Taxonomy & naming
Australoheros ykeregua was described in 2011 by Oldřich Říčan, Lubomír Piálek, Adriana Almirón and Jorge Casciotta in Zootaxa (volume 2982, pages 1–26), in a paper that erected two new species at once — Australoheros ykeregua from Argentina and Australoheros angiru from Brazil — and used morphology together with molecular phylogenetics to take stock of the whole genus. The holotype, an adult of 4 in standard length, is catalogued at the Museo Argentino de Ciencias Naturales in Buenos Aires as MACN-ict 9467; it was collected in December 2007 from the arroyo Paraiso (also called Canal Muerto), a small tributary of the río Uruguay in Misiones province. A series of paratypes from neighbouring streams ranges up to about 5.5 in SL. The genus Australoheros — erected by Říčan and Kullander in 2006 for the southern "chanchito" cichlids long lumped under Cichlasoma — sits in the subfamily Cichlinae, tribe Heroini, the great radiation of Central and South American substrate-spawning cichlids. Within that genus, ykeregua belongs to the inland Uruguay–Paraná lineage rather than the coastal Atlantic species of southeastern Brazil. Its closest relative is Australoheros forquilha, described from the rio Uruguai drainage in Rio Grande do Sul, Brazil; the two had been treated as a single species (the informal "Australoheros sp. Forquilha") until fresh 2007 material showed the Argentine and Brazilian populations were morphologically and genetically distinct. The describers proposed that the barrier separating them is the Salto Moconá itself, the long lava-ledge waterfall on the río Uruguay just below the confluence with the río Pepirí Guazú — Australoheros ykeregua holds the lower, Argentine reaches, Australoheros forquilha the upper Brazilian ones, with no gene flow across the falls. Earlier authors had recorded the fish under provisional labels — "Cichlasoma cf. tembe" in Casciotta, Körber & Stawikowski (2003) and Stawikowski & Werner (2004) — that now fall to ykeregua.
Morphology
This is a modestly sized cichlid: FishBase gives a maximum of 5.5 in standard length, and the largest type specimen measured about 5.5 in SL, so an adult is a palm-sized fish rather than a tankbuster. The body is the compact, fairly deep, laterally compressed shape typical of Australoheros — body depth averages about 45% of standard length — but ykeregua is actually one of the slenderer, narrower-headed members of the genus, with head width less than half the head length. The mouth is subterminal with notably thick, fleshy lips, the lower jaw a touch shorter than the upper. Fin counts run to roughly XV–XVII dorsal spines with 9–11 soft rays and 5–6 anal spines with 7–9 soft rays. The standout features are in the colour, and they are diagnostic. Living fish carry a single interrupted line of opalescent pale blue dots running below the eye along the lower edge of the suborbital bones; a checkerboard of red dots scattered across the interradial membranes of the dorsal, anal and caudal fins; and a wash of red to orange over the branchiostegal membrane, the mouth and lower head, and the base of the pectoral fin. Over the flanks sit six or seven broad vertical bars, a dark mid-lateral blotch centred on the fourth bar, a bar across the caudal peduncle, and a spot at the base of the tail — the standard Australoheros markings — overlaid with many fine horizontal stripes most visible on the lower body. The near-identical sister species Australoheros forquilha is told apart at a glance because in that fish the blue dots are not confined to a line under the eye but cover the whole head and every body scale, and the red spreads back across the entire belly to the end of the anal fin rather than staying on the head. Sexual dimorphism has not been formally documented for this species; in Australoheros generally, males grow somewhat larger and develop more produced, pointed dorsal and anal fins, while ripe females are rounder-bodied and the sexes intensify their barring and colour when breeding.
