Taxonomy & naming
Gymnogeophagus caaguazuensis was described by the German ichthyologist and aquarist Wolfgang Staeck in 2006, in the journal Zoologische Abhandlungen of the Dresden natural-history museum, from material collected in the drainage of the lower río Paraguay. The holotype (museum register MTD F 30367) and a series of ten paratypes (MTD F 30384–93) are held in the Dresden collection. The type locality is the upper río Tebicuary-mí, a tributary of the río Tebicuary, at roughly 25°28'S, 56°10'W where Route 7 crosses the stream about 16 mi east of Coronel Oviedo in what was then Caaguazú department, Paraguay. Staeck diagnosed the species against all other described Gymnogeophagus by a combination of features: a comparatively deep body, a short caudal peduncle, a distinctly lyreate (lyre-shaped) caudal fin in males, 26–28 scales in the longitudinal (E1) row, and its small adult size. The genus Gymnogeophagus was erected by the Brazilian zoologist Alípio de Miranda Ribeiro in 1918 for Geophagus-like eartheaters of the subtropical La Plata region; within the cichlid family it sits in the eartheater tribe Geophagini of the New-World subfamily Cichlinae. Molecular work has since placed caaguazuensis firmly within the genus's mouthbrooding lineage: a 2019 study by Alonso and colleagues, erecting the related Gymnogeophagus jaryi from the middle Paraná in Argentina, recovered caaguazuensis as the sister species of jaryi and a close relative of Gymnogeophagus australis.
Morphology
This is one of the smallest species in its genus. The original description gives a maximum of about 3.5 in standard length (the body without the tail fin); wild-caught adults typically run to roughly 3.5 in SL, which works out to around 4.5 in in total length once the caudal fin is included — figures of 6 in sometimes quoted in the hobby are not supported by the description and are best disregarded. The body is relatively deep and short, with a correspondingly short caudal peduncle. The ground colour is a light beige to yellowish tan, usually carrying a row of darker blotches along the flank. The head and gill covers are marked with iridescent blue spots and stripes, and the scales over much of the body and the fins bear pale, pale-blue glints; the dorsal, caudal and anal fins are edged in a translucent reddish margin. Sexual dimorphism is conspicuous — Staeck described it as a defining trait — and is the easiest way to sex the fish: males grow larger and develop the characteristic lyre-shaped caudal fin, with the upper and lower tail lobes drawn out into points, while females stay smaller and plainer with a more rounded tail. As in all Gymnogeophagus, the cheek and the midline of the back ahead of the dorsal fin are scaleless ('naked'), the feature that gives the genus its name.
Habitat
Gymnogeophagus caaguazuensis is endemic to Paraguay and, on present records, restricted to the upper río Tebicuary-mí system within the lower río Paraguay basin — a very small known range that lies behind its Endangered listing. It is a fish of small rivulets and streams that swing between extremes with the seasons. In the rainy season these watercourses can run with a strong current over bare sandy bottoms set among rocky areas; in the dry season, as the flow drops, the species retreats into pools and ponds of more or less stagnant water, where the substrate is sand, pebbles and rock strewn with decaying leaf litter and roots. The single set of in-situ measurements published with the description, taken at the type locality in February, recorded near-neutral to slightly alkaline water at pH 7.9, an electrical conductivity of about 240 µS/cm and a temperature of 76 °F — modest, moderately mineralized water typical of the subtropical La Plata streams the genus inhabits, and cooler and harder than the soft blackwaters of the Amazon proper. The associated fish fauna at the type locality included the livebearer Phalloceros caudimaculatus, a Rineloricaria armoured catfish and two small characins. As a benthopelagic eartheater it works the sandy shallows; depth is not separately documented, but these are shallow stream-and-pool habitats only a metre or so deep.
Feeding
Like its eartheater relatives, Gymnogeophagus caaguazuensis is a benthic forager that takes mouthfuls of sand and fine sediment and sifts them through the gill rakers, swallowing small invertebrates and organic particles and expelling the cleaned substrate. FishBase places it at a trophic level near 3.3, consistent with an omnivore that leans carnivorous — feeding on small bottom invertebrates and the organic detritus of the leaf-litter and sand it processes. No detailed wild gut-content study has been published for the species, so its diet is inferred from the foraging mode shared across the genus and tribe rather than measured directly. In the stream-and-pool communities it inhabits it functions as a small generalist of the sandy benthos, working alongside catfish and characins rather than competing as a specialist.
