Taxonomy & naming
Hisonotus maculipinnis was described by C. T. Regan in 1912 in the Proceedings of the Zoological Society of London (pt 3, art. 36, p. 668, Pl. 77 fig. 3), from the type locality Rio de la Plata, South America. The original combination was Microlepidogaster maculipinnis; subsequent placement in Otocinclus produced the synonym Otocinclus maculipinnis. The current valid combination Hisonotus maculipinnis is recognised by the Catalog of Fishes (Eschmeyer, CAS) and by FishBase (Ref. 36885 = Burgess, 1989).
The genus Hisonotus belongs to subfamily Hypoptopomatinae within Loricariidae — one of the five subfamilial groupings of the armoured catfishes (Armbruster, 2004). Hypoptopomatinae contains the diminutive, algae-grazing loricariids historically grouped around Otocinclus and its relatives; they differ from the larger Hypostominae (bristlenose plecos, royal plecos, etc.) in their smaller size, more slender bodies, and typically social habits. The genus name Hisonotus derives from the Greek isos (equal) and noton (back), according to FishBase — a reference to the relatively even dorsal profile of the body.
Morphology
Hisonotus maculipinnis is a tiny fish, with a maximum recorded standard length of 1.5 in SL. The body is slender and dorsoventrally compressed — more so than the bulkier Ancistrus or Hypostomus, and closer in gestalt to Otocinclus. The armour of bony scutes covers the flanks as in all Loricariidae, but the overall impression is of a delicate, fine-scaled animal rather than the heavily plated appearance of the larger plecos.
The species name maculipinnis means 'spotted fin' in Latin, pointing to the patterned finnage: the fins bear spots or markings that distinguish it from plain-finned relatives. Body colouration is not described in detail in the available sources, but Hypoptopomatinae typically display cryptic, brownish patterning with spots or reticulations that provide camouflage against leaf litter and biofilm-covered substrates.
Sexual dimorphism is not described for H. maculipinnis specifically. In many small hypoptopomatine species, females are visibly wider-bodied than males when gravid, but detailed dimorphism information is absent from the literature consulted.
Habitat
Hisonotus maculipinnis occurs in the Paraná/Paraguay and La Plata River drainages of southern South America, as confirmed by the IUCN assessment (2020) and consistent with the type locality of Rio de la Plata. This places the species in subtropical river systems — a distinct biogeographic setting from the tropical Amazonian loricariids, with seasonal water temperatures that can drop considerably below typical tropical aquarium ranges.
In nature, small hypoptopomatines such as H. maculipinnis occupy vegetated margins, leaf litter accumulations, and submerged aquatic plant beds in rivers and streams, where they graze periphyton (algae and biofilm) from leaf and stem surfaces. The species is listed as benthopelagic in FishBase — it moves through the water column to reach grazing surfaces rather than remaining strictly on the bottom. The genus as a whole has facultative air-breathing capacity, useful during seasonal low-oxygen periods. Population trend is assessed as stable, with the species occurring across a wide river basin system.
Feeding
Hisonotus maculipinnis is an aufwuchs micro-grazer — it feeds on the thin layer of algae, diatoms, cyanobacteria, and associated organic detritus that coats submerged surfaces in its riverine habitat. The suctorial mouth is adapted for gentle rasping of soft biofilm from leaves, plant stems, rocks, and wood rather than the more forceful excavating of large plecos. This makes the species functionally analogous to Otocinclus in the hobby, though not closely related to it.
In captivity, similar small hypoptopomatines require a continuous supply of algae or algae-substitute foods. Algae wafers, spirulina flakes or tablets, and blanched soft vegetables (zucchini, spinach, cucumber) are suitable. A tank with established algae growth on hard surfaces is beneficial. These fish graze almost continuously and should not be kept in an algae-free, heavily cleaned aquarium where food availability between feedings is negligible.
Mating
No specific mating observations for H. maculipinnis appear in the available literature. The species is shoaling in habit, making it likely that courtship, where documented in the genus, occurs within a group context rather than through strongly territorial pair-bonding. Small hypoptopomatines as a group tend not to show the pronounced cave-guarding male territoriality of Ancistrus and larger plecos; their mating systems are less well studied.
In related Otocinclus and Hisonotus, brief chasing of females by males prior to spawning has been described, with spawning occurring on plant leaves or glass surfaces. Whether H. maculipinnis conforms to this pattern is not documented.
Breeding
No breeding records for Hisonotus maculipinnis are listed on PlanetCatfish, and the broader literature does not document aquarium spawning of this species specifically. General information from closely related hypoptopomatines — including some Otocinclus — suggests that eggs are deposited on flat surfaces (plant leaves, aquarium glass, broad leaves of aquatic plants) and are not guarded by either parent after spawning. The eggs are small and adhesive.
Conditioned adults in a heavily planted aquarium with soft, slightly acidic subtropical-range water and a good algae supply would likely represent the most promising conditions for observing spawning, based on analogies with related species.
In the aquarium
At 1.5 in maximum SL, H. maculipinnis is among the smallest Loricariidae and is suited to relatively modest aquarium volumes. Its social nature — like other small hypoptopomatines, it is a shoaling species rather than a solitary or territorial fish — means it should be kept in groups rather than as a lone individual; a group of six or more will behave more naturally and show less stress-related hiding.
Water conditions should reflect the subtropical La Plata origin: temperature in the range 64–75 °F is appropriate, and the species is likely intolerant of the high tropical temperatures (82–86 °F) suited to Amazonian species. Soft to moderately hard water with a slightly acidic to neutral pH is appropriate. Good filtration and water quality are important, as with all small loricariids.
A planted aquarium with a range of broad-leaved plants (Amazon swords, Anubias, Java fern) on which algae can develop is ideal. The fish should not be expected to perform algae control in the manner of larger plecos — at 1.5 in they are grazers of soft film, not scrapers of thick algae growth. Avoid keeping with large, boisterous, or predatory tankmates. Compatible companions include small tetras, rasboras, small Corydoras, and other peaceful subtropical community fish.
Conservation
The IUCN Red List assessed Hisonotus maculipinnis as Least Concern in November 2020. The species occurs across the Paraná/Paraguay and La Plata river basins — a large, ecologically diverse drainage covering much of southern South America — and no specific threats or population declines have been identified.
Detailed ecology and threat data beyond the LC status confirmation are limited. The Paraná–La Plata system faces habitat pressures from agricultural runoff, damming (the Paraná is among the world's most dammed rivers), and riparian clearance, but these are broad-scale pressures not specifically assessed against H. maculipinnis. The stable population trend and wide basin-scale distribution support the current Least Concern categorisation.