Taxonomy & naming
The species was described by Albert Günther in 1864 as Nemacheilus galilaeus, taking its name from the Sea of Galilee (Lake Tiberias), the type locality on the Israel–Syria border region. For much of its taxonomic history it circulated under Nemacheilus or Noemacheilus, the old catch-all genera into which most Eurasian stone loaches were once lumped.
A broader revision of the Nemacheilidae split those catch-all genera apart, and Nun galilaeus is the combination now recognised. Oxynoemacheilus is itself a large genus — on the order of 41 valid species — ranging from Albania in the west to central Iran in the east, of which this species is one of the westernmost, tied to the Jordan River drainage. The family Nemacheilidae is the largest of the loach families, the group of small, generally scaleless "stone" or "brook" loaches distinct from the spined Botiidae and the armoured Cobitidae.
This fish was long listed as Oxynoemacheilus galilaeus; Eschmeyer's Catalog of Fishes places the species in Nun, and the current combination Nun galilaeus is followed here.
Morphology
This is a small, elongated, subcylindrical loach, reaching only around 2.2 in in standard length. The body depth runs to roughly 15.5% of standard length and the head about 21.2% — a slim, low-slung profile typical of a fish that lives tight against the substrate. The lateral line runs the full length of the body to the anal fin, a detail used in separating it from close relatives.
Like other Nemacheilidae it is effectively scaleless — the skin is naked or bears only minute, deeply embedded scales — which makes it, as with all stone loaches, sensitive to copper and full-strength medications. Genus members typically carry the small erectile suborbital spine characteristic of loaches generally, used defensively rather than for any active role in feeding or territory.
Habitat
Nun galilaeus is endemic to the Jordan River basin, historically known from Lake Tiberias (the Sea of Galilee) and Lake Hula in Israel and from Lake Muzairib in southern Syria. It has since been extirpated from Lake Hula, and current records place the only surviving population in Lake Muzairib. FishBase describes it as a demersal, subtropical, non-migratory freshwater fish found in shallow water near the shore, among stones and submerged roots rather than in open water or deep channels.
Lake Muzairib itself is under real pressure: moderate pollution and a declining water level driven by over-abstraction and reduced rainfall have degraded the lake in recent years, even as the species is still reported to be locally abundant there. Because the entire known population is confined to one small, shallow, subtropical lake, its fate is now essentially tied to the fate of that single water body.
Feeding
FishBase lists a trophic level for the species of around 2.8, consistent with a herbivore/detritivore leaning diet rather than a pure invertebrate hunter — grazing algae, biofilm and organic detritus off stones and root surfaces in the shallows it inhabits, likely supplemented opportunistically with small invertebrates as in most Nemacheilidae. Detailed dietary studies are not available for this species; the picture here is inferred from its trophic level and from the foraging habits typical of small stone loaches in similar habitat.
As with its relatives, feeding is a bottom-oriented, low-intensity activity carried out over stones and root tangles in shallow, near-shore water rather than in open water or on the wing.
Mating
No published account of courtship or spawning behaviour for Nun galilaeus was found. Data on reproduction in this species is genuinely sparse — unsurprising for a Critically Endangered fish confined to a single lake and effectively absent from scientific aquaculture and the aquarium trade alike.
By analogy with other Nemacheilidae stone loaches, which typically pair or group loosely over gravel and stones in flowing or shallow water without forming lasting bonds or defended territories, courtship in this species is presumed to be a brief, non-elaborate affair tied to suitable water conditions rather than a seasonal migration of the kind seen in botiid loaches.
Breeding
Breeding biology has not been documented for this species specifically. Based on the pattern general to stone and brook loaches (Nemacheilidae), it is presumed to be an egg-scatterer that releases eggs over gravel or among stones in shallow water, with no parental care afterward — the same pattern seen across the family, from European Barbatula to Asian Schistura.
There is no record of this species being bred in captivity, and given its restriction to a single Syrian lake and its near-total absence from the aquarium trade, no captive breeding programme is known to exist. As with most stone loaches, the honest position is that reproduction has never been observed or managed by aquarists, and the species' survival currently depends entirely on the condition of its one remaining wild population.
In the aquarium
Nun galilaeus is not a hobby fish. Its extreme rarity, single-lake distribution and Critically Endangered status mean it is not collected, traded or kept in aquaria, and no species-specific care data exists in the aquarium literature. Nothing here should be read as an invitation to seek the species out for a tank.
What can be inferred from its habitat and its Nemacheilidae relatives is a fish that would need cool-to-warm, clear, well-oxygenated, shallow flowing or lake-edge water over stones and fine gravel with minimal vegetation, and — being effectively scaleless like all stone loaches — would be highly sensitive to copper-based medications and full-strength treatments generally, a point worth remembering for its close relatives that do occasionally appear in the trade.
Conservation
The IUCN Red List classifies Nun galilaeus as Critically Endangered, an assessment last updated in the 2022.2 Red List cycle. The species has already been lost from Lake Hula and from its type locality, Lake Tiberias, and the only population known to persist today is in Lake Muzairib in southern Syria.
Even that last refuge is under strain: moderate pollution and a falling water level, driven by over-abstraction and low rainfall, are degrading Lake Muzairib, though the species is still reported locally abundant there for now. With its entire global population dependent on the water quality and water level of a single small lake, Nun galilaeus is a stark illustration of how a wide historical range can collapse to one fragile remnant — and how directly a fish's fate tracks the health of its water body.