Taxonomy & naming
Australoheros kaaygua was described in 2006 by the Argentine ichthyologists Jorge R. Casciotta and Adriana E. Almirón together with Sergio E. Gómez, in the journal Zoologische Abhandlungen of the Dresden natural-history museum (vol. 55: 77–83). The type locality is the arroyo Ñandú, a small stream in the Río Iguazú basin of Misiones Province, Argentina; the holotype is held in the fish collection of the Museo Argentino de Ciencias Naturales in Buenos Aires under register MACN-ict 8917, with paratypes deposited in La Plata, Buenos Aires and Dresden (MTD F). It was the first species of Australoheros recorded from the Iguazú basin above the Iguazú Falls, and the authors named it for the Ka'aỹgua, a Guaraní group who in the 16th century refused to submit to the rule of the Jesuit Missions. The genus Australoheros itself was erected by Oldřich Říčan and Sven O. Kullander in 2006 to hold the southern South American 'chanchitos' long lumped in the catch-all genus Cichlasoma. Two years later, Říčan and Kullander's 2008 monograph on the Uruguay and Paraná Australoheros redescribed kaaygua in detail from both Argentine and Brazilian material — a paper sometimes mistaken for the original description, but it is a revision that confirmed, not coined, the name. Within the genus kaaygua anchors its own informally named 'kaaygua group' (with the slimmer, red-flushing Australoheros minuano). A taxonomic twist followed in 2011, when Říčan, Piálek, Almirón and Casciotta described Australoheros angiru from the same Río Iguazú drainage: kaaygua and angiru had previously been confused as a single species, but molecular and morphological data show they are two non-sister-group species that share the drainage without being truly sympatric — a reminder that the small cichlids of these streams are more diverse than their similar looks suggest.
Morphology
Australoheros kaaygua is a small, comparatively deep-bodied cichlid: FishBase lists a maximum of 3.5 in standard length (the snout-to-tail-base measure) for a female, so even a well-grown adult is a hand-sized fish of roughly 4.5 in in total length. The body is moderately deep (mean body depth about half the standard length), the snout short and straight in profile, and the mouth small with isognathous, evenly aligned jaws — the upper lip sits horizontally level with the lower edge of the eye, a subtle but diagnostic feature separating it from the lower-mouthed Australoheros charrua. The teeth are all caniniform and slightly curved, increasing in size toward the front of the jaws, the equipment of a generalist grasper rather than a scraper or crusher. Fin and scale counts give the cleanest diagnosis: sixteen (rarely seventeen) dorsal-fin spines, modally seven anal-fin spines, six or seven branched anal-fin rays, modally twelve pectoral rays, about twenty-four E0-row scales, three cheek-scale rows, twenty-six vertebrae, and only five to six gill rakers on the lower limb of the first arch. The species' signature trait is colour: the ground colour is yellowish to yellow or orange, and although many Australoheros are yellowish, the hue is most strongly developed in kaaygua. The flanks carry six dark vertical bars and a midlateral stripe punctuated by a prominent dark midlateral blotch; the caudal fin has red margins and corners (a feature it shares with some Uruguayan Australoheros facetus). In preserved fish the lateral band resolves into two rows of dark-centred scales separated by a narrow light line — the character the describers leaned on. Juveniles are more heavily spotted than adults, with nearly every body scale dark-based, while the dorsal, anal and caudal fins lack the conspicuous spots seen in some look-alikes. Sexual dimorphism follows the genus pattern, made plain in breeding dress: ripe females develop a distinct black blotch in the dorsal fin and, once they begin guarding free-swimming young, lose the yellow ground colour for a pale greyish-white, whereas males keep their colour and grow somewhat larger.
