Taxonomy & naming
Oxynoemacheilus fatsaensis was formally described in 2021 by Saygun, Ağdamar and Özuluğ, with the type locality given as the Elekçi Stream, a coastal Black Sea drainage in northern Anatolia — the specific epithet references Fatsa, the district on that coast. It sits within the genus Oxynoemacheilus, part of the family Nemacheilidae, the "stone" or "brook" loaches, which is by far the largest loach family and dominates the freshwater fish fauna of Turkey and the wider Middle East.
The description places the species in the O. bergianus species group and reports a COI barcode p-distance of 3.3% from its closest relative, O. banarescui — a small but diagnostic genetic gap that, together with distinguishing morphological characters, supported recognizing it as a new species rather than a population of an existing one. Turkish inland waters are unusually rich in this genus: Oxynoemacheilus alone accounts for 41 of the 49 nemacheilid species known from the country, a reflection of how thoroughly — and how recently — this fauna has been revised. As with many young splits within Cobitoidea, expect the genus-level picture here to keep shifting as more populations are barcoded; for now the valid combination is Oxynoemacheilus fatsaensis.
Morphology
The original description characterizes O. fatsaensis by a set of fine anatomical details typical of the genus: a suborbital groove present in males (a feature used to distinguish sex and species within Oxynoemacheilus), an axillary lobe at the base of the pelvic fin, no dorsal adipose crest along the caudal peduncle, a deeply emarginate (forked) caudal fin, small rostral barbels, and a mottled pattern across the flanks rather than bold banding. This is the conservative body plan shared across the Nemacheilidae — an elongate, sub-cylindrical body, a flattened belly suited to holding station on the streambed, and cryptic camouflage rather than the bright colours of the botiid loaches sold in shops.
As a true loach it is expected to carry the small, erectile suborbital spine typical of the group and to have the near-scaleless, or very finely scaled, skin characteristic of Nemacheilidae — a feature that matters for medication sensitivity even though it has not been documented specifically for this species. Detailed measurements and colouration beyond the diagnostic description are not available in the sources consulted; this is a recently described, poorly photographed species and further published detail is sparse.
Habitat
Oxynoemacheilus fatsaensis is known from the Elekçi Stream, its type locality on Turkey's Black Sea coast, and more widely from the Yeşilırmak drainage, a substantial river system in northern Anatolia. Both are cool, fast-flowing, well-oxygenated freshwater systems typical of the region's coastal and inland drainages, running over gravel and stone substrates rather than still or vegetated water.
As with the great majority of Oxynoemacheilus species, this is a fish of clear, current-swept streams rather than warm, still, or blackwater habitats — the genus as a whole is adapted to cool-to-mild flowing water across Anatolia, the Middle East and into the Caucasus and Central Asia. No specific water chemistry or temperature data have been published for this species, but its stream habitat is consistent with the cool, oxygen-rich conditions typical of Turkish nemacheilids.
Feeding
No feeding data have been published specifically for Oxynoemacheilus fatsaensis. Nemacheilid stone loaches as a group are bold benthic micro-predators, foraging over gravel and stone in fast water for small invertebrates — aquatic insect larvae, small crustaceans and other invertebrate prey dislodged by the current — rather than grazing algae or taking large amounts of plant matter.
In the absence of species-specific data, the safest statement is that O. fatsaensis most likely follows this genus-typical pattern: an active bottom forager taking small live prey items from fast, clear water. This should be treated as an inference from its relatives rather than a documented fact.
Mating
No courtship or spawning behaviour has been documented for this species — unsurprising for a fish described only in 2021 and never reported in aquarium literature. Stone and brook loaches in Nemacheilidae are, as a rule, egg-scatterers with no pair-bonding or nest-building behaviour recorded; spawning is typically associated with rising water and increased flow in the wild.
Sexual dimorphism has not been described for O. fatsaensis beyond the diagnostic note that the suborbital groove is present in males — a taxonomic character used to help distinguish the sexes and separate this species from close relatives, rather than a fully described secondary sexual trait.
Breeding
There is no record of Oxynoemacheilus fatsaensis being bred in captivity, and this is consistent with the genus and family as a whole: stone and brook loaches are egg-scatterers that release eggs over gravel and among stones in fast-flowing water, with no parental care, and the great majority of Nemacheilidae species are simply never spawned by hobbyists. This is compounded here by the species' complete absence from the aquarium trade.
Breeding data for this species should be treated as data sparse rather than absent by omission: nothing has been published, and nothing should be assumed beyond the general pattern for its family — an egg-scattering, non-guarding reproductive mode typical of fast-water Nemacheilidae.
In the aquarium
Oxynoemacheilus fatsaensis has no aquarium-keeping history; it is not part of the ornamental trade and no husbandry data have been published. It should not be assumed to tolerate warm, still community-tank conditions — Anatolian stone loaches of this genus generally come from cool, fast, highly oxygenated streams, and any speculative care would need to replicate that: strong flow, high dissolved oxygen, a hard gravel/stone substrate, and cooler temperatures than a typical tropical community tank.
As with other near-scaleless Nemacheilidae, a precautionary approach to medication (avoiding copper and full-strength treatments) would be prudent if the species were ever kept, though this has not been specifically documented. In practice this fish belongs to the large, quiet majority of loach species that exist only in the ichthyological literature and river-survey records, not in the hobby.
Conservation
Oxynoemacheilus fatsaensis has not been assessed on the IUCN Red List and is recorded here as Not Evaluated. As a species described only in 2021 and known from just two river systems — the Elekçi Stream and the Yeşilırmak drainage on Turkey's Black Sea coast — it has a narrow documented range, which is common among recently split Anatolian nemacheilids but also means very little is known about population trends or threats.
It carries no history of aquarium collection or trade, so it faces no documented ornamental-trade pressure. Any conservation concern would instead track the general pressures on small Anatolian coastal and inland drainages — water abstraction, damming and pollution — but no specific assessment of this species' status against those pressures has been published.