Taxonomy & naming
Australoheros guarani was described by the Czech ichthyologist Oldřich Říčan and the Swedish cichlid specialist Sven O. Kullander in 2008, in their monographic revision of the Australoheros species of the Uruguay and Paraná River drainages (Zootaxa 1724). The original combination is the one still in use; there are no junior synonyms. The holotype, an adult of 5 in standard length, is held at the Muséum d'histoire naturelle de Genève as MHNG 2237.58, with paratypes split between Geneva (MHNG 2237.56, 2582.98) and the Swedish Museum of Natural History in Stockholm (NRM 33498, 42215). The type locality is the Río Guyray-gua, a stream in the Río Paraná drainage reached at kilometre 197 of the old Asunción–Puerto Stroessner (now Ciudad del Este) road, in the department of Caaguazú, Paraguay — part of the Monday River basin. The genus Australoheros, erected by Říčan and Kullander in 2006, is the southernmost group of heroine cichlids and the only one living entirely south of the Amazon, spread across the La Plata system (the Paraná, Paraguay and Uruguay rivers) and the coastal drainages of southern Brazil and Uruguay. Within that genus, molecular work based on the mitochondrial cytochrome b gene places Australoheros guarani in the facetus species group and, more tightly, in a clade of three Middle Paraná endemics — Australoheros guarani, Australoheros tembe and Australoheros kaaygua — each isolated above a waterfall on its own tributary. (Note: Australoheros guarani is sometimes loosely filed under the 'upper Paraná'; the primary literature and the IUCN assessment both place it specifically in middle Paraná tributaries, and it should not be confused with its Argentine sister Australoheros tembe, which the Catalog of Fishes lists from the upper Tembey/arroyo Urugua-í.) In the older aquarium literature the whole facetus complex circulated under Cichlasoma; that name is no longer valid for these fish.
Morphology
This is a small cichlid: the largest measured specimen, the holotype, is 5 in in standard length, and FishBase carries no larger record. Its single most distinctive feature is shape — Australoheros guarani is the most deep-bodied species in the entire genus, with a body depth exceeding roughly half its standard length, a steep forehead and a notably short caudal peduncle. It also carries some of the lowest fin-ray and vertebral counts in the genus (dorsal with about 15–17 spines and 8–10 soft rays, anal with 6–7 spines and 7–8 soft rays, 26 vertebrae of which only 13 are caudal) and the largest body scales, with just 2½ scale rows between the dorsal fin and the upper lateral line where most congeners have 3½. The flank markings are understated for a chanchito: a small, often inconspicuous midlateral blotch and, when present, a straight midlateral stripe running low along the side. Living fish — first photographed only in 2016, nearly a decade after the species was named — show a pattern unique among Australoheros: each scale on the body bears a short orange to dark-yellow vertical bar at its base, set against a whitish ground that warms to yellow in adults. The lower head, pectoral fins and the soft parts of the median fins are yellow to orange, the pelvic fins and fin spines carry a violet tint, and the rear of the caudal fin and the tip of the soft dorsal are red; the iris is red to violet, ringed in yellow. Breeding colours have never been observed, but by analogy with virtually all heroine cichlids they are expected to involve a darkening of the black markings against a paler body.
Habitat
Australoheros guarani is a narrow-range endemic of eastern Paraguay, recorded only from two tributaries of the middle Paraná — the Monday River (the type locality) and the Tembey River — which lie in separate basins separated by intervening drainages and, critically, by waterfalls that isolate each tributary from the Paraná channel itself. The 45-metre Monday Falls in particular form a hard barrier to upstream fish movement. The species is genuinely uncommon and poorly collected: the type series came with almost no habitat data, and for years there were no field observations at all. The one well-documented capture, by Říčan and Říčanová in September 2016, is revealing. The fish were taken not in the clear, rocky, running streams that dominate the region and that suit some of their relatives, but in a quiet side-lagoon roughly 131 ft long and 20–33 ft wide, connected to a Tembey tributary only by a small, shallow overflow channel, at about 837 ft above sea level. Seven fish were netted there in minutes, while repeated, much harder sampling of the adjacent rocky-and-muddy stream pool over two visits years apart produced none. The collectors concluded that stagnant or slow-flowing, shallow, soft-bottomed lagoons are probably the species' typical habitat — a reading reinforced by its lentic-adapted body plan and echoed by the IUCN assessment, which lists it from shallow, stagnant side-lagoons off main streams. No in-situ temperature, pH or hardness measurements have been published for Australoheros guarani; the collection depth where the fish were caught was about one metre. Given the subtropical lowland setting and the chemistry typical of this part of the Paraná basin, the water it lives in is most likely warm and roughly neutral, but those specific figures remain genuinely unmeasured rather than reported (see the fact box notes).