Habitat
Australoheros kaaygua is a fish of clear subtropical streams in the upper Paraná drainage of southern South America. Its core range is the Río Iguazú basin above the Iguazú Falls, spanning Misiones Province in Argentina and the state of Paraná in Brazil, plus tributaries of the upper Río Uruguay in the Brazilian states of Santa Catarina and Rio Grande do Sul; Argentine material is also recorded from the Río Soberbio in the Uruguay drainage. The Iguazú Falls form a real biogeographic wall — many fishes above the falls are found nowhere below them — which is part of why an endemic little cichlid like kaaygua exists there at all. The IUCN puts its extent of occurrence at about 59,348 mi² but stresses that the wider distribution is genuinely uncertain. These are not warm Amazonian lowlands but the cooler, seasonal subtropics of the Atlantic-forest and grassland interior, and the fish is a benthopelagic inhabitant of permanent rivers, streams and creeks — the type and paratype series come from modest arroyos (Ñandú, Deseado, Yacuí, Lobo). Neither FishBase nor the original description tabulates in-situ temperature, pH, hardness or a depth range for the species, so the values used for the temperature, pH and depth tiles here are defensible habitat- and genus-typical figures rather than measured ones: a subtropical-stream temperature band of roughly 64–82 °F (reflecting the seasonal climate above the falls and the cool-tolerance documented across the genus's southern chanchitos), a near-neutral, slightly soft-to-moderate pH of about 6.5–7.5, and a shallow small-watercourse depth of about 0–7 ft. All three are flagged as inferred; the genuine field chemistry of this fish remains unpublished.
Feeding
No dedicated diet study has been published for Australoheros kaaygua, but its trophic role can be read confidently from its anatomy and its relatives. FishBase places it near a trophic level of 3.2 — the band of a small omnivore that leans toward animal prey. Its caniniform, slightly recurved teeth, enlarging toward the front of the jaws, and its small terminal mouth are the toolkit of a benthic generalist that picks and grasps small prey rather than rasping algae from rock or crushing snails. Like other chanchitos it works the stream bottom and the margins of submerged structure, taking aquatic insect larvae and other small invertebrates along with incidental algae, plant fragments and detritus gathered while foraging through the substrate. In its clear upper-Iguazú and upper-Uruguay streams it functions as a mid-level benthic omnivore — neither a specialist predator nor a committed herbivore — and as such is a minor but typical member of the small-stream fish community above the falls.
Mating
Reproductive behaviour has not been described for Australoheros kaaygua in the wild, but the genus is consistent enough, and captive observations of the species exist (Staeck, who photographed and kept it, reported on its behaviour and spawning in aquaria). Australoheros are pair-forming, territorial substrate spawners: a male and female establish and jointly defend a territory and intensify into the genus's breeding livery — the dark vertical bars interrupted across their upper portion between the gill cover and the midlateral blotch, while the yellow interbar spaces stay bright all the way down to the belly. In kaaygua the female's onset of breeding condition is marked by a black blotch that appears in her dorsal fin. As spawning nears, pairs become markedly more site-attached and aggressive toward intruders, conspecific or otherwise. As with related southern chanchitos, the natural trigger is seasonal: the cooler subtropical winter followed by spring warming and rising water brings fish into condition, the cue keepers reproduce with a cool rest period followed by a temperature rise.
Breeding
Australoheros kaaygua is a biparental, open substrate spawner in the manner of its genus. No clutch count specific to kaaygua has been published, so the figure given here is labelled as genus-typical, drawn from the well-documented breeding of relatives such as the original chanchito Australoheros facetus, which lay a few hundred adhesive eggs (commonly on the order of 200–500) on a cleaned firm surface — a flat stone, a root, or the floor of a shallow pit. Both parents share the work, but in the documented division of labour for this fish the female does most of the direct care: she tends and fans the eggs and then guards the free-swimming fry, while the male holds the wider territory. The female's yellow ground colour persists until the fry become free-swimming, at which point she switches to a pale whitish-grey guarding dress — a colour change typical of cichlasomatine cichlids in which the female carries the bulk of brood defence — while the male usually keeps his colour as he helps shepherd the young. Spawning is cued by the seasonal warming that follows the cool subtropical winter, and the fry, once free-swimming, are not difficult to grow on small live and prepared foods under parental guard.
In the aquarium
Australoheros kaaygua is rarely encountered in the hobby outside of specialist cichlid circles, and most practical guidance must be drawn from the well-documented care of its closest relatives — the small Uruguayan and Argentine chanchitos, especially Australoheros facetus and Australoheros minuano. Where species-specific keeper reports exist (Staeck photographed and bred the fish in captivity), they confirm that kaaygua behaves much as expected for a cool-tolerant, small-stream chanchito, and there is no reason to think its requirements differ markedly from the genus norms described below. The following should be understood as representing genus-typical care for small Australoheros rather than verified kaaygua husbandry data.
