Taxonomy & naming
This loach was described by Erk'akan & Kuru in 1986, originally as Orthrias angorae ercisianus, a subspecies of the widespread Anatolian stone loach Orthrias (now Oxynoemacheilus) angorae. It has since been raised to full species status and placed in Oxynoemacheilus, one of several genera carved out of the old catch-all Noemacheilus/Nemacheilus as the family Nemacheilidae was revised through DNA and morphological work. The Catalog of Fishes is the governing authority for the valid combination, Oxynoemacheilus ercisianus.
The species epithet ercisianus refers to Erciş, the district on Lake Van's northeastern shore where the type locality — the Ercis (Erciş) stream — is found. It belongs to family Nemacheilidae, the stone and brook loaches, the largest family within the loach superfamily Cobitoidea and one dominated by small, often narrowly endemic stream species across Eurasia.
Morphology
Oxynoemacheilus ercisianus is a small, slender stone loach, reported to reach about 3 in in total length — modest even by the standards of this diminutive family. Like other Nemacheilidae it has an elongate, cylindrical body, a flattened underside suited to hugging the streambed, and small barbels around the mouth for feeling out food on the bottom. Detailed colour-pattern and fin-ray descriptions specific to this species are not covered in the available sources, so we avoid guessing at fine identification marks here.
As in other members of the genus, the skin carries the family's typically small, deeply embedded scales rather than the more heavily armoured covering of some other fish groups, and members of this family generally lack the prominent erectile suborbital spine that characterizes the true spined loaches (Botiidae and Cobitidae) — that spine is not a Nemacheilidae trait. Fine anatomical detail beyond what the sources confirm is left unstated rather than invented.
Habitat
This is a strict endemic of the Lake Van basin in eastern Turkey, recorded from the tributary streams Zilan, Deli, Bendimahi and Karasu, which flow into the lake's eastern shores from its type locality at the Ercis stream. In an unusual twist for a freshwater stream loach, a 2018 study also documented the species living within Lake Van itself, among microbialite formations on the lake bed — a genuinely striking record, since Lake Van is the world's largest saline soda lake, with water around pH 9.8, far more alkaline than almost any water a freshwater fish tolerates.
Its core habitat, however, is the clear, fast-flowing, well-oxygenated mountain streams that drain into the lake — classic Nemacheilidae water. Precise temperature and pH figures for these tributary streams are not given in the available sources, so exact numbers are left as data-sparse rather than guessed; what is documented is that this is a cool, fast, clear-stream fish first, with the Lake Van population representing a remarkable exception rather than the rule.
Feeding
Specific dietary data for Oxynoemacheilus ercisianus was not found in the available sources. Based on its family, it is presumed to be a benthic micro-predator in the pattern typical of Nemacheilidae, picking small invertebrates, insect larvae and crustaceans from the streambed of the fast, clear water it inhabits.
In the aquarium, a fish of this type would be expected to take small live and frozen foods such as bloodworm and daphnia, along with sinking pellets or wafers, and may show little interest in flake left to float. As with its wild diet, this is an inference from the family rather than a documented fact for this exact species, and should be treated as a starting point rather than a settled feeding regime.
Mating
No documentation of courtship, sexing or spawning behaviour for Oxynoemacheilus ercisianus was found in the available sources. What is known is that the species is oviparous, laying eggs rather than bearing live young, in keeping with the rest of the family.
Given how little is recorded even about its basic biology beyond distribution and conservation status, mating behaviour for this species should be treated as essentially undocumented. Nothing here should be read as evidence of a cave-guarding, mouthbrooding or pair-bonding strategy — none of those patterns are known in stone and brook loaches, which as a family are egg-scatterers.
Breeding
As with feeding, no captive-breeding record for this species was found. In line with its family, Oxynoemacheilus ercisianus 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 pattern for Nemacheilidae and quite different from the cave-spawning or mouthbrooding strategies seen in unrelated fish groups.
The honest picture is that this species, like the great majority of stone and brook loaches, is rarely if ever bred in the home aquarium; it is essentially unknown in the hobby at all given its extreme range restriction to a single lake basin in eastern Turkey, and any breeding account here would be invented rather than sourced. Data is sparse to the point of nonexistent for aquarium spawning.
In the aquarium
Oxynoemacheilus ercisianus is not an established aquarium fish; no keeping accounts, husbandry notes or trade records were found in the available sources, and given its Endangered status and single-basin endemism it should not be collected or traded. What can be said, extrapolated cautiously from its family and habitat, is that any stone loach of this kind would need cool, clear, fast-flowing, highly oxygenated water reflecting the mountain streams it comes from — not a warm, still community tank — plus a fine gravel or sand substrate to forage over and a lid, since small benthic loaches are accomplished escape artists.
As with all Nemacheilidae, the near-scaleless skin typical of the family would be expected to make it sensitive to copper-based medications and full-strength treatments generally, meaning any hypothetical care would call for gentle, reduced-dose medicating and close disease monitoring. Given the total absence of a hobby track record, however, this section is necessarily speculative and this fish is best understood as a conservation subject rather than an aquarium candidate.
Conservation
Oxynoemacheilus ercisianus is listed as Endangered (EN) on the IUCN Red List, assessed in 2013. It is restricted to the Lake Van basin of eastern Turkey — a drainage of roughly 2,000 square kilometres — with no other known population anywhere, making it exceptionally vulnerable to any single localized threat. Recorded threats include habitat degradation, stream modification and pollution within its narrow tributary range, though detailed population trend data is not available.
The 2018 discovery of a population living within Lake Van itself, among microbialite structures despite the lake's extreme alkalinity, underscores both the fish's unusual physiological tolerance and how much remains to be learned about it. As a single-basin endemic with no aquarium trade to speak of, its conservation is a matter of protecting the Zilan, Deli, Bendimahi and Karasu streams and the wider Lake Van watershed, not of managing collection pressure.