Taxonomy & naming
Gymnogeophagus pseudolabiatus was described in 2015 by Luiz R. Malabarba, Maria Claudia Malabarba and Roberto E. Reis in a revision that named five new Gymnogeophagus from the rio Uruguay drainage (Neotropical Ichthyology 13(4): 637–662). The holotype is an adult male, 3.5 in standard length (MCP 35027, in the collection of the Museu de Ciências e Tecnologia da PUCRS, Porto Alegre), collected from rio Sarandi III, a tributary of the rio Quaraí, on the road between Santana do Livramento and Quaraí, Rio Grande do Sul, Brazil, in December 1992 by R. E. Reis, P. H. Wimberger and J. F. P. Silva. A large paratype series spans both banks of the border river, from Rio Grande do Sul in Brazil to the Department of Artigas in Uruguay. The species had been recognized as distinct long before it received a name: the molecular phylogeny of Wimberger, Reis & Thornton (1998) carried it as 'Gymnogeophagus sp. C', and Pereyra & García (2008) placed it within their Clade 2 of the Gymnogeophagus gymnogenys species complex. Within the genus it is therefore not closely allied to the other thick-lipped species, Gymnogeophagus labiatus, despite the strong resemblance — its swollen lips are a convergence, a recurring rock-foraging adaptation seen across cichlid radiations. The genus Gymnogeophagus was erected by Alípio de Miranda Ribeiro in 1918 for Geophagus-like eartheaters distinguished by a naked (scaleless) cheek and predorsal midline and a characteristic forward-pointing spine on the first dorsal pterygophore.
Morphology
This is a small to mid-sized eartheater. The largest measured specimens in the type series reach about 3.5–4 in standard length, so a maximum a little over 4 in SL is well documented; FishBase's listed 4 in SL is at the low end of that range. The body is elongate and laterally compressed, with a pointed snout and a dorsal head profile that runs nearly straight from the mouth to the interorbital region before rising convexly to the dorsal-fin origin; adult males develop a moderate adipose nuchal hump. The single most conspicuous feature is the mouth: upper and lower lips are thick and extremely well developed, with the lower lip deeply notched at the midline — the trait that gives the fish its 'false-lipped' name and that it shares only with Gymnogeophagus labiatus among congeners. The fin counts are XII–XV dorsal spines with 9–11 soft rays, and three anal spines with 7–9 soft rays. In life, breeding males are spectacular: an olivaceous back carrying rows of bright blue spots, a yellowish-green head, red marks scattered over the nape and gill cover, and red dorsal, anal and caudal fins peppered with rounded hyaline (clear) spots. Two markings separate it from Gymnogeophagus labiatus on sight — a dark bar that runs down and slightly back from the dorsal-fin origin (absent in labiatus), and fins covered with well-defined circular dots rather than the longitudinal stripes labiatus shows. Sexual dimorphism is marked: mature males grow larger, carry the adipose hump, produce a pointed dorsal fin with an elongated fourth ray, and are more intensely coloured, while females and juveniles are paler, keep a rounded dorsal fin, and show a conspicuous dark vertical band across the cheek below the eye.
Habitat
Gymnogeophagus pseudolabiatus is a narrow-range endemic of the rio Quaraí (rio Cuareim on the Uruguayan side) and its tributaries, a single river that forms the international boundary between Rio Grande do Sul, Brazil, and the Department of Artigas, Uruguay, in the middle rio Uruguay drainage. Its entire known range is small — the original describers estimated an extent of occurrence of only about 4,500 square kilometres. Within that range it is a rock-stream specialist: the type description records the species 'exclusively in stone bottom habitats, covered with all sizes of stones and rocks' along the Quaraí and its feeder creeks, rather than over the open sand favoured by some of its relatives. These are clear, flowing subtropical waters draining the grassland country of the Brazil–Uruguay frontier. Like the rest of the genus, it lives at the cool southern edge of the cichlid world, where streams are warm in the austral summer but can fall toward the low teens (Celsius) in winter; the smaller Cuareim-basin watercourses also shrink to isolated pools in the dry season. No formal in-situ chemistry has been published for this species, but the genus as a whole occupies near-neutral, soft to moderately mineralized water, and a rocky-substrate riffle fish of this drainage sits in shallow water on the order of a metre or so deep.
Feeding
Like all eartheaters, Gymnogeophagus species feed by taking mouthfuls of substrate and sifting them for edible particles, but pseudolabiatus is built for rock rather than sand. Its hypertrophied, fleshy lips are the same kind of structure that has evolved repeatedly in cichlids that forage in rocky crevices — pressed against stone, thick lips help seal over irregular surfaces so the fish can suck out the small invertebrates and organic matter sheltering there. The lower pharyngeal tooth plate backs this up: it is broad, with enlarged, blunt, molar-like teeth on the posterior and medial rows, a crushing mill suited to hard-shelled prey and tough food items rather than the fine grinding plate of a pure sand-sifter. Direct gut-content studies for this species have not been published, but its congeners are generalist omnivores taking small crustaceans, insect larvae, other invertebrates and some algae and detritus, and there is no reason to think pseudolabiatus differs in that broad guild. FishBase places it at a trophic level of roughly 3.3. In its stony streams it functions as a benthic invertivore-omnivore working the rock surfaces and interstices.
