Taxonomy & naming
Gymnogeophagus jaryi was described in 2019 by Felipe Alonso, Guillermo E. Terán, Gastón Aguilera, Oldřich Říčan, Jorge Casciotta, Wilson S. Serra, Adriana Almirón, María de las Mercedes Benítez, Ignacio García and Juan Marcos Mirande, in the open-access journal PLoS ONE (volume 14, article e0210166). The genus Gymnogeophagus was erected by Alipio de Miranda Ribeiro in 1918 for the 'bare-headed' eartheaters of the subtropical La Plata system, a group set apart from Geophagus proper by their naked (scaleless) supraorbital region and a forward-pointing spine at the front of the dorsal fin. Within Gymnogeophagus, species are informally sorted into two groups defined by reproductive mode: the Gymnogeophagus rhabdotus group of substrate brooders and the Gymnogeophagus gymnogenys group of mouthbrooders. Gymnogeophagus jaryi belongs to the latter, and the authors note it is the first new species of the gymnogenys group to be described from this part of Misiones. A molecular (mtDNA) phylogeny in the description recovers it as the sister species of Gymnogeophagus caaguazuensis of the Paraguay basin, with that pair sister to Gymnogeophagus australis of the lower La Plata estuary. The holotype is an adult male, 4.5 in SL, catalogued CI-FML 7463 in the Fundación Miguel Lillo collection in Tucumán, collected in the Cuña Pirú stream about 5 mi from Aristóbulo del Valle on provincial route 7, Paraná River basin (roughly 27°05′S, 54°57′W), in November 2016. There are no synonyms; the species has been valid since its original publication.
Morphology
This is a modestly sized eartheater. FishBase lists a maximum of 5 in standard length, and the type series reflects this: the male holotype measured 4.5 in SL and the largest paratype 5 in SL, while mature females in the series were considerably smaller, around 2.5–3 in SL. The body is laterally compressed with the steep, blunt head typical of the genus. The dorsal fin carries 13–14 spines and 10–11 branched (soft) rays, the anal fin 3 spines and 8–9 soft rays. In life, mature males are strikingly marked: the central area of each scale on the upper trunk and tail bears a light iridescent blue blotch, the unpaired-fin bases are yellow grading to orange and red distally, and the soft part of the dorsal fin shows large white spots arranged as transverse stripes. A diagnostic feature shared with only two relatives (Gymnogeophagus australis and Gymnogeophagus caaguazuensis) is that the front of the dorsal fin is hyaline to grey rather than coloured. Adult males develop a pronounced nuchal hump that begins at the level of the eyes, giving a concave forehead profile in side view; the lips, branchiostegal membrane and isthmus are grey, and there is no conspicuous dark oblique bar running from the eye to the front of the head. The fish is separated from its close ally Gymnogeophagus caaguazuensis by a longer caudal peduncle and a non-lyrate (rather than lyre-shaped) tail, and from Gymnogeophagus australis by the blue scale-centres and the absence of scales on the soft dorsal fin. Sexual dimorphism is therefore marked: only males build the head hump and the brightest blue-and-white nuptial dress, and they reach a larger size than females. Juveniles and stressed fish show a more muted, marbled brownish pattern over a whitish belly.
Habitat
Gymnogeophagus jaryi is a fish of small, clear running waters in the southern tributaries of the Middle Paraná basin. Its core range lies on the forested sierra of central Misiones around Aristóbulo del Valle, in streams such as the Cuña Pirú and Ñacanguazú. On the strength of photographs and mtDNA, the describers also report the species from the Manduviyú, Pirapó and Ype Curú drainages just across the river in Paraguay, so the documented range spans Argentina and Paraguay. The type habitat is described as streams with sandy and rocky bottoms and abundant marginal vegetation; the fish are most often caught over rocky areas or among marshy bankside plants. The setting is subtropical rather than equatorial: the climate on the ridge at Aristóbulo del Valle is classed as warm-temperate, with high year-round rainfall (about 1,26.5 in annually) and a strong seasonal temperature swing, the mean monthly air temperature ranging from roughly 75 °F in January to 58 °F in June (annual mean about 66 °F). Water level and turbidity vary with the rains, spiking muddy after storms and clearing between them; the lowest water and clearest conditions come in the December–February dry window. These are clear, well-oxygenated hill streams of moderate mineral content; the genus as a whole occupies soft to moderately hard, near-neutral water (roughly pH 6.5–7.5). The description does not report measured in-situ pH, conductivity or depth, but the documented biotope is shallow stream and stream-margin water on the order of a metre or less.
Feeding
Like its congeners, Gymnogeophagus jaryi is a benthic-feeding eartheater that works the stream bottom for small invertebrates and the organic film of the substrate. FishBase places it at a trophic level of about 3.4, the figure expected for a small carnivore-leaning omnivore, and hobby commentary on the species and the genus emphasises an insectivorous, invertebrate-based natural diet. Gymnogeophagus species characteristically take in mouthfuls of sand or fine sediment and sort edible items from grit using the gill rakers, expelling the cleaned substrate through the gill openings — the 'eartheater' habit that gives the group its name. Detailed gut-content work specific to Gymnogeophagus jaryi has not been published, so the dietary picture is inferred from its morphology, its rocky-and-sandy stream habitat and the well-documented feeding of related Gymnogeophagus. In its community it functions as one of the bottom-foraging consumers of stream invertebrates and detritus.
