Taxonomy & naming
Gymnogeophagus che was described in 2000 by Jorge R. Casciotta, Sergio E. Gómez and Nelly I. Toresanni in the Revista del Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", from material collected in the arroyo Urugua-í and the arroyo Uruzú in the Paraná drainage of northeastern Argentina. The holotype (MLP 8747, deposited in the Museo de La Plata) came from the arroyo Urugua-í at provincial route 19, near the footbridge in the Islas Malvinas provincial park, Misiones Province; numerous paratypes are held at La Plata (MLP) and the Museo Argentino de Ciencias Naturales (MACN). Eschmeyer's Catalog of Fishes and FishBase both treat the name as valid in its original combination, Gymnogeophagus che, and place it in the subfamily Cichlinae, tribe Geophagini. The genus Gymnogeophagus Miranda Ribeiro, 1918 is diagnosed by two osteological synapomorphies established in Reis & Malabarba's 1988 revision: the complete absence of supraneural bones, and a distinctive forward-directed spine on the first dorsal-fin pterygiophore. Within the genus, two lineages are recognized — a substrate-breeding clade (often called the Gymnogeophagus rhabdotus group) and a mouthbrooding clade (the Gymnogeophagus gymnogenys group). Gymnogeophagus che belongs firmly to the substrate-breeding clade, and within it to the tightly defined Gymnogeophagus setequedas species group, a trio of allopatric Paraná-system endemics — Gymnogeophagus setequedas, Gymnogeophagus taroba and Gymnogeophagus che — separated from one another by the waterfalls of the region (Casciotta, Almirón, Piálek & Říčan 2017). The specific name is a deliberate piece of regional wordplay: "che" is a Mbya-Guaraní word meaning "mine" or "my", and as a colloquialism it so marks Argentine speech that it distinguishes Argentines from other Spanish-speakers — a nod to the fish being endemic to Argentina.
Morphology
Gymnogeophagus che is a modestly sized eartheater, reaching about 4.5 in in standard length (FishBase, after Kullander 2003), with the largest examined specimens just over 4 in SL. The body is comparatively low and slender for the genus — body depth around 37–41% of standard length, shallower than its relative Gymnogeophagus taroba — with a low head profile, a feature the describers cited as diagnostic. Like all Gymnogeophagus it carries the genus's hidden hallmarks (no supraneurals, a forward-pointing spine at the front of the dorsal fin) rather than any single obvious external badge. In life the fish is an olive-to-brassy ground colour with reddish-brown scale centres on at least the rear of the body and caudal peduncle, a conspicuous dark round blotch on the midflank, and a dark bar running through the eye from the nape to the lower edge of the gill cover. Bright pale-blue iridescent spots and lines decorate the cheeks, gill cover and rear body. The fins are the most useful field characters within its species group: the dorsal fin is heavily patterned with broad, dominant white bands on the spiny portion, the soft dorsal and anal fins carry a combination of rounded spots at the base and bolder stripes distally, and the caudal fin shows relatively few hyaline (clear) stripes — the combination of these fin patterns, plus the lack of small rounded spots across all the unpaired fins, separates it from its congeners. Sexual dimorphism is present but moderate: mature males grow somewhat larger and longer-bodied than females, and some males develop drawn-out filamentous tips on the dorsal, anal and pelvic fins, with the pelvic fins reaching noticeably farther back than in females.
Habitat
Gymnogeophagus che is, on present knowledge, a single-drainage endemic. It is restricted to the arroyo Urugua-í and its tributaries (including the arroyo Uruzú and the arroyo Grapia) in the Paraná basin of Misiones Province, northeastern Argentina — a small, forested Atlantic-Forest catchment that drains into the upper Paraná a short distance below the Iguazú confluence. This places it in the Lower Parana freshwater ecoregion. Its isolation is geological: the 28-metre Urugua-í falls (replaced in 1989 by a hydroelectric dam) historically cut the Urugua-í off from the neighbouring Paraná tributaries, and that barrier is thought to be what separated Gymnogeophagus che from its nearest relative, Gymnogeophagus setequedas, in the classic pattern of waterfall-driven speciation that defines the whole Gymnogeophagus setequedas group. Field workers report it as common throughout the Urugua-í basin and as an ecological generalist within that basin — it occupies essentially all the habitat types present, from flowing stream reaches to the standing water of the artificial Urugua-í reservoir created by the dam. These are subtropical waters with a strong seasonal temperature swing typical of the genus's far-southern range, running cool in winter and warm in summer rather than holding the steady tropical warmth of Amazonian eartheaters. Published in-situ chemistry specific to the Urugua-í is sparse, but the surrounding upper-Paraná tributaries are generally clear, soft and near-neutral; recorded depths are those of a shallow stream-and-margin fish, on the order of the first metre or two of the water column.
Feeding
Like its genus, Gymnogeophagus che is an eartheater in the literal sense — a benthic sifter that takes mouthfuls of sand and fine sediment, sorts edible particles in the buccal and pharyngeal cavities, and expels the cleaned grit through the gill openings and mouth. No dedicated gut-content study of this species has been published, but its trophic ecology can be read confidently from its anatomy and from its better-studied relatives: Gymnogeophagus are omnivores that work the bottom for small benthic invertebrates (chironomid and other insect larvae, microcrustaceans, worms), supplemented by detritus and a lesser amount of algae and periphyton. FishBase places Gymnogeophagus che at an estimated trophic level of about 3.3, consistent with an invertebrate-leaning omnivore rather than a herbivore or a piscivore. In the Urugua-í community it functions as one of several substrate-foraging cichlids, sharing the bottom with characins, loricariid catfishes and pike cichlids (Crenicichla), and feeding activity in these temperate-zone eartheaters typically slackens through the cooler months.