Mating
Gymnogeophagus pseudolabiatus belongs to the gymnogenys complex, the mouthbrooding lineage of the genus, and its reproductive social structure follows the pattern documented for that clade rather than the monogamous pair-bonding of the substrate-spawning gymnos. In the mouthbrooders the system is essentially polygynous and female-dominated in care: a male holds and defends a territory while courting and spawning with several females, and the female alone takes on incubation. This decoupling — the male freed to keep breeding and guarding ground while the female carries the brood — is exactly what field observers report for close relatives such as Gymnogeophagus gymnogenys and Gymnogeophagus labiatus, where in clear streams a single male can be seen dominating the breeding territory of multiple egg-tending females. Specific courtship observations for pseudolabiatus itself have not been published, but its membership in Clade 2 of the gymnogenys complex (Pereyra & García 2008; Wimberger et al. 1998) makes the female-mouthbrooding, male-territorial system the firmly expected mode. Breeding in the genus is strongly seasonal, concentrated in the austral summer.
Breeding
No spawning of Gymnogeophagus pseudolabiatus has been described in either the wild or the aquarium, so what follows is inferred from its place in the mouthbrooding gymnogenys lineage and from the well-studied biology of its relatives. Gymnogeophagus mouthbrooders are delayed (larvophilous to ovophilous) maternal mouthbrooders: a pair cleans a hard surface — typically a flat stone — and the eggs are laid and fertilized there, after which the female takes them up into her mouth, in some species immediately and in others (as documented in Gymnogeophagus balzanii) only shortly before hatching, roughly a day or so after spawning. From that point the male departs and the female incubates and guards the brood alone, and the free-swimming fry continue to bolt back into her mouth when threatened, the alarm cue apparently being the dark circle of the parent's open mouth. Clutch sizes in the genus's mouthbrooders run to several hundred eggs (around 500 reported for the larger Gymnogeophagus balzanii); pseudolabiatus, a smaller fish, would be expected to produce a correspondingly smaller clutch, but no count has been published. Spawning in wild Gymnogeophagus of this region is tied to the warming water of the austral summer, with brooding females captured between roughly November and January.
In the aquarium
Gymnogeophagus pseudolabiatus is essentially an ichthyologist's fish rather than an aquarium staple — described only in 2015, narrowly endemic to a single border river, and not part of the established ornamental trade. It is realistic to say it is rarely if ever kept, and there is no published captive-care or breeding account for the species specifically. That said, the husbandry that works for the genus translates directly, and it is worth setting out honestly for anyone who might encounter wild stock. The cardinal rule for southern Gymnogeophagus is temperature: these are subtropical fish from cool, seasonal water, and they are slowly killed by being kept in a heated tropical tank. Keepers of the genus run them unheated or lightly heated, broadly in the high-teens to mid-20s Celsius (roughly 64–79 °F), with a genuine winter cool-down — fish overwintered in mild outdoor ponds and then warmed in spring colour up and breed best. Aim for near-neutral, soft to moderately hard water. Because this is a rock-stream species rather than a sand-sifter, the tank should be floored with smooth sand or fine gravel but built around a structure of stones and cobbles that provides crevices, sightline breaks and territory boundaries; a 50-gallon-class (around 55 US gal) footprint is a sensible minimum for an active, territorial eartheater of this size. Good current and high oxygen suit a fish of flowing streams. Diet is undemanding: the genus accepts dry, frozen and live foods readily, and a varied menu including small invertebrates keeps condition and colour. As a thick-lipped rock-forager it will appreciate food it can work off surfaces. Mouthbrooding gymnos are generally less aggressive than the substrate-spawning members and make reasonable community fish with robust, similarly cool-water tankmates, but a breeding male will hold and defend a territory, so give subordinate fish room to escape. The honest summary: a handsome fish for the specialist who can source it and is prepared to keep it cool — not a beginner's cichlid, and not one to expect in the trade.
Conservation
The IUCN Red List assesses Gymnogeophagus pseudolabiatus as Least Concern (assessed 14 April 2023 by E. Fernando of the IUCN Freshwater Biodiversity Assessment team, published 2023; population trend unknown). The original describers had already recommended Least Concern, noting that the species is relatively frequent and abundant in the rio Quaraí basin and that no specific threats were detected, even though its extent of occurrence is only about 4,500 square kilometres. The cautionary note is that this is a true narrow-range endemic confined to one small border river, which leaves it more exposed to localized harm than a wide-ranging species would be. The rivers of this Brazil–Uruguay grassland frontier face the familiar pressures of the region — agricultural land use (including rice farming, which is documented as degrading habitats elsewhere in the gymnogenys complex's range), water abstraction, and the seasonal drying of the smaller Cuareim-basin streams — none of which is presently flagged as a population-level threat to this species, but all of which form the backdrop against which a single-river endemic's status should be read. It carries no CITES or CMS listing.