Feeding
No gut-content or feeding study has been published for Australoheros guarani specifically, so its diet has to be read from its genus and its build. Australoheros are generalized heroine cichlids — opportunistic omnivores that root through soft substrate and pick at the bottom for small invertebrates, plant material and detritus, in the manner that earned the group its 'chanchito' (little pig) nickname for the way the fish uproot plants while foraging. FishBase places Australoheros guarani at an estimated trophic level of about 3.2, consistent with a mid-level invertivore-leaning omnivore rather than a specialist predator or a dedicated herbivore. Its deep body and short caudal peduncle are the hallmarks of a slow-water fish that manoeuvres around structure rather than chasing prey in current, and the soft-bottomed lagoons where it has been found would offer exactly the chironomid larvae, micro-crustaceans and organic litter such a forager exploits. This trophic placement should be treated as inference from genus biology and FishBase's model estimate, not as a measured result.
Mating
The courtship and pair-forming behaviour of Australoheros guarani has not been observed or described — the species has never, as far as the published record shows, been spawned in captivity or watched breeding in the wild. What can be said comes from the well-documented biology of its close relatives. Australoheros are monogamous, biparental cichlids that form pairs at breeding time, with both sexes becoming territorial and, in the chanchito tradition, distinctly pugnacious around a nest site; keepers of congeners such as Australoheros facetus and Australoheros oblongum consistently describe pairs that fight 'like young pigs' and harry tankmates or each other when in breeding condition. Sexual dimorphism in the genus is modest, with males tending to grow somewhat larger and develop slightly more extended fins, but no sex-specific differences have been documented for Australoheros guarani itself; the present account treats its dimorphism as genus-typical and unconfirmed. In relatives, spawning is triggered by a seasonal warming and by clean, slightly acidic water — the kind of cue a lowland Paraguayan backwater would deliver at the start of the warm, wet season.
Breeding
There is no published account of reproduction in Australoheros guarani, in the wild or in aquaria, and breeding colours have never been seen. The species is confidently inferred to be a biparental substrate spawner because every studied member of the genus is one. In the chanchito group, a pair selects and cleans an open surface — a flat stone, a submerged root, a broad leaf or, in tanks, a clay pot — and lays an adhesive clutch on it, typically in the range of a couple of hundred to around five hundred eggs in well-grown fish. Both parents then guard the eggs and, after they hatch, the wriggler and free-swimming fry, moving them between pits and defending them aggressively; hobbyists keeping Australoheros oblongum and similar species report attentive, capable parents that may, however, turn on the brood once they prepare to spawn again. Clutch size, egg and larval development times, and parental-care duration for Australoheros guarani are unknown and the figures given here are explicitly genus-typical values rather than measurements of this species. Should the fish ever enter the hobby in numbers, the standard chanchito triggers — large water changes, a few degrees of warming and a richer live-food diet — would be the rational starting point.
In the aquarium
Australoheros guarani is effectively unknown in the hobby. As of the mid-2020s there is no record of the species being kept, imported commercially, or bred in captivity; it has never appeared in a European or North American livestock list. Any account of its aquarium care is therefore built on what is well established for its near relatives — particularly the common chanchito Australoheros facetus, the closely related Australoheros oblongum, and the genus as a whole — and should be read as reasoned extrapolation rather than species-specific experience.