Mating
Gymnogeophagus caaguazuensis belongs to the mouthbrooding lineage of Gymnogeophagus, and its reproductive behaviour reflects that. The genus as a whole spans a striking range of breeding systems — from monogamous substrate-spawning in some species to polygynous mouthbrooding in others — a diversity mapped onto the genus's phylogeny by Wimberger, Reis and Thornton in their 1998 mitochondrial study of parental care and mating systems. In caaguazuensis the mating system is polygynous: a dominant male spawns with several ripe females in a season and holds a loose hierarchy among the group. Courtship is marked by a behaviour Staeck flagged as a hallmark of the mouthbrooding species complex — courting males, facing a ripe female, perform a conspicuous quivering of the mouth, opening and closing it at very high frequency. The fish naturally live in small groups within which a clear pecking order forms, with display 'parades' and minor scuffles until rank is settled; outside of these contests they are comparatively peaceful for a cichlid.
Breeding
The species is a delayed, larvophile maternal mouthbrooder — a two-stage strategy shared with several of its congeners and observed directly in aquaria. A pair begins like an open substrate-spawner: the eggs are laid on a flat surface such as a stone and tended in the open, exactly as a non-mouthbrooding cichlid would do. Then, shortly before the larvae hatch, the female takes the developing eggs into her mouth and orally broods the larvae and young until they are free-swimming, after which she continues to escort and shelter them, taking the fry back into her mouth at any threat. Parental care is essentially maternal; the male's role is largely confined to defending the spawning area. Specific clutch sizes and incubation times have not been formally published for this species — fecundity in a fish this small is modest, on the order of a few dozen to perhaps a hundred eggs as in comparable small Gymnogeophagus — and the figures circulated in the hobby are approximate. The transition from substrate care to mouthbrooding is the diagnostic reproductive trait of the lineage rather than an aquarium artefact.
In the aquarium
Gymnogeophagus caaguazuensis reaches the hobby only occasionally, almost always as wild-caught imports from Paraguay rather than tank-bred stock, and it is not a beginner's fish. It is best kept as a group of five or more, because it naturally lives in small social groups and a lone or paired fish tends to grow shy, sicken and languish; a group also lets a dominant male's attention spread among several females rather than fixing on one. That social need pushes the tank size up: a footprint of around 59 in with a volume in the region of 105 US gal is a sensible minimum for a proper group, with open swimming space balanced against broken sightlines from wood, rock and plants so subordinate fish can stay out of view and aggression stays at the level of bluster.
The substrate is the one non-negotiable: fine sand. These fish feed by taking sand into the mouth and sifting it, and sharp gravel both frustrates that behaviour and risks abrading the gills. A flat stone or two should be provided as spawning sites. Filtration should be generous and the water kept clean, but strong constant current is unnecessary and the fish are not adapted to it; gentle to moderate flow suits them.
Water chemistry need not be exotic so long as it is stable and clean: the type-locality data (pH near 7.9, conductivity ~240 µS/cm, ~75 °F) point to near-neutral to slightly alkaline, moderately hard water around the mid-20s Celsius, and a maintenance range of roughly 72–79 °F and pH 6.5–8.0 is comfortable. Unlike the soft-water Amazonians it is sometimes shelved alongside, it does not want acidic blackwater. As a subtropical La Plata fish it tolerates and arguably benefits from a cooler winter period; sustained tropical heat is less appropriate than for an equatorial cichlid. Diet is straightforward — a varied menu of quality sinking pellets or flake plus frozen and live foods such as mysis, brine shrimp, daphnia and bloodworm — the main pitfalls being overfeeding and an over-monotonous dry-food-only regime.
Tankmates should be peaceful, similarly sized fish that share the same calm, moderately hard water; large boisterous or strongly territorial species will bully it into hiding. One specific caution carries weight for a fish this scarce in the wild: do not house it with other Gymnogeophagus or closely related eartheaters, as the genus hybridizes readily and crosses would compromise the integrity of an already-threatened species in captivity. Keeping a clean group, sexing the lyre-tailed males from the plainer females and conditioning on live foods are the keys to seeing — and perpetuating — this little mouthbrooder.
Conservation
The IUCN Red List assesses Gymnogeophagus caaguazuensis as Endangered, under criteria B1ab(iii)+2ab(iii), assessed on 16 December 2020 by Felipe Alonso and published in 2022; the population trend is recorded as unknown. The 'B' criteria reflect a very small geographic range — the species is known only from the upper río Tebicuary-mí system in eastern Paraguay — coupled with inferred continuing decline in the extent and quality of that habitat. Small subtropical stream fish of this kind are exposed to the familiar pressures on the lower Paraguay basin: agricultural expansion and the associated clearing, siltation and water abstraction, which degrade exactly the sand-bottomed rivulets and seasonal pools the species depends on. It carries no CITES listing and is of no commercial fishery interest. For aquarists the conservation note is practical as well as legal: because the fish enters the trade mainly as wild imports from a tiny range, keeping clean, non-hybridized groups has a genuine value beyond the tank.