Taxonomy & naming
Oxynoemacheilus merga was described by the Polish-Russian naturalist Johann Heinrich (Ivan Andreyevich) Krynicki in 1840, hence the parentheses around his name — a signal that the species has since been moved out of its original genus. Like most western Palaearctic stone loaches, it spent long stretches of taxonomic history parked in the old catch-all genera Nemacheilus or Noemacheilus before the western Palaearctic lineage was split out into Oxynoemacheilus, the genus it is placed in today under the Catalog of Fishes.
It belongs to the family Nemacheilidae, the stone or brook loaches — by species count the largest family of loaches, and a group still being actively revised as genetic and morphological work continues to redraw genus boundaries across Anatolia, the Caucasus and Central Asia. O. merga is one of only four Oxynoemacheilus species that reach into Europe, the rest of the genus being concentrated in Anatolia and the Middle East, which makes this Caspian-basin population something of an outlier for the genus.
Morphology
This is a small loach, reported to reach only around 4 in standard length, with the elongate, sub-cylindrical body typical of Nemacheilidae — built for holding position on the bed of a fast stream rather than for open swimming. The caudal fin is emarginate (shallowly forked), and the dorsal fin carries around seven and a half branched rays, details that matter more to a taxonomist's key than to a keeper but that help confirm identification within a genus full of similar-looking small brown loaches.
As in other members of the family, expect the near-scaleless, thin skin typical of stone loaches and, where present in the genus, a small erectile spine below the eye — features that make Nemacheilidae as a whole sensitive to handling and medication even when a given species' fine anatomical detail hasn't been fully documented. Coloration and pattern for this species are not detailed in the available literature; like much of its biology, that gap likely reflects how rarely it has been studied outside faunal surveys rather than any lack of a real answer.
Habitat
Oxynoemacheilus merga is confined to the western Caspian Sea basin, recorded from the Kuma, Terek, Sulak, Shura-ozen and Samur river drainages spanning Azerbaijan, Georgia and Russia. Within those systems it lives in upper montane streams — cool, fast-flowing, well-oxygenated mountain water rather than the warm lowland channels many aquarium loaches come from.
Recorded water temperatures for the species run from about 50–72 °F, squarely on the cool side and a reminder that this is a temperate mountain-stream fish, not a tropical one. Nothing in the available sources suggests it tolerates still or warm water for any length of time, and its narrow, high-gradient range makes it the kind of stream endemic that is easy to overlook in survey work — part of why it is flagged as scarce but possibly under-recorded rather than confidently common or rare.
Feeding
FishBase places Oxynoemacheilus merga at a trophic level around 2.7, consistent with a benthic micro-predator that forages over the streambed for small invertebrates — aquatic insect larvae, worms and other tiny prey items typical of what a small Nemacheilidae loach picks out of gravel and cobble in fast water. No further detail on its specific diet has been documented.
In practice, a fish built this way should be treated as a bottom-feeding invertebrate hunter: it would be expected to take small live and frozen foods such as bloodworm or daphnia readily, with less interest in flake or plant matter, though this species itself has not been kept and observed closely enough in the hobby to confirm feeding behaviour firsthand.
Mating
No courtship or spawning behaviour has been documented for Oxynoemacheilus merga specifically. Stone loaches in this family are not known for any elaborate pairing display; where breeding has been observed in related Nemacheilidae, it typically involves fish gathering over gravel or among stones in fast-flowing water during a seasonal window, with no evidence of pair bonding or nest-building.
Given how little fieldwork exists on this particular species — it is known chiefly from faunal checklists and museum-style surveys of the Caspian drainages rather than dedicated life-history studies — any account of its mating behaviour beyond the general pattern for the genus would be speculation rather than documented fact.
Breeding
As in other stone and brook loaches, Oxynoemacheilus merga is presumed to be an egg-scatterer, releasing eggs over gravel or among stones in fast-flowing water with no parental care afterward — the standard reproductive mode across Nemacheilidae. No aquarium spawning of this species has been reported, and it is essentially unknown in the hobby, so there is no captive-breeding record to draw on.
Data on this species' reproduction is genuinely sparse rather than merely under-summarized here: it has not been the subject of targeted breeding studies, and what can be said responsibly is limited to the family-level pattern of gravel-scattering with no guarding of eggs or fry.
In the aquarium
Oxynoemacheilus merga is not an established aquarium fish — it does not appear to be traded, and any account of its care would be extrapolated from its wild habitat rather than from hobbyist experience. That habitat is unambiguous: cool, fast, clear, well-oxygenated mountain-stream water in the 50–72 °F range, over a gravel or cobble substrate, which is a very different set of conditions from the warm, still setups suited to many tropical loaches.
If this species were ever kept, the same caution that applies to Nemacheilidae generally would apply here: near-scaleless stone loaches tend to be sensitive to copper and to full-strength medications, so any treatment should be conservative. A keeper attracted to this species should recognise it as a cool-water specialist rather than try to fit it into a standard tropical community, and should expect to source very little husbandry information beyond what is given here.
Conservation
The IUCN Red List assesses Oxynoemacheilus merga as Least Concern, an assessment carried out in 2008. That status sits alongside notes in the literature that the species is scarce within its range but possibly overlooked in surveys — a combination that is common for narrow-range mountain-stream fish that are hard to sample rather than genuinely secure in large numbers.
No specific threats to the species are documented in the sources available here. Its restriction to a handful of western Caspian drainages in Azerbaijan, Georgia and Russia means that any future degradation of those upper montane streams — through damming, water abstraction or pollution — would be worth monitoring, but no such decline has been recorded for this species specifically.