A pair of adults at roughly 4–4.5 in total length can be housed in a tank of about 25–30 US gal (roughly 35.5 in length), though a larger footprint makes territorial management noticeably easier, particularly around spawning. Because this is a fish of clear subtropical streams above the Iguazú Falls — a seasonally cool, geographically elevated drainage — it is one of the more temperature-flexible cichlids a keeper is likely to own. It tolerates conditions in the range documented for the habitat: roughly 64–82 °F, with 68–77 °F a comfortable everyday band. Cooler water through a rest period in winter (down to around 64 °F) is not merely tolerable but recommended as a natural cue for the breeding cycle. Heating is not strictly necessary in warm-temperate climates. Water chemistry should be near-neutral to mildly acidic: pH 6.5–7.5, soft to moderate hardness, with good oxygenation and regular partial water changes the most important husbandry variable. These fish come from clear streams and do not thrive in chronically dirty or stagnant water.
The aquascape should reference the fish's natural stream habitat: a sand or fine gravel substrate, smooth river stones of various sizes, a few pieces of driftwood, and robust rooted or potted plants along the margins. Flat stones and broad-leafed plants provide potential spawning sites and should be offered deliberately. Chanchitos are diggers, particularly during breeding preparation, so plants anchored in pots rather than bare substrate fare better. Subdued to moderate lighting suits the fish well. Strong filtration with a gentle return current is preferable to high-flow turbulence; these are stream fish, not torrent inhabitants.
For tankmates, the guiding principle is that Australoheros kaaygua — like all Australoheros — becomes a determined and potentially aggressive territory holder once a pair has bonded and spawning is imminent or underway. In a community setting before breeding, it is compatible with comparably sized, robust South American fish that share the cool-subtropical temperature range: small characins, loricariid catfish, and other non-cichlid benthos workers. Conspecifics other than the paired partner should be removed or given ample space and visual breaks. Mixing with other cichlid pairs in small tanks rarely ends well once spawning starts. Keeping a single pair in a species tank or a lightly stocked South American community is the most reliable approach.
Breeding follows the open-substrate pattern described in the Breeding section, and pairs that have settled and been given a cool rest period followed by gradual warming generally spawn without much further prompting. The main keeper mistakes with this species and its relatives are: keeping the water too warm year-round (which skips the seasonal cue and leaves fish in perpetual breeding dress without actually spawning), neglecting water quality (chanchitos tolerate poor conditions longer than many cichlids but fertility and fry survival drop markedly), and housing the pair in a tank too small to allow the male's territory-holding function without persistent harassment of the female. Fry are not difficult once free-swimming and will accept Artemia nauplii, finely crushed flake, and similarly small prepared foods under steady parental guard.
Conservation
Australoheros kaaygua is listed as Data Deficient on the IUCN Red List (assessed 4 November 2020 by R.G. Frederico; assessment published 2023). That category is not a verdict of safety but an admission of ignorance: the assessment notes that the species' exact range, its population size and the threats acting on it are all poorly known, that it 'appears to be uncommon,' and that further research on its distribution, ecology and population status is needed. It carries no CITES listing and is of no fishery interest. The fish is not a pinpoint endemic — it spans the Iguazú basin in two countries plus the upper Uruguay tributaries — but its extent of occurrence is modest and uncertain, the profile of a fish that could prove either secure or quietly vulnerable once properly surveyed. The pressures on its habitat are the familiar ones of the upper Paraná and upper Uruguay: the Iguazú and Uruguay drainages have been heavily dammed for hydropower (the upper Iguazú alone carries a cascade of large reservoirs), and the surrounding Atlantic-forest and grassland country has been widely cleared for agriculture, bringing siltation, riparian loss and agro-chemical run-off into the small clear streams this cichlid favours. None of these is documented as an active threat to kaaygua specifically, but they are the backdrop against which any future, better-informed assessment of a small, range-restricted stream cichlid above the Iguazú Falls should be read.