Taxonomy & naming
Oxynoemacheilus theophilii was described in 2006 by Stoumboudi, Kottelat and Barbieri, part of the wave of revisionary work that carved the old, catch-all genus Noemacheilus (and later Nemacheilus) into a series of narrower western Palaearctic genera. Oxynoemacheilus is one of those genera, erected for a group of stone loaches distributed across Anatolia, the Levant, the Caucasus and the eastern Mediterranean, and this species sits within it as a Büyük Menderes drainage and Lesbos endemic.
It belongs to the family Nemacheilidae, the stone or brook loaches — by far the largest loach family, encompassing hundreds of small, often narrowly-distributed species across Eurasia. The species epithet honours Theophilus Chatzimichael (1873–1934), the celebrated folk painter of Lesbos, reflecting the island population's discovery locality on the Evergetoulas Stream. The Catalog of Fishes is the reference for the valid combination, and no synonymy issue affects the current name.
Morphology
This is a small, slender stone loach, with FishBase recording a maximum length of about 3 in total length in males and unsexed individuals, and around 2.5 in standard length reported for the largest specimens examined. Like other nemacheilids it has an elongate, only mildly compressed body, a ventrally flattened head suited to life on the streambed, and small barbels around the mouth for detecting food by touch and taste.
As in its relatives, the skin bears very small, deeply embedded scales rather than large exposed ones, leaving it functionally close to naked — a trait shared across the stone loaches that makes them sensitive to harsh water conditions and medications. Detailed morphometric and meristic data for this species come from the length-weight relationship studies and the genus-wide revision of Özkurt and colleagues, which situate it among the western Palaearctic Oxynoemacheilus assemblage rather than describing dramatic ornamentation; most of its story is one of quiet, cryptic adaptation to fast clear water rather than showy pattern.
Habitat
Oxynoemacheilus theophilii is confined to a genuinely small range: the Büyük Menderes (Meander) River drainage of western Anatolia in Turkey, and the Evergetoulas Stream on the Greek island of Lesbos — an unusual trans-Aegean distribution for a freshwater fish with no ability to cross open sea, likely reflecting an older land connection or drainage history between Anatolia and Lesbos. FishBase describes it occupying the upper reaches of streams with current, in clear, cold fresh water.
This places it firmly among the cool, fast-flowing, well-oxygenated headwater habitats favoured by many Oxynoemacheilus species, rather than warm, still, or lowland water. It is a demersal fish, living in close contact with the substrate of gravel and stone over which the current runs. Because its entire known range amounts to one river drainage and one small stream, the species is intrinsically vulnerable to any change in flow or water quality at those specific sites.
Feeding
Stomach content examination of a female specimen found aquatic insect larvae, consistent with the general pattern for stone loaches as bold, benthic micro-predators that comb the streambed for small invertebrate prey. Like other Nemacheilidae, Oxynoemacheilus theophilii most likely also takes other small invertebrates and crustaceans encountered on the substrate of its fast, clear home water, though comprehensive dietary data for the species remain sparse.
In captivity, stone loaches of this type generally do well on small sinking live and frozen foods — bloodworm, brine shrimp, small chopped invertebrates — and often ignore floating flake, reflecting their bottom-oriented, current-adapted foraging style. No aquarium feeding data specific to this species exist; the above follows from its confirmed insect-larvae diet and general Oxynoemacheilus biology.
Mating
No courtship or spawning behaviour has been documented for Oxynoemacheilus theophilii in the wild or in captivity. Stone loaches in this genus are not known for pronounced external sexual dimorphism, and nothing species-specific has been published distinguishing males from females beyond general body condition.
As with the great majority of Nemacheilidae, reproduction in this species is presumed to follow the family's general pattern: a seasonal spawning tied to flow and temperature conditions in its cold, fast headwater streams, timed to favourable conditions for egg and larval survival. Given how narrow and little-studied its range is, this remains an inference from the wider genus rather than a directly observed account for theophilii itself.
Breeding
There is no record of Oxynoemacheilus theophilii being bred in the home aquarium, and the species is essentially unknown to the hobby. Stone and brook loaches of this genus are, in general, egg-scatterers that release eggs over gravel and among stones in fast-flowing water, with no parental care afterward — they are not cave-guarding male-brooders, and they are not mouthbrooders.
For this species specifically, data are sparse to the point that even that general pattern is an inference from its relatives rather than a direct observation. A keeper attempting to work with this fish should expect to be operating with almost no precedent: breeding.mode here is best summarised as egg-scattering over gravel in fast water with no parental care, and rarely if ever achieved in captivity.
In the aquarium
Oxynoemacheilus theophilii is not established in the aquarium trade and is not a fish a hobbyist is likely to encounter for sale; what follows is drawn from its confirmed biology rather than any body of keeper experience. Its small adult size (under 3 in) and requirement for cool, clear, fast-moving, well-oxygenated water point toward a specialised streamtank setup — strong flow, high dissolved oxygen, a gravel or stone substrate, and cooler temperatures than the average tropical community tank — rather than a standard warm, still aquarium.
As with other nemacheilids, its near-scaleless skin means it should be treated cautiously with medications and copper-based treatments, and it would be expected to be an early indicator of poor water quality or parasite outbreaks in a mixed setup. Given its Endangered status and total lack of an established hobby population, the responsible position is that this is a species to admire in the scientific literature rather than to seek out for a tank — any specimens in captivity should trace to legitimate research or conservation-breeding programmes, not wild collection from its fragile home streams.
Conservation
The IUCN Red List assesses Oxynoemacheilus theophilii as Endangered, an assessment dated 18 September 2024 and based on criteria B2ab(iii,v) — a small, fragmented range with continuing decline in habitat quality and population. This reflects the fish's extreme geographic restriction to the Büyük Menderes drainage in Turkey and the Evergetoulas Stream on Lesbos, Greece.
Water abstraction for agriculture and other uses, compounded by climate-change-driven drought across the eastern Mediterranean, are identified as the pressures acting on its habitat — a combination that reduces flow, raises temperature and can dry up the small headwater streams this species depends on. With no aquarium trade to speak of, the entire conservation picture for this species rests on protecting the hydrology of a small number of specific watercourses in western Anatolia and Lesbos.