Taxonomy & naming
Gymnogeophagus lipokarenos was described in 2015 by Luiz R. Malabarba, Maria Cláudia Malabarba and Roberto E. Reis, in a paper in Neotropical Ichthyology that erected five new species of the genus at once from the rio Uruguay drainage (the others being Gymnogeophagus missioneiro, Gymnogeophagus constellatus, Gymnogeophagus pseudolabiatus and Gymnogeophagus mekinos). The holotype is an adult male of 4.5 in standard length (MCP 23522), taken in December 1992 from the rio Dourados near Aratiba, Rio Grande do Sul, Brazil, by P. H. Wimberger, R. E. Reis and C. S. Fontana; an extensive paratype and non-type series held at MCP, UFRGS, UMMZ and ZVCP backs the description. The species had in fact been recognized long before it was named: in Wimberger, Reis & Thornton's 1998 mitochondrial phylogeny of the genus it appears as 'Gymnogeophagus sp. E.' The genus Gymnogeophagus itself was raised by Miranda Ribeiro in 1918 and comprehensively revised by Reis & Malabarba in 1988. Within it, lipokarenos sits in the gymnogenys clade — a group of mouthbrooding species united by shared derived features (including the loss of a dark oblique preorbital bar) and centred on the widespread Gymnogeophagus gymnogenys. The five 2015 species form a set of largely allopatric, regionally replacing members of this clade along the Uruguay's tributaries.
Morphology
This is a modestly sized cichlid: the largest measured type specimens reach about 5 in standard length, while FishBase carries a maximum of 5 in SL, so a fully grown adult is roughly hand-sized. The body is elongate and laterally compressed, with large ctenoid flank scales and a fin formula of XIII–XIV dorsal spines and 9–11 soft rays, three anal spines and 7–9 anal soft rays, and 28 vertebrae. The single most arresting feature is sexual: mature males develop a very large adipose (fatty) nuchal hump that swells the predorsal profile and, in big males, can rise above the tips of the longest dorsal-fin spines, sometimes wrapping around the dorsal-fin origin. It is this exaggerated 'fat head' that gives the species its name and most readily distinguishes it from the more modest humps of its relatives. The colour pattern is the other diagnostic. The unpaired fins are bright yellow — not the red typical of most congeners — and are dusted with white spots aligned between the rays; the outer edge of the dorsal fin and the upper and lower margins of the caudal are red with thin pale stripes. In life the body of a breeding male is bluish-green with rows of bright spots along the scale rows, the mid-dorsal region and hump dark blue, and the head largely greenish-blue with a little red on the cleithrum. Females and juveniles are less ornate, retain a conspicuous dark vertical band across the cheek below the eye that males lose, and never develop the hump. The lower pharyngeal tooth plate carries enlarged, blunt, molar-like posteromedial teeth, fitting a diet that includes hard-shelled prey.
Habitat
Gymnogeophagus lipokarenos is a fish of the upper Río Uruguay and its larger tributaries, on the border of Rio Grande do Sul and Santa Catarina in southern Brazil; the original description treated it as endemic to the Upper Uruguay freshwater ecoregion. In 2017 Casciotta and colleagues recorded it for the first time from Argentina, in the province of Misiones, extending the known range downstream into the Lower Uruguay ecoregion. Type and reference material comes from clear-running rivers and streams such as the rio Dourados, rio Jacutinga, rio Turvo, rio Passo Fundo and the Uruguay mainstem, including several reservoirs (Itá, Monjolinho) impounded on the system. These are subtropical waters at the cool southern margin of the cichlid range: unlike Amazonian eartheaters, Gymnogeophagus live through a genuine seasonal cycle in which river temperatures swing widely between a warm summer and a cold winter, and the genus as a whole is intolerant of being held permanently warm. Published in-situ chemistry specific to this species is scant; collection records place it in shallow river and stream habitat (museum elevation data fall in the 0–1640 ft band, with collections from depths within the upper few metres). By the standards of its genus the water is near-neutral, of moderate hardness, and clear over sand and rock.
Feeding
Like all members of its genus, Gymnogeophagus lipokarenos is an eartheater — a benthic sifter that takes mouthfuls of sand and fine sediment, sorts edible particles in the buccal cavity and expels the rest through the mouth and gills. No dedicated gut-content study of this species has been published, but its morphology is informative: the lower pharyngeal plate bears enlarged, blunt, molariform posteromedial teeth, the dental signature of a fish that crushes hard items such as small molluscs and other shelled invertebrates in addition to soft prey. FishBase estimates a trophic level near 3.3, consistent with an invertivore-leaning omnivore. Field accounts of congeneric Uruguayan gymnos describe them as opportunistic omnivores taking small crustaceans, insect larvae, tiny fish and some algae from sandy and rocky bottoms in flowing water, and lipokarenos is expected to forage in the same generalist, substrate-sifting fashion within its riverine community.