Habitat
Australoheros ykeregua is a narrow-range endemic, known only from Argentina — from a cluster of small tributaries of the río Uruguay below the Salto Moconá, in the southern interior of Misiones province. The streams that hold it have been named in the literature: arroyos Fortaleza, Paraiso, Shangai (Pindaiti), Guerrero, Tamandua and Soberbio, plus the Yabotí system. This is upper Río de la Plata / río Uruguay country at the edge of the Atlantic Forest. The high and middle reaches where the fish lives are described as crystal-clear waters running in narrow channels through deep basalt valleys, where rapids, pools and small falls alternate, shaded by a narrow gallery forest of jungle trees that drops food into the water and provides cover and spawning structure. The in-situ water chemistry is genuinely surprising for a cichlid a hobbyist might expect to want soft, acidic, tropical water: limnological measurements from the arroyo Paraiso (Araya 2017, as cited in the IUCN assessment) gave a water temperature of 66–77 °F, a near-neutral to mildly alkaline pH of 7.3–7.9, low conductivity of 15–38 ppm, and high dissolved oxygen of 7.4–9.6 mg/L. The relatively cool, subtropical temperature regime and the oxygen-rich, neutral-to-hard water reflect this fish's place in the temperate southern reaches of the cichlid family, not the warm blackwaters of the Amazon. No published depth band exists for the species; given that it is confined to these shallow, fast, gallery-forest streams, the working figure here of roughly 0–7 ft is inferred from that stream habitat rather than measured, and is flagged as such in the facts block.
Feeding
There is no published gut-content or trophic study specific to Australoheros ykeregua, so its diet is reconstructed from its genus and habitat rather than from direct observation — a point worth stating plainly. Australoheros are generalist benthic omnivores: the well-studied congener Australoheros facetus is described as an opportunistic diurnal feeder taking detritus and plant material and preying on small aquatic animals including the occasional small fish, with an estimated trophic level around 3.3, and FishBase places ykeregua at a comparable level near 3.2 based on its size and close relatives. In a clear, gallery-forested stream the realistic menu is aquatic insect larvae and other small invertebrates, crustaceans, worms, plant matter and the detritus and allochthonous material (leaf litter, terrestrial insects) that the overhanging forest delivers — a flexible, forage-as-you-go strategy rather than any narrow specialization. The thick, fleshy lips shared with Australoheros tembe may help it work prey from crevices and rocky surfaces. Ecologically it sits as a mid-level invertivore-omnivore within a stream fish community whose structure, the regional literature notes, is shaped strongly by that fringing gallery forest.
Mating
The courtship and pair behaviour of Australoheros ykeregua have not been described from the wild or, as far as the published record goes, from aquaria — another genuine gap. What can be said is grounded in the consistent biology of Australoheros and the broader Heroini. These are not harem or mouthbrooding cichlids; they are monogamous, biparental substrate spawners that form a pair bond for the duration of a breeding cycle, with both sexes becoming territorial and defending a chosen spawning site. In the closely studied Australoheros facetus, fish pair off at spawning time and both members hold and defend the territory together. Colour change is part of the ritual: breeding Australoheros typically darken their vertical bars and intensify their reds and yellows, and in this species the red-orange head wash and blue suborbital dots are the likely signalling features. As with most South American substrate spawners, expect site selection and cleaning of a hard surface, mutual display, and escalating defence of the nest area as the pair commits — behaviour inferred here from the genus, since species-specific accounts are lacking.
Breeding
No breeding account exists for Australoheros ykeregua specifically; what follows is the genus-typical pattern, flagged as such. Australoheros are open, biparental substrate spawners — the opposite of the cave-hiding or mouthbrooding strategies seen elsewhere in the cichlid family. The model species Australoheros facetus is an easy open spawner that deposits its eggs on an exposed hard surface such as a flat stone or a piece of submerged wood; both parents then guard and fan the eggs and shepherd the larvae and free-swimming fry, with parental care lasting on the order of one to three weeks. Fecundity in the genus runs to a few hundred up to roughly a thousand eggs depending on female size, so a clutch of several hundred is the reasonable expectation for a fish of ykeregua's modest dimensions; the specific clutch size for this species is not documented. In Australoheros facetus, spawning is triggered as water warms past about 82–86 °F, which — read against ykeregua's cooler 66–77 °F home streams — suggests a seasonal, warm-season spawning window for the wild population. The species' Vulnerable status, its restriction to a few small streams, and the near-total absence of it from the aquarium literature mean this is a cichlid whose reproduction is essentially undocumented and would be genuinely worth recording.
In the aquarium
Australoheros ykeregua is a genuine rarity in the hobby — as of this writing, it has not established any foothold in the ornamental trade outside Argentina, and published aquarium accounts are essentially absent. Care notes here are drawn from the broader Australoheros genus, in particular the closely related and more frequently kept Australoheros facetus and Australoheros forquilha, adjusted for ykeregua's documented wild chemistry and subtropical provenance. Anyone keeping this species would be among the first to do so in a systematic way, and notes from any breeding attempt would be a genuine contribution.
