Taxonomy & naming
The species was described by the German naturalist Reinhold Friedrich Hensel in 1870 as Heros acaroides, from material collected at Porto Alegre and the Rio Cadeia in Rio Grande do Sul, southern Brazil (Catalog of Fishes type locality: 'Rio Cadea [Cadeia], Porto Alegre'). Its history since then is a textbook case of a good species being swallowed by a more famous one. As early as 1875 Franz Steindachner treated acaroides as a synonym of the widespread chanchito Chromis facetus Jenyns, 1842, and Pellegrin (1904) repeated the synonymy under Cichlasoma; for most of the twentieth century the name simply disappeared into facetus. The genus Australoheros itself did not exist until Říčan & Kullander (2006) split the southern 'austral Heros' off from the Central American heroine cichlids, the name combining Latin australis ('southern') with Heros, the type genus of the tribe Heroini. Heros acaroides was revalidated by Schindler, Ottoni & Cheffe (2010, Vertebrate Zoology 60: 139–146), who examined Hensel's surviving syntypes in the Berlin museum, designated a lectotype (ZMB 7455), and recognised the Patos-Mirim fish as a distinct species of Australoheros rather than a synonym of Australoheros facetus. The most recent revision — Ottoni, Santos de Lucena, Kullander, Norén & Calegari (2022, PLOS ONE 17(12): e0261027) — used combined morphology and DNA across the coastal radiation, recognised seventeen valid species in the genus, and confirmed Australoheros acaroides as one of eight species in the coastal rivers; in that same work Australoheros taura is treated as a junior synonym of Australoheros acaroides. The taxonomy of the genus remains genuinely difficult: the coastal species are morphologically conservative, Australoheros acaroides and Australoheros facetus were historically conflated and overlap in the Rio Jacuí drainage, and species limits in Australoheros are still actively debated, so any single name in this group should be read as provisional.
Morphology
Australoheros acaroides is a small to mid-sized cichlid: FishBase gives a maximum of about 4.5 in standard length, and specimens in the revisionary material run to roughly 5.5 in SL at the upper end for the genus. The body is the typical chanchito shape — deep, strongly laterally compressed, with a steep forehead and a rounded snout — built for slow, structured water rather than open current. Coloration follows the genus pattern: a tan to olive or grey-brown ground crossed by a series of dark vertical bars down the flanks, the most prominent of which sits at mid-body, often expressed as a darker blotch where the bar crosses the lateral midline; the bars can intensify or fade rapidly with mood, a chameleon-like lability shared across the chanchitos. Diagnosis within Australoheros rests on fine meristic and counts rather than on bold markings: Australoheros acaroides shows a relatively low scale count (modally fewer than 25 scales in the E1 row, typically around 24), a low vertebral count (13+13 or 13+14), about XVI dorsal-fin spines (XVI.9 and XVI.10 nearly equally frequent) and a modal anal-fin formula near VII.8. In the largest individuals the lower jaw thickens and becomes somewhat angled, a feature it shares with big Australoheros facetus. Sexual dimorphism is modest and of the usual cichlid kind — males grow larger and develop more produced soft-dorsal and anal-fin tips, while ripe females are deeper- and rounder-bodied — but the sexes are not reliably separable on color alone.
Habitat
The species is restricted to the temperate coastal lowlands of the southern Atlantic slope. Its core range is the Laguna dos Patos drainage — the lower and middle Rio Jacuí, the Lago Guaíba and its tributaries around Porto Alegre — together with the coastal streams that drain into the Laguna Mirim (Lagoa Mirim) to the south, a system that straddles the Brazil–Uruguay border; the Catalog of Fishes accordingly lists the species from both Rio Grande do Sul (Brazil) and Uruguay. This is the Laguna dos Patos freshwater ecoregion, a flat barrier-and-lagoon landscape of coastal lagoons, swamps, ponds and slow rivers. Australoheros acaroides is a subtropical-to-temperate freshwater fish that also tolerates the mildly brackish water of the lower lagoons and estuarine margins. Within these waters it favours streams of moderate current with sandy-clayey bottoms and marginal vegetation; during the dry season it withdraws into deeper, shaded, vegetation-rich pools. Published in-situ chemistry for this particular species is sparse, so the water envelope is best read from its habitat and its close relatives: these are cool-temperate waters, broadly circumneutral to slightly acidic, that swing seasonally — the genus is notably cold-hardy, with congeners comfortable from the upper teens in winter into the high twenties Celsius in summer. As a shallow lagoon-and-stream fish it lives within the top metre or two of the water column rather than in any deep zone.
