. Data are too sparse to offer any species-specific account.
Breeding
No breeding data are available for Hisonotus heterogaster. Neither captive breeding attempts nor field reproductive data have been published. The species' Data Deficient IUCN status extends to basic biological parameters including reproduction.
In the aquarium
Hisonotus heterogaster is not present in the ornamental fish trade and has no documented captive history. No husbandry information can be offered based on the available literature. Its subtropical origin in upland Rio Grande do Sul streams, combined with what is known from the subfamily broadly, would suggest cool, well-oxygenated water with ample biofilm-covered surfaces, but these are inferences from ecology rather than observations from maintenance.
Conservation
The IUCN Red List assessed Hisonotus heterogaster as Data Deficient in November 2018. This assessment reflects insufficient distributional and population data to determine the species' threat status — not an assurance that the species is secure. It is known only from tributaries of the rio Jacuí in Rio Grande do Sul and may have a narrow range.
Rio Grande do Sul's upland stream systems face ongoing pressures from agricultural conversion of native vegetation, sedimentation from soil erosion on cleared slopes, and water extraction. For a narrow-range endemic known from a single drainage and with sparse collection records, these regional pressures are a relevant concern even in the absence of specific population trend data. The Data Deficient category signals that basic inventory and monitoring work is needed before meaningful conservation assessments can be made.