A single pair or a small group of three to five fish is comfortable in a 120-litre (roughly 120 × 40 × 15.5 in) aquarium; a larger footprint of 40 US gal or more is preferable if breeding is intended, since the pair requires a defended territory and tankmates need room to withdraw. The substrate should be fine sand or fine gravel — the species forages at the bottom, and coarse substrate can damage the thick lips. Furnish the tank with flat stones and smooth rocks for potential spawning sites, a scattering of driftwood, and some hardy background planting (Vallisneria, Sagittaria or potted species if the fish uproot them), which replicates the structured gallery-forest stream habitat. Strong filtration and good oxygenation matter: the wild streams run clear, fast and well-oxygenated (7–9 mg/L DO), and the species will not thrive in warm, still, low-oxygen conditions.
Water chemistry should stay in the range the species occupies in the wild: temperature 66–77 °F, pH 7.3–7.9, and low to moderate hardness reflecting the soft but not acidic basalt-stream water (conductivity of 15–38 ppm in the wild suggests very soft water despite the neutral-to-slightly-alkaline pH). This is a critically important point for keepers used to South American cichlids in general: do not keep ykeregua in warm Amazonian conditions. The 66–77 °F window makes it compatible with unheated or minimally heated tanks in temperate homes during the warmer months, and it is almost certainly intolerant of sustained temperatures above 81–82 °F. Cooler, oxygen-rich water should be the target, not the compromise.
In terms of tankmates, Australoheros ykeregua is a moderately assertive cichlid — chanchitos are not brawlers by cichlid standards, but a breeding pair will hold and defend territory vigorously. Peaceful or mid-water community fish that can tolerate the same cool, neutral conditions and will not compete for bottom territory are the best candidates: medium-sized tetras, larger Corydoras species, or similarly sized subtropical schoolers. Avoid slow-moving ornamental fish that cannot hold their own against cichlid aggression, and avoid other bottom-dwelling or cave-claiming species that will contest territory. The fish is not known to be a dedicated plant destroyer in the Australoheros genus, though some digging at spawning time should be expected.
The most common keeper mistakes with subtropical chanchitos are keeping them too warm (inadvertently placing them in a tropical community tank running 82–86 °F), neglecting oxygenation in favour of temperature, and underestimating the pair's territorial needs at spawning time. Feed a varied omnivore diet — quality sinking pellets, frozen or live invertebrates, occasional vegetable matter — which mirrors the generalist benthic foraging the species undertakes in the wild. Breeding follows the standard biparental open-substrate pattern of the genus: the pair will clean a flat stone, deposit and fertilise several hundred eggs on the surface, and defend the site together, fanning the eggs and subsequently herding free-swimming fry for one to three weeks. Raising fry presents no special difficulty beyond that of any medium-sized substrate-spawning cichlid, though the very limited existing population outside its home range means any captive reproduction is of genuine conservation value.
Conservation
Australoheros ykeregua is listed as Vulnerable on the IUCN Red List under criteria B1ab(iii)+2ab(iii) — assessed by Jorge Liotta on 20 November 2020 and published in 2022. The listing turns on how little range the fish has: an extent of occurrence estimated at about 17,460 mi² but an area of occupancy under 311 mi², spread across only six locations, with a continuing decline in habitat quality. The driver is land use. The basalt-valley streams of southern Misiones are hemmed by patches of agriculture, above all tobacco cultivation, and the assessment singles out the misuse of agrochemicals — agrotoxins causing water pollution, wildlife die-offs and watercourse degradation — alongside dams and water management, sedimentation from soil erosion, and small-scale harvesting (the species is taken in a small way for the ornamental trade nationally). Protection is patchy: the Yabotí and Fortaleza streams fall within a biosphere reserve and El Soberbio is marginally protected, but the Paraiso, Shangai, Guerrero and Tamandua basins have no protection at all, and some have been flagged as needing "urgent protection and restoration." The recommended actions are habitat protection, management of the ornamental harvest, and basic research into a poorly known fish. Placed in its wider setting, ykeregua is a small-range Atlantic Forest stream endemic in one of South America's most fragmented and agriculturally pressured landscapes — the kind of narrowly distributed cichlid for which catchment-scale forest and water protection, not species-level measures, will decide its future. Unlike the widespread, weedy Australoheros facetus (assessed Least Concern and even invasive in parts of Europe), this neighbour has nowhere else to go.