Feeding
A dedicated feeding-ecology study of Australoheros acaroides in southern Brazil (Studies on Neotropical Fauna and Environment, 2024) examined 88 individuals and recorded 27 food-item types, painting the species as an omnivore with a relatively narrow but flexible trophic niche. The diet is dominated by aquatic insect larvae — dipteran (midge/chironomid) larvae are the principal prey — alongside annelids, chelicerates (aquatic mites), crustaceans such as cladocerans (water fleas), molluscs, leeches (Hirudinea), occasional small fish, plant material and macrophytes, plus fine particulate and digested organic matter. FishBase places it at a trophic level near 3.2, consistent with a benthic-leaning, insect-and-invertebrate omnivore that also takes vegetation and detritus. The study found that diet composition shifted significantly both by season and by body-size class (a PERMANOVA result): the fish reorganises what it eats as resources change through the year and as it grows. It is described as a diurnal forager. In its community Australoheros acaroides functions as a generalist benthic-to-midwater omnivore of vegetated lagoon and stream margins rather than a feeding specialist.
Mating
Direct field observations of courtship in Australoheros acaroides are scarce, but its reproductive biology is that of a typical chanchito and is well documented in its close relative Australoheros facetus. These cichlids form monogamous pairs for spawning, and both members of the pair become territorial, jointly selecting and defending a patch of bottom centred on a spawning surface. Pair formation runs through the usual cichlid sequence of mutual display, lateral spreading of the bars and fins, and substrate-cleaning before eggs are laid. Outside of breeding the fish are comparatively social and unfussy; territoriality sharpens markedly once a pair commits to a site, and as in most substrate-spawning cichlids intraspecific aggression around the nest can be intense, with the male patrolling the wider territory while the female works close to the eggs. Spawning in the wild tracks the warm season, with reproductive activity rising as temperatures climb through spring and summer in this temperate setting.
Breeding
Australoheros acaroides is a biparental, open substrate spawner — the standard chanchito mode. The pair cleans a firm surface, typically a flat stone or submerged wood (and readily a flat rock or slate in captivity), and the female deposits an adhesive clutch of eggs in neat rows over it; both parents then tend the spawn. A species-specific clutch count for Australoheros acaroides has not been published, but in the genus a vigorous pair lays on the order of a few hundred eggs (congeners such as Australoheros facetus produce clutches in the hundreds). Care is shared and prolonged for a fish of this size: the eggs hatch in roughly two to four days at warm temperatures, the wrigglers are moved to pits, and once free-swimming the fry are shepherded and guarded by both parents — care of eggs and young together commonly running on the order of three weeks before the brood disperses. Breeding is temperature-cued: like other Australoheros it is cold-tolerant and goes quiet in cool water, with spawning triggered as the water warms into the high twenties Celsius. The combination of pair-bonded biparental care, an open-substrate nest and broad temperature tolerance makes it, like the wider chanchito group, a robust and uncomplicated breeder under the right seasonal cue.
In the aquarium
Australoheros acaroides is rarely encountered in the hobby outside of specialist circles interested in South American cichlids and, in particular, in cold-tolerant subtropical species. It has never achieved wide popularity in the aquarium trade — most chanchito keepers in Europe and North America work with the more familiar Australoheros facetus, and Australoheros acaroides is typically only available through cichlid society fish swaps or specialist breeders with a South American focus. Those who do keep it generally find it straightforward, though not especially showy by cichlid standards.
