Taxonomy & naming
Styxichthys gejiuensis was described by Chu and Chen in 1979, one of many Triplophysa species named from the karst cave systems and cold streams of the Qinghai-Tibet Plateau and surrounding southern China. The species epithet gejiuensis simply means "of Gejiu," naming the prefecture-level city in Yunnan Province near which the type locality lies.
It belongs to the family Nemacheilidae — the stone or brook loaches, the largest family within the loach superfamily Cobitoidea — in the genus Triplophysa, a large Eurasian genus concentrated on the Tibetan Plateau and in Central Asia that includes many other cave-adapted and surface-stream species. As with much of this genus, older literature may use the now-superseded name Noemacheilus or Nemacheilus for related fish, but the current valid combination for this species is Styxichthys gejiuensis.
This fish was long listed as Triplophysa gejiuensis; Eschmeyer's Catalog of Fishes places the species in Styxichthys, and the current combination Styxichthys gejiuensis is followed here.
Morphology
This is a small loach, reaching only about 2 in in standard length, and it shows the unmistakable signature of a cave-dwelling (troglobitic) fish. Its eyes are degenerated — reduced and functionally blind — and its body lacks pigment, leaving it pale rather than patterned. Three pairs of barbels around the mouth take over the sensory role the eyes have given up, letting the fish feel its way over rock and sediment in total darkness.
Like other Nemacheilidae it is essentially naked: the body carries no meaningful scale covering, a trait shared across the family and one with real consequences for how sensitive the skin is to chemical treatment. Data on other morphological details — fin ray counts, precise proportions — are sparse for this species; most of what has been documented centers on the diagnostic cave adaptations rather than a full anatomical description.
Habitat
Styxichthys gejiuensis is known only from a single underground river at Bajianjing, near Qifang, close to the city of Gejiu in Yunnan, China — a subtropical karst region roughly between 21° and 25° N. It has not been recorded from any other cave system, making it a true single-site endemic in the strictest sense.
The water in this cave system is described as freshwater to brackish, cool and mineral-rich in the way karst groundwater typically is, but as an obligate cave species it has no meaningful history in surface streams. The critical habitat problem is that the water at the type locality has been progressively withdrawn for industrial and domestic use, drying or degrading the very cave system the species depends on — the reason FishBase describes wild populations as almost extinct.
Feeding
No dietary studies specific to Styxichthys gejiuensis appear in the available sources, and none should be assumed. Cave-adapted nemacheilids in general are thought to forage as opportunistic micro-predators and scavengers on whatever organic material and small invertebrates drift or fall into the cave system — there being no photosynthetic food web to speak of underground — but this is inference from the family's ecology rather than a documented fact for this species.
Given that this fish has essentially no aquarium history, there are no husbandry feeding notes to draw on. Data is sparse, and that gap should be stated honestly rather than filled with a generic loach diet.
Mating
No courtship or spawning behaviour has been documented for Styxichthys gejiuensis. Nothing in the available sources describes sexual dimorphism, seasonal cues, or pairing behaviour for this species, which is unsurprising for a single-cave endemic that has never been kept or studied under aquarium or laboratory conditions.
Breeding
There is no record of this species being bred in captivity, and essentially nothing published on its reproductive biology in the wild. Based on the pattern seen across the stone and brook loaches (Nemacheilidae) generally, the expected mode would be an egg-scatterer producing eggs with no parental care, but this has not been confirmed for Styxichthys gejiuensis specifically.
The honest picture is simply that this is a data-sparse cave species with no aquarium-breeding history whatsoever — a fish known from field surveys of one cave system, not from any breeding program or hobbyist account.
In the aquarium
Styxichthys gejiuensis has no meaningful presence in the aquarium hobby. It is not traded, not kept, and given its status as a Critically Endangered to Vulnerable single-site cave endemic in China, it should not be collected for the trade — any specimens taken from its one known population would represent a direct threat to the species' survival.
What can be said, by extension from its relatives, is that as a scaleless nemacheilid it would be expected to be highly sensitive to copper and full-strength medications, and as a cave fish it would need to be kept in complete darkness and cool, clean, mineral-rich water rather than the brightly lit, planted setups typical of most stream loaches. None of this is a care recommendation, since the species simply is not available — it is a note on what its biology implies, not a husbandry guide.
Conservation
Conservation status for this species is unusually pointed for a loach: FishBase's assessment, dated August 31, 2010, lists it as Critically Endangered under criteria B1ab(iii)+2ab(iii) — a small range, a single location, and continuing decline in habitat quality — and describes the species as "almost extinct." The stated driver is the abstraction of water from its type-locality cave system for industrial and domestic use, which has degraded or drained the only habitat this fish is known to occupy. A later IUCN Red List entry (2022.2) instead lists the species as Vulnerable; the two assessments disagree, and this article records both rather than picking one to state as settled fact.
Whatever the precise category, every source agrees this is a single-cave endemic with no other known population, no aquarium trade to relieve or add pressure, and a habitat under direct, documented threat from water extraction. It is as clear a case as exists among loaches of a species whose entire fate rests on the water quality and quantity of one specific place.