Mating
Direct field observations of courtship in Gymnogeophagus che have not been published, so its mating behaviour is inferred from the substrate-breeding clade to which it belongs and from the genus's well-documented general pattern. Substrate-breeding Gymnogeophagus form monogamous pairs that hold and defend a territory together; the pair selects and cleans a spawning surface, and both parents take part in defending the eggs and young, with the female generally the more intensive guardian of the brood. This contrasts with the polygamous, female-only care of the mouthbrooding Gymnogeophagus gymnogenys group, where one male courts and spawns with several females in succession. In the temperate climate of its range, pair formation and breeding are seasonal, tied to the warming water of the southern spring and summer rather than occurring year-round. Males in breeding condition can be told by their larger size and the filamentous extensions of the unpaired fins. As with most cichlids, territoriality intensifies sharply around spawning, when an otherwise unremarkable little eartheater becomes a determined defender of its patch of streambed.
Breeding
Gymnogeophagus che is a biparental substrate spawner — it does not brood in the mouth. The reproductive distinction between the two halves of the genus is sharp and phylogenetically informative: the clade containing Gymnogeophagus che (together with Gymnogeophagus rhabdotus, Gymnogeophagus meridionalis, Gymnogeophagus setequedas, Gymnogeophagus taroba and Gymnogeophagus terrapurpura) comprises species "whose females deposit adhesive eggs on a surface and one or both parents guard and tend them" (Turcati, Serra-Alanís & Malabarba 2018), whereas the Gymnogeophagus gymnogenys clade brood eggs and larvae orally. Specific clutch and developmental figures have not been published for Gymnogeophagus che itself, but its close substrate-breeding relatives give a reliable picture: pairs clean a flat surface, lay and fan a clutch of adhesive eggs (congeners such as the meridionalis/rhabdotus assemblage lay on the order of a few hundred eggs — roughly 300 in documented cases), and both parents guard the developing eggs and then the free-swimming fry, the female tending most closely while the male patrols the territory. Spawning is seasonal, triggered by the rising water temperatures of spring and early summer that bring these far-southern cichlids into condition. Because the species has essentially no aquarium history, no captive spawning account specific to Gymnogeophagus che is on record; any successful breeding would be expected to follow the proven template for the substrate-spawning gymnos — a cleaned flat stone or pit, biparental care, and a cool-water seasonal cue.
In the aquarium
Gymnogeophagus che is, for practical purposes, not an aquarium fish. It is a narrow-range Argentine endemic that does not appear in the ornamental trade; the gymnos that hobbyists actually keep are the wider-ranging, more colourful species (Gymnogeophagus balzanii, Gymnogeophagus labiatus, the meridionalis/rhabdotus complex, and the Gymnogeophagus gymnogenys forms). Anyone who did obtain wild Gymnogeophagus che would be keeping a fish for which no published husbandry exists, and the only honest guidance is to extrapolate from the well-understood requirements of its genus. The cardinal rule for all southern Gymnogeophagus is temperature: these are subtropical, seasonally cool-water fish, and they fare badly in a heated tropical tank. Long-term keeping of the southern species is best done unheated, with water in roughly the 64–79 °F band and a genuine cool winter rest in the mid-teens Celsius — sustained tropical warmth leaves them listless, faded and short-lived. A fine sand substrate is essential for a fish that feeds by sifting; a deep enough bed of soft sand lets it express normal foraging and protects its gills. Water should be soft to moderately hard and near-neutral (pH roughly 6.5–7.5), with strong filtration but gentle flow and the clean, well-oxygenated conditions of its stream habitat. Given its small adult size (around 4.5 in) a single pair would suit a tank of about 30 US gallons, larger for a group. Diet is undemanding — sinking foods, frozen and live invertebrates such as bloodworm and daphnia, with some vegetable content. Tankmates, by analogy with its relatives, should be peaceful temperate or subtropical South Americans; the fish is unlikely to trouble companions outside the breeding season, when, like all cichlids, it becomes territorial. The overriding caution is conservation, not husbandry: this is a single-drainage, Near Threatened endemic, and it should not be collected from the wild for the hobby.
Conservation
The IUCN Red List assesses Gymnogeophagus che as Near Threatened (criterion B1a), assessed on 3 February 2021 by Héctor S. Vera-Alcaraz and published in the 2023 update. The Near Threatened listing reflects the species' very restricted geographic range — a single river basin, the arroyo Urugua-í in Misiones, Argentina — which gives it a small extent of occurrence even though it is reported to be common and an ecological generalist within that basin. It carries no CITES or CMS listing. The most obvious large-scale alteration to its habitat is the Urugua-í hydroelectric dam, completed in 1989, which drowned the historic Urugua-í falls and converted a stretch of the river into a reservoir; notably, the fish persists in the artificial reservoir as well as in the flowing reaches, which tempers the immediate concern. The broader risks are those facing the Atlantic Forest streams of Misiones generally — deforestation, agricultural runoff, river impoundment and the spread of introduced fishes in the wider Paraná system. None is documented as driving a decline in this particular species, and its population trend is recorded as unknown; the conservation flag it raises is the classic one for a single-drainage endemic, that a fish found in only one small place is intrinsically vulnerable to any change to that place.