A pair of adults, which top out below 5 in standard length, would likely manage in a 25–30 US gal aquarium, but a 150-litre or larger tank is a safer choice for two reasons: the species comes from stagnant, shallow lagoons, so it expects stable, well-oxygenated water without strong flow, and the chanchito breeding temperament means a pair can become viciously territorial — toward each other as much as toward other fish — when conditioning or guarding a brood. A roomy footprint with good sightline-breaking is more valuable than raw volume. The substrate should be fine sand or rounded gravel to allow the natural bottom-rooting behaviour, and the decor ought to include several potential spawning sites: flat stones, upturned clay pots and sturdy pieces of driftwood all work. Hardy, low-light plants such as Anubias or Java fern, anchored to decor rather than planted loose, can be left in, but any plant simply pushed into sand will be excavated; fine-leaved or soft-stemmed species will be shredded.
Water chemistry need not be extreme. Based on the Paraná drainage origin and the parameters inferred for the type-locality lagoons, the fish should tolerate a pH of 6.0–7.5 and a temperature of 72–82 °F. Australoheros are famously cool-tolerant for tropical cichlids — Australoheros facetus survives down to roughly 59 °F in southern European feral populations — so Australoheros guarani is likely equally hardy in the cold direction, and a room-temperature subtropical setup without a heater may be workable in mild climates. Hardness is not a priority: soft to moderately hard water appears accepted across the genus. Regular water changes of 25–30 % weekly are, as always, more important than hitting an exact chemistry target. Filtration should be adequate but flow gentle, to replicate the still-water lagoon conditions this species naturally inhabits.
Suitable tankmates are limited. Given the pair aggression typical of Australoheros at breeding time, the safest approach is to keep the fish as a species-only pair or in a very large tank where robust, fast-moving dithers — larger tetras, robust barbs, or similarly sized non-cichlid fish that can escape harassment — occupy the upper and mid water while the pair claims the bottom. Keeping multiple pairs in the same tank, or pairing Australoheros guarani with comparably sized, similarly tempered cichlids, risks uncontrolled fighting. The fish is not a community-tank candidate. Bottom-dwelling catfish that compete for the same floor space, or any fish small enough to be eaten or harassed to death, should be excluded.
The main keeper mistakes recorded for genus-level husbandry apply squarely here. Overestimating compatibility is the most common: a pair that is peacefully co-existing today can attempt to kill each other within hours of coming into breeding condition, and a divider or second tank should be on hand. Underestimating the root-and-dig drive is another; any aquascape will be redecorated, and fighting this tendency by jamming plants back in is futile. Finally, given the species' conservation status (Near Threatened, very small range in eastern Paraguay) and its complete absence from the ornamental trade, any individual that does reach a hobbyist almost certainly came via scientific collection or a specialist breeder — responsible record-keeping and a willingness to share offspring with other specialist keepers is strongly encouraged.
Conservation
The IUCN Red List assesses Australoheros guarani as Near Threatened (criterion B1a), in an assessment by H.S. Vera-Alcaraz with a date of 3 February 2021 and a publication year of 2023. The basis is its very small range: the documented extent of occurrence is only about 7,509 mi² and the area of occupancy roughly 5,92 mi², across at most three locations in the Río Monday and Río Tembey tributaries of the Paraná in Paraguay. That range sits squarely inside one of South America's most intensively farmed landscapes. The dominant pressure flagged by the assessment is the spread of soybean and other agro-industrial monoculture, together with the land-use change, road building, water abstraction and small irrigation dams that accompany it, and the agricultural runoff — sediment, fertilizer and pesticides — that follows. Whether these are presently driving a population decline is unknown, because there is essentially no monitoring of the species; the Red List explicitly calls for basic research into its taxonomy, population, ecology and threats. The fish is taken in the ornamental trade, as most cichlids are, but at the national/international level rather than as a documented population-level threat. The broader context is the eastern Paraguayan Atlantic-forest frontier, where the conversion of native forest and gallery vegetation to cropland has fragmented and degraded exactly the small, shallow lagoon habitats this cichlid depends on — a generalized basin pressure that bears directly on a fish with nowhere else to go.