Mating
Direct field observations of courtship in Gymnogeophagus jaryi are limited, but its placement in the Gymnogeophagus gymnogenys species group tells most of the story, because that group is defined by maternal mouthbrooding. In mouthbrooding Gymnogeophagus the sexes pair only briefly to spawn; the female then takes the eggs into her mouth, so there is no long biparental tending of a fixed nest as there is in the substrate-brooding rhabdotus group. The pronounced nuchal hump and the vivid blue-and-white finnage of mature male Gymnogeophagus jaryi are classic male advertisement structures, used in display to females and in contests with rival males. Consistent with brief pair formation around a spawning site, the holotype (a male) was collected together with a female paratype under marginal vegetation, and the two were accompanied by fry in November 2016 — a snapshot of the species reproducing in early austral summer, as water levels fall and temperatures rise. As with the genus generally, spawning is tied to the warm season after the cool subtropical winter.
Breeding
Gymnogeophagus jaryi is a maternal mouthbrooder. The original description assigns it to the Gymnogeophagus gymnogenys group, whose members carry the eggs and larvae in the mouth rather than guarding them on a substrate, and the only direct breeding note in the literature is the November 2016 capture of the holotype and a female paratype together with free fry beneath bankside vegetation. Spawning thus occurs in the warm half of the year. No clutch count, egg size or brooding duration has been published specifically for Gymnogeophagus jaryi; in mouthbrooding Gymnogeophagus generally the female picks up a comparatively small clutch (on the order of a few dozen to perhaps a hundred eggs) and incubates them orally for one to a few weeks before releasing free-swimming young, which she may continue to shelter in her mouth when threatened. Parental care is therefore essentially maternal and oral. A pronounced cool-then-warm seasonal cycle is the natural reproductive trigger for the genus — a winter rest at low temperature followed by spring warming — and replicating that cycle is widely regarded as the key to inducing spawning in captive Gymnogeophagus. The specifics for Gymnogeophagus jaryi remain to be documented, as the fish is barely present in aquaculture or the hobby.
In the aquarium
Gymnogeophagus jaryi is, for practical purposes, not an aquarium fish. It originates in streams of Argentina and Paraguay, two countries with little ornamental-fish export, and at the time of its description aquarium editors were already cautioning hobbyists not to expect it in the trade. The genus as a whole has only a small following, partly because its members need a genuine cool season to thrive and breed and so do not suit a permanently tropical community tank. Any husbandry advice for Gymnogeophagus jaryi is therefore extrapolated from its close relatives and from general Gymnogeophagus keeping, not from kept specimens.
If it were maintained, the requirements would mirror other subtropical eartheaters. A bottom of fine sand is essential so the fish can sift naturally; sharp gravel risks damaging the gills of a substrate-sorting feeder. Given a maximum size around 5 in and the genus's moderate activity, a footprint of roughly 47 in (about 65–80 US gal) would be a sensible minimum for a small group, furnished with smooth rockwork and some marginal-style planting to echo the rocky, vegetated streams it comes from, plus clean, well-oxygenated water and a brisk current.
The single most important and most commonly mishandled parameter is temperature. Naked eartheaters are repeatedly kept too warm year-round; Gymnogeophagus jaryi comes from water that cools markedly in the austral winter, and a seasonal regime — letting the tank drift down toward the mid-teens Celsius in winter and back up to the mid-20s in summer — both keeps the fish healthy and is the recognised trigger for reproduction in the group. Aim for soft to moderately hard, near-neutral water (about pH 6.5–7.5). Diet should lean on small invertebrate foods — frozen and live bloodworm, daphnia and brine shrimp, with a quality sinking prepared food — reflecting its insectivorous natural feeding. Tankmates, were the fish available, are best chosen from other subtropical South Americans that tolerate the same cool winter; standard warm-water tropical community fish are a poor match for its temperature needs.
Conservation
The IUCN Red List assesses Gymnogeophagus jaryi as Endangered (EN) under criterion B1ab(iii), assessed on 16 December 2020 by F. Alonso and published in 2022; the listed range is Argentina (Misiones) and Paraguay, and the population trend is recorded as unknown. The Endangered listing rests on the species' small and fragmented distribution in the streams of the Middle Paraná basin and the inferred decline in habitat quality there. The 2019 description itself had suggested the species might qualify only as Least Concern, reasoning that it appeared 'widely distributed' across the southern Middle Paraná tributaries — so the formal Red List category and the describers' provisional view differ, and the EN assessment reflects the more conservative, criterion-based evaluation of a genuinely restricted range. The species has no CITES listing. The Middle Paraná region of Misiones combines notable local endemism with mounting pressure from deforestation, agriculture and the alteration of small watercourses, and as a small-stream specialist of that area, Gymnogeophagus jaryi is exposed to exactly the kind of localised habitat degradation that the B-criterion is designed to capture. It is of no fisheries interest and is harmless to humans.