Taxonomy & naming
Geryichthys sterbai was described by Manfred Zarske in 1997, who erected the new genus Geryichthys specifically to hold this species and described a second new species, Microcharacidium geryi, in the same paper. The type locality is in the Rio Ucayali drainage of Peru, part of the upper Amazon system. The genus and species sit in the family Crenuchidae — the South American darters and sailfin characins — within the order Characiformes.
Subsequent taxonomic work on the darter characins, notably a phylogenetic review by Graça et al. (2008), treated the genus-level distinctions among the small Characidium-relatives critically, and some sources have discussed Geryichthys in relation to (or as a synonym within) the broader Characidium group; this profile follows the original combination Geryichthys sterbai Zarske, 1997, as used by GBIF and the Catalogue of Life, without asserting a resolution to that debate one way or the other. The species epithet honours the German ichthyologist and aquarist Dr. Rudolf Zajíc, better known under the pen name "Sterba" — Dr. Günther Sterba, the author of the classic Freshwater Fishes of the World — a common dedication pattern in Zarske's characin descriptions of this period.
Morphology
No species-specific size or meristic data for Geryichthys sterbai were found in the sources consulted for this profile. Based on its placement among the small darter characins of the family Crenuchidae, and comparative maximum lengths reported for related Characidium species (roughly 2.5–3.5 in standard length across several congeners on FishBase), G. sterbai is inferred to be a similarly small, slender-bodied nano fish in the same general size range — but this figure is an inference from relatives, not a measurement of the species itself, and should be read as such.
Like other crenuchids, darter characins of this type typically show an elongate, faintly compressed body, a relatively large eye, and a build suited to perching on the substrate and darting between resting points rather than sustained swimming. No description of colour pattern, fin shape or sexual dimorphism specific to G. sterbai was available in the cache for this profile.
Habitat
Geryichthys sterbai is known from the Rio Ucayali drainage in Peru, part of the upper Amazon basin. Beyond the type locality, no detailed habitat account specific to this species was found — its precise microhabitat (stream size, substrate, current) has not been documented in the sources reviewed here.
By inference from the drainage and from the general ecology of Crenuchidae darters, which typically occupy clear-to-tea-coloured Amazonian streams and tributaries with soft, acidic-to-neutral water, G. sterbai likely lives in similar upper-Amazon stream habitat. This is a reasonable inference from the basin, not a confirmed water-chemistry reading for the species, and should be treated accordingly.
Feeding
No diet study specific to Geryichthys sterbai was found. Darter characins in general, including Characidium species and the related sailfin characins, are micro-predators that pick tiny invertebrates — chironomid (midge) larvae, other insect larvae and micro-crustaceans — off the substrate and surrounding cover, and they frequently ignore dry flake food entirely.
Were this species ever kept, the reasonable approach based on its relatives would be small live or frozen foods sized to a nano predator's mouth — microworm, baby brine shrimp, cyclops and finely crushed frozen bloodworm — rather than flake or large pellets.
Mating
No courtship or pairing behaviour has been documented for Geryichthys sterbai specifically. Within the family Crenuchidae, the sailfin characins (Crenuchus spilurus and Poecilocharax species) are known to court and pair around a cave or leaf surface, with the male displaying to attract a female to the chosen spawning site; true Characidium darters, by contrast, are generally understood to scatter eggs over the substrate without an elaborate pairing ritual. Given that Geryichthys was erected as distinct from Characidium and shows some affinity with the darter lineage, its mating behaviour is presumed to be closer to the Characidium pattern, but this has not been confirmed for the species and should be read as inference, not observation.
Breeding
No aquarium or field breeding record for Geryichthys sterbai was found in the sources reviewed. This species has essentially no history in the hobby, so nothing can be said with confidence about its reproductive mode from direct observation.
By analogy with its family, two patterns are known among crenuchid relatives: the sailfin characins (Crenuchus, Poecilocharax) are cave- or leaf-spawners in which the male briefly tends and guards a small clutch, a genuinely breedable pattern at nano scale in specialist aquaria; true Characidium darters are generally described as substrate egg-scatterers with no meaningful parental care. Without a direct record for G. sterbai, this profile does not assign it definitively to either mode — data is sparse, and that gap is stated rather than filled with a guess.
In the aquarium
Geryichthys sterbai is not established in the aquarium trade and no care reports specific to the species were found. It is not offered under a trade name and there is no meaningful hobbyist track record to draw on.
If a specimen were ever encountered — most plausibly as an incidental catch mixed in with wild-collected Characidium or other small Peruvian stream fish — the sensible starting point, based on its darter relatives, would be a quiet, well-filtered nano setup with fine sand or gravel, some current, and cover from wood or leaf litter, fed on small live or frozen invertebrate foods rather than flake. This is a general extrapolation from the family, not a tested care regime for this species.
Conservation
Geryichthys sterbai has not been evaluated by the IUCN Red List; no assessment page for the species could be located. Given how little is documented about its range, abundance or population trend beyond the original 1997 description and a single Peruvian drainage record, a Data Deficient or Not Evaluated status is unsurprising for a narrow-range, taxonomically obscure stream fish of this kind.
No specific threats to the species have been documented in the sources reviewed. As with other small, poorly studied fish restricted to upper Amazon tributaries, general watch items would include habitat degradation from deforestation and water-quality change in the Rio Ucayali drainage, but this profile does not assert any specific, documented pressure on G. sterbai itself.