Mating
Direct observations of courtship in Gymnogeophagus lipokarenos have not been published, so its mating system is read from its clade. It belongs to the gymnogenys group, whose members are mouthbrooders, and in mouthbrooding Gymnogeophagus the social structure is markedly different from the monogamous substrate spawners of the genus. The male holds and defends a territory, develops the species' spectacular breeding dress — here the bright yellow fins and the towering fatty nuchal hump — and courts a succession of females, while each female ultimately rears the brood alone. Strong sexual dimorphism is the rule in these mouthbrooders: ripe males are larger, vividly coloured and hump-headed, females smaller and plainer, exactly the pattern seen in lipokarenos. Such polygynous, male-display systems make the nuchal hump and fin colour signals of male quality, which is consistent with the extreme development of both in this species. Spawning across the Uruguayan gymnos is strongly seasonal, concentrated in the warm summer months when females in breeding condition (and, in mouthbrooders, females carrying eggs) are found.
Breeding
Breeding of Gymnogeophagus lipokarenos has not been documented in the wild or, as far as the published record shows, in the aquarium; what follows is inferred from its membership in the mouthbrooding gymnogenys clade. The mouthbrooding gymnos are delayed (larvophilous) mouthbrooders: a pair cleans a firm surface and the female lays her eggs on it, the male fertilizes them, and the eggs are tended in the open for roughly a day before — typically just before hatching, after about 24–36 hours — the female takes them up into her mouth to incubate. At that point the male leaves to court and spawn with other females, and the female alone broods the developing young and, once they are free-swimming, lets them forage near her, the fry darting back into her mouth at any threat (the dark circle of the open mouth seems to be the refuge cue). Clutch sizes in related mouthbrooding gymnos run to a few hundred eggs — figures of roughly 500 are reported for Gymnogeophagus balzanii — so a comparable order of magnitude is the reasonable expectation for this similarly sized species, though no count specific to lipokarenos exists. Spawning is tied to the warm season.
In the aquarium
Gymnogeophagus lipokarenos is essentially an ichthyologist's fish rather than a hobby staple: it is rarely if ever offered in the international trade, and almost all of what is known about it comes from museum specimens. There are no published captive-breeding accounts. Anyone who did obtain it should treat it as a typical subtropical gymno, and the cardinal rule for these southern eartheaters is temperature. They are not tropical fish; kept permanently warm they become listless, lose colour and are short-lived. A cool, seasonally varied regime suits them — broadly the low-to-mid 20s °C in the active season with a cooler winter rest, never held at high-20s tropical temperatures for long. A genus-typical setup means a tank of at least 30–50 US gallons (the lower end only for a single fish or a small group of a small species, more for a breeding group), a deep bed of fine sand the fish can sift without damaging its gills, and clear, near-neutral, moderately hard water with gentle to moderate current echoing its home streams. Smooth rocks and driftwood to break sightlines and define territories help, and securely anchored plants are generally left alone. Diet is undemanding: sinking pellets, frozen and live foods such as bloodworm, daphnia and small crustaceans all suit an omnivorous sifter, and a varied menu brings out colour. Because the species is a polygynous mouthbrooder, the sensible social plan — by analogy with its better-known relatives — is one male with several females in a tank large enough to spread aggression, rather than a single pair. Suitable tankmates would be peaceful, similarly cool-water South Americans of comparable size; the genus is generally a good community member outside the breeding season. None of this is documented for lipokarenos specifically, so a keeper would be working it out from genus experience.
Conservation
The IUCN Red List assesses Gymnogeophagus lipokarenos as Least Concern (assessed and published 14 April 2023, assessor E. Fernando; population trend unknown). The assessment lists the species from Brazil (Rio Grande do Sul and Santa Catarina) and Argentina (Misiones) and characterises its habitat simply as inland wetlands. The original 2015 description reached the same conclusion, noting the species is frequent and abundant in the upper rio Uruguay and that, despite a relatively restricted extent of occurrence of roughly 7,373 mi², no specific threats were detected. That said, much of its range lies along a river that has been heavily dammed — type and reference material comes from several hydroelectric reservoirs (Itá, Monjolinho and others) on the upper Uruguay — and impoundment, altered flow and the agricultural land use of southern Brazil are the kinds of pressures that bear on the region's stream fishes generally. None is documented as an active threat to this particular cichlid, and its present status is secure, but as a comparatively narrow-range subtropical endemic it warrants ongoing attention.