The most useful single fact about this species in captivity is that it does not need a heater in any temperate household. Its natural waters in the Patos-Mirim lagoon system and lower Jacuí run seasonally from the mid-teens to the high twenties Celsius, and the species handles anything in the range of roughly 61–82 °F without difficulty. Keeping it at the cooler end of that band — in the low-to-mid twenties — is entirely appropriate and may extend its life and suppress the aggression that comes with constant tropical warmth. In practice this means a room-temperature tank is fine through most of the year in a temperate climate, which is genuinely unusual for a New World cichlid. Water chemistry is not demanding: circumneutral to slightly acidic pH (6.0–7.5) and moderate softness suit it well, though — like Australoheros facetus — it is a tolerant fish that adapts to harder or more alkaline tap water without obvious distress in most cases.
A tank of at least 25–30 US gal is appropriate for a pair; larger is better once breeding begins and the pair becomes territorial. The substrate should be sandy or fine-gravel — this is a benthic species of shallow coastal lagoons and slow streams, and it roots through the bottom foraging for invertebrates. Flat stones, slate pieces, or submerged wood give spawning surfaces and break sightlines, both of which become critical once a pair bonds. Marginal plant cover around the edges (hardy species such as Java fern or Vallisneria tolerate the temperature range) replicates the vegetation-rich margins the species inhabits in the wild and gives subordinate fish refuge. The fish will dig and may uproot plants not tied to wood or rock.
Tankmate selection requires care, particularly when the pair is breeding. Australoheros acaroides is a moderately assertive substrate spawner that will vigorously defend its nest and fry; during breeding the pair can be lethal to anything it considers a threat, including fish considerably larger than itself. Outside of breeding it is a reasonably community-tolerant cichlid by South American standards, and can be housed with robust, similarly sized species — other South American cichlids of comparable disposition, large tetras, or loricariid catfish — provided the tank is large enough to allow retreat. Keeping two males together in a small tank invites trouble; a single pair in a species tank, or a pair in a large community with plenty of structure, are the safest arrangements. Avoid delicate small fish, especially when the pair has eggs or fry.
In captivity it is an unfussy feeder that accepts good-quality dried foods (pellets or granules of suitable size) readily, but its natural diet of insect larvae, small invertebrates and plant matter is reflected in better colour and condition when supplemented with frozen or live chironomid larvae (bloodworm), brine shrimp, daphnia, or similar. Vegetable matter — blanched spinach, spirulina-enriched foods — rounds out the diet. Breeding in captivity follows the same pattern as in the wild: a temperature rise into the mid-to-upper twenties, combined with a pair that has settled in and accepted each other, usually triggers spawning on a flat stone or slate. Both parents guard the clutch and the subsequent wrigglers and free-swimming fry with considerable intensity; the main common mistake is removing one parent too early, depriving the pair of their shared guarding dynamic, or failing to provide enough flat surfaces and broken sightlines so that a submissive partner has somewhere to retreat when the dominant fish becomes overbearing. Provided the tank is adequate and the pair compatible, Australoheros acaroides is a capable, reliable breeder.
Conservation
The IUCN Red List assesses Australoheros acaroides as Least Concern (assessed 7 November 2018), reflecting a species that is reasonably widespread and locally common across the Patos-Mirim coastal plain and the lower Jacuí–Guaíba system, with no evidence of major decline. It carries no CITES listing and is not a significant aquarium-trade target. The risks it faces are the general pressures on the densely settled coastal lowlands of Rio Grande do Sul and adjacent Uruguay rather than anything specific to the fish: the Laguna dos Patos basin drains a large, heavily farmed and increasingly urbanised catchment (including the Porto Alegre metropolitan area), so habitat conversion, agricultural run-off, water-quality decline and modification of marginal wetland vegetation are the plausible long-term concerns. As a tolerant, brackish-capable lagoon-and-stream generalist it is far better buffered against these pressures than the many narrow-range endemics of the same coastal system, and none of these factors is currently documented as a population-level threat to Australoheros acaroides specifically.