Taxonomy & naming
Mustura sijuensis was described by the Indian ichthyologist A. G. K. Menon in 1987, in the volume Fauna of India: Pisces, as Noemacheilus sijuensis. Noemacheilus (and the closely related spelling Nemacheilus) was for decades the catch-all genus for South and Southeast Asian stone loaches; large-scale revisions since the 1990s have split it apart, and most Indian and Himalayan species — this one included — are now placed in Schistura, a genus of well over 200 recognized species of small stream and cave loaches. The Catalog of Fishes is the current authority for this combination, and the species remains classified under Schistura rather than the older Noemacheilus/Nemacheilus names still occasionally seen in older literature.
The species belongs to Nemacheilidae, the stone and brook loaches — the largest family within the loach superfamily Cobitoidea. The specific epithet sijuensis is toponymic, naming the type locality directly: Siju Cave in the Garo Hills of Meghalaya, India, the only place the species was originally collected.
This fish was long listed as Schistura sijuensis; Eschmeyer's Catalog of Fishes places the species in Mustura, and the current combination Mustura sijuensis is followed here.
Morphology
Mustura sijuensis is a small loach: the holotype measures 2.03 in standard length, with paratype specimens ranging from about 0.98–1.63 in SL, putting it among the smaller members of its genus. Like other Schistura it has an elongate, only moderately compressed body, and males possess the erectile suborbital spine characteristic of the family — a small defensive spine folded into a groove beneath the eye that can be flicked out, a feature shared with its botiid cousins though on a far smaller scale. The dorsal fin carries roughly eight-and-a-half branched rays, and the lateral line runs complete along the flank. Recorded body pattern includes eight to ten brown saddle markings crossing the back.
As a species collected from within a cave system, M. sijuensis shows the troglobitic traits typical of cave-adapted fishes — populations living in permanent darkness in this group are frequently reduced in pigmentation and eye development compared to surface relatives, though detailed comparative data on the degree of reduction in this particular species is sparse in the available literature. Its skin, like that of loaches generally, is essentially scaleless to the touch, a point that carries directly into how it must be handled and, if ever kept, medicated.
Habitat
The type locality is Siju Cave in the Garo Hills of Meghalaya, northeastern India — a limestone cave system with a clear, fast-flowing subterranean stream, high in dissolved oxygen. This is a genuinely dark, running-water habitat rather than the still, silted pools sometimes associated with cave fishes, and it places M. sijuensis among a small number of true cave-dwelling (troglobitic) loaches known worldwide. A population also occurs, or has been recorded, in the nearby epigean (surface) Pathar Syllang Stream in West Khasi Hills; whether these represent the same species or a related population is treated as uncertain in the source literature.
Precise water chemistry data — temperature, pH, hardness — has not been published for this species; cave streams of this type in the region are generally assumed to run cool relative to surface rivers at the same latitude, but that figure has not been directly measured and confirmed for Siju Cave itself, so it is reported here as an estimate rather than a verified reading. The known range is tightly restricted, on the order of 249 mi², making this a genuine single-system endemic.
Feeding
As a member of Schistura, M. sijuensis is presumed to be a benthic micro-predator in the pattern typical of the genus, taking small invertebrates, insect larvae, and crustaceans from the streambed. No species-specific dietary study has been published, and nothing is documented about its foraging behaviour in total darkness versus the epigean population, if the two prove to be the same species.
Given the near-total absence of husbandry records, there is no verified information on how this species would take to captive diets. Other small Schistura in the trade generally accept small live and frozen foods such as bloodworm and daphnia readily, along with sinking micro-pellets, but this cannot be stated as fact for M. sijuensis itself — it is noted here as a reasonable inference from the genus, not a confirmed observation.
Mating
Nothing has been published on courtship or spawning behaviour in Mustura sijuensis. Cave-dwelling fishes as a group often show unusual or attenuated reproductive strategies compared to surface relatives, but no observations — wild or captive — exist for this species to describe. Sexual dimorphism has not been documented; with specimens known only from a handful of type and paratype individuals, no reliable pattern of external sex differences has been reported.
Breeding
There is no record of Mustura sijuensis being bred, or even reliably observed spawning, in the wild or in captivity. Based on the reproductive pattern typical of Nemacheilidae generally, the presumed mode is egg-scattering over gravel or stream substrate with no parental care — the pattern seen across stone and brook loaches — but this has not been directly confirmed for this species. Given that it has essentially no history in the aquarium trade and is a narrow-range cave endemic under conservation concern, it should be treated as a species that is not bred in captivity, and any acquisition of the fish should raise serious welfare and conservation questions rather than breeding ambitions.
In the aquarium
Mustura sijuensis has essentially no presence in the aquarium hobby, and for good reason: it is a narrow-range, Endangered cave endemic collected — at most — from a single Indian cave system and one nearby stream. There is no established husbandry record, no verified temperature or water-chemistry range from captive care, and no track record of long-term survival in aquaria to draw on. Any general expectations have to be borrowed cautiously from other small Schistura: cool-to-moderate, clean, well-oxygenated, flowing water, a fine soft-to-gravel substrate, and — because loaches are essentially scaleless — caution with copper-based medications and full-strength treatments, half-dosing where medication is unavoidable.
In practice, this is not a species to seek out for a home tank. It is not established in the trade, its wild population is small and localized, and keeping pressure on a cave-restricted Endangered endemic runs directly counter to its conservation needs. Its value is as a subject of scientific and conservation interest rather than as an aquarium fish.
Conservation
Mustura sijuensis is assessed by the IUCN as Endangered, under criterion B1ab(iii), reflecting an extent of occurrence estimated around 249 mi² and an inferred ongoing decline in habitat quality. As a single-cave (or single-cave-plus-one-stream) endemic, it is inherently vulnerable: the entire known population depends on the integrity of Siju Cave and, if it is the same species, the Pathar Syllang Stream nearby.
The principal threats identified are limestone mining, quarrying, and deforestation in and around the Garo Hills — activities that can alter cave hydrology, silt the stream, and degrade the surrounding catchment that feeds it. Because troglobitic species like this one cannot simply relocate if their cave system is disturbed, habitat protection at Siju Cave itself is central to the species' survival; there is no aquarium trade pressure on this species to speak of, so its conservation story is a straightforwardly habitat-driven one.