Taxonomy & naming
Oxynoemacheilus phasicus was formally described in 2021 by Jörg Freyhof, Cüneyt Kaya, Levan Epitashvili and Matthias Geiger, in a paper in Zootaxa dedicated to Caucasian Oxynoemacheilus. It sits within the O. brandtii species group, a cluster of superficially similar stone loaches from the Caucasus that has only recently been resolved into distinct species using a combination of fine anatomical detail and DNA barcoding. The genus Oxynoemacheilus itself is one of several segregates carved out of the old, unwieldy Noemacheilus/Nemacheilus in recent decades — a reminder that Cobitoidea taxonomy has been reshuffled repeatedly, and the Catalog of Fishes remains the authority for which combination is currently valid.
The species epithet phasicus refers to the Phasis, the Greek name for the Rioni River, which together with the neighbouring Enguri drainage forms the entire known range of the species. Genetically it is distinguished from O. brandtii of the Kura drainage by a 7.5% divergence in the mitochondrial COI barcode, a substantial split for such superficially similar fish, and morphologically it is separated from the closely related O. cemali by a caudal fin that is only slightly emarginate rather than deeply forked, a pelvic fin that falls short of the genital papillae, and a pelvic-fin origin set below the first branched dorsal ray.
Morphology
This is a small, moderately slender stone loach, reaching about 3 in in standard length. The body is deepest at the midline between the nape and the origin of the dorsal fin, and the head is roundish and flattened on its underside — a shape typical of benthic loaches that spend their lives pressed against streambed gravel. Adults show a mottled or marbled pattern across the flanks, one of the diagnostic features of the wider O. brandtii group, while juveniles instead carry a series of distinct flank bars that presumably fade with age.
Like its relatives in the O. brandtii group the caudal fin is only slightly emarginate, with a comparatively deep caudal peduncle, and the pelvic fins are short, not reaching forward to the genital papillae. As a nemacheilid, it almost certainly carries the family's characteristic near-scaleless skin, though the original description does not go into skin or scale detail beyond separating it from close relatives on fin shape and colour pattern; published data on this species do not describe a suborbital spine, and unlike the botiid loaches, true nemacheilids do not carry one.
Habitat
Oxynoemacheilus phasicus is a narrow-range endemic, known only from the Rioni and Enguri river drainages of the eastern Black Sea basin in Georgia — river systems draining the southern slopes of the Caucasus into the Black Sea. It is a demersal, bottom-dwelling fish of freshwater streams in a temperate climate zone, consistent with the cool, flowing, well-oxygenated mountain and foothill streams typical of Caucasian stone loaches generally.
Published sources do not report specific water-chemistry measurements — temperature, pH or flow rate — for this species, so figures here are inferred from the habits of the O. brandtii group rather than measured for O. phasicus itself. Caucasian stone loaches of this kind typically live in cool-to-mild, fast-flowing, clear streams rather than warm, still, or blackwater habitats; this is a coldwater-to-cool-temperate fish, not a tropical one.
Feeding
No diet study has been published for Oxynoemacheilus phasicus specifically. Based on its family and its close relatives, it is almost certainly a benthic micro-predator, picking small invertebrates, insect larvae and crustaceans from the streambed of the fast, clear water it inhabits, in the manner typical of Nemacheilidae.
In the near-total absence of an aquarium history for this species, there is no keeper-reported feeding regime to draw on; any care approach would need to follow the general stone-loach pattern of small sinking live, frozen or pelleted foods taken off the bottom, rather than surface or flake feeding.
Mating
No courtship or spawning behaviour has been recorded for Oxynoemacheilus phasicus. Even basic reproductive data such as size or age at maturity are unknown for this recently described species — the original description and subsequent sources do not report a maturity range. This is honestly a gap in the literature rather than a case where data can be inferred with confidence.
Breeding
There are no published breeding observations for this species, in the wild or in captivity. Going by the pattern typical of stone and brook loaches (Nemacheilidae) generally, it is presumed to be an egg-scatterer, releasing eggs over gravel or among stones in flowing water with no parental care, but this has not been documented for O. phasicus itself.
The species has essentially no aquarium history to draw a breeding account from — it is rarely, if ever, encountered in the trade — and stone and brook loaches as a group are only occasionally bred even when better known. The honest summary is that breeding data for this fish is sparse to the point of being absent, and any keeper should treat it as effectively unbred in captivity.
In the aquarium
Oxynoemacheilus phasicus has no meaningful aquarium-keeping history; it is not established in the ornamental trade and there is no body of hobbyist experience to draw on. What can be said follows from its biology and its family: as a Nemacheilidae species from cool, fast, well-oxygenated Caucasian streams, any attempt to keep it would need to replicate that — cooler water than a typical tropical community tank, strong flow, high dissolved oxygen, and a fine gravel or stony substrate rather than a still, planted setup.
As with most true loaches, caution around medications is a sensible default even without species-specific data, since scaleless or near-scaleless skin is a general vulnerability across the family. Given how little is documented, and its narrow, single-country natural range, this is a species better left to specialist or scientific interest than treated as an aquarium candidate at present.
Conservation
Oxynoemacheilus phasicus has not been assessed on the IUCN Red List; it is listed here as Not Evaluated. This is unsurprising for a species only formally described in 2021 — it is simply too recent to have gone through a Red List assessment cycle.
What is known of its range is a cause for caution: the species is confirmed only from the Rioni and Enguri river drainages in Georgia, a genuinely narrow endemic range for a freshwater fish. FishBase records medium resilience, with a population doubling time of 1.4 to 4.4 years, which offers some buffer against short-term pressure, but a single-country, two-drainage range leaves any species exposed to habitat degradation, damming or pollution in those specific rivers. There is no evidence this species is targeted by the aquarium trade; its conservation profile, such as it is, rests entirely on the state of its home rivers rather than on collection pressure.