Taxonomy & naming
Paracanthocobitis triangula was described by Singer, Pfeiffer & Page in 2017 as part of a revision that split the previously broad Paracanthocobitis zonalternans species complex into several distinct species, of which P. triangula was one of three described as new. It belongs to the genus Paracanthocobitis, within the family Nemacheilidae — the stone or brook loaches, the largest family within the loach superfamily Cobitoidea and a group whose taxonomy has been repeatedly reshuffled as older, broadly defined genera such as Nemacheilus and Noemacheilus have been split apart on closer inspection.
The species epithet triangula is Latin for "triangular," referring to a small black triangular blotch within the ocellus (eye-spot) marking on the upper margin of the caudal peduncle — the diagnostic feature that helps separate it from its close relatives in the zonalternans complex. No English common name is in general circulation; "Wedged zipper loach" appears as a vernacular label in occurrence databases but is not an established trade name, since the species is not known to be traded.
Morphology
This is a small, slender true loach built on the standard Nemacheilidae plan: an elongate, only lightly compressed body, a subterminal mouth adapted for picking at the substrate, and — where present in the genus — an erectile suborbital spine tucked in a groove beneath the eye, used defensively rather than for feeding. The skin bears the family's characteristically reduced or embedded scalation, a trait that leaves nemacheilids sensitive to medications and copper in the same way as their spined-loach relatives.
The original description records a maximum size of just 1 in standard length in a female specimen, making Paracanthocobitis triangula one of the smallest loaches known — a genuine micro-species rather than a small-for-its-family fish. Diagnostic features noted in the type description include an incomplete lateral line, the specific dorsal-fin ray count, an axillary pelvic lobe, and a pattern of black blotches culminating in the small triangular mark within the caudal-peduncle ocellus that gives the species its name. Beyond these type-description characters, detailed morphological data — growth curves, sexual size differences, live coloration in the field — has not been documented.
Habitat
Paracanthocobitis triangula is endemic to northeastern Bangladesh, known only from the Nur Nody Stream (upstream of the Lubha River near Nihalpur Village) and the Dauki/Piyain Gang River system in the Sylhet region, both part of the wider Brahmaputra (Jamuna) and Meghna drainages. This narrow, two-stream range is typical of the many nemacheilid loaches described in recent taxonomic revisions across South and Southeast Asia, where a single clear-water stream system can hold a genetically distinct, geographically isolated species.
As with much of its family, the species is presumed to favor clear, flowing streams with a rocky or gravelly bottom, consistent with the demersal, bottom-hugging habits typical of stone loaches. Specific data on flow rate, substrate composition, temperature and water chemistry at the type localities were not captured in the sources available for this profile, and should be treated as unconfirmed rather than assumed — this is a genuinely data-sparse species outside its original description.
Feeding
No dedicated dietary study exists for Paracanthocobitis triangula, but its trophic level estimate and its place within Nemacheilidae point to the same general strategy shared by nearly all stone and brook loaches: an omnivorous bottom-feeder picking small invertebrates, insect larvae and crustaceans, along with some organic detritus, off the streambed. This micro-predatory habit is consistent across the family and is the safest inference in the absence of direct field observation for this species.
Nothing has been documented about feeding behavior in captivity, since the species has no known aquarium history. Any care approach would need to follow the conservative pattern used for other small nemacheilids — small sinking live, frozen or prepared foods offered at the bottom — rather than any species-specific record.
Mating
No information on courtship, spawning triggers, or sexual behavior has been documented for this species. Stone and brook loaches as a family are not known for elaborate courtship or pair-bonding; where reproduction has been observed in related Nemacheilidae, it typically follows the broad pattern of egg-scattering over gravel in flowing water with no pairing behavior beyond the act of spawning itself. Whether Paracanthocobitis triangula follows a seasonal or flow-triggered spawning cue, as many stream-dwelling Asian loaches do, has not been studied.
Breeding
There are no aquarium breeding records for Paracanthocobitis triangula, and the species is not present in the ornamental trade. Based on the general pattern for Nemacheilidae, natural reproduction is presumed to be egg-scattering over gravelly or rocky substrate in flowing water, with no parental care — the typical mode for stream-dwelling stone loaches. This should be read as an inference from family-level biology rather than a documented fact for this species specifically.
Like the great majority of small nemacheilid stone loaches, Paracanthocobitis triangula has never been bred in captivity, and given its narrow endemic range and total absence from the hobby, it is unlikely that a keeper will encounter it to attempt breeding in the foreseeable future.
In the aquarium
Paracanthocobitis triangula is not established in the aquarium trade, and there is no reliable husbandry record to draw on. Nothing in the available sources documents tank size, social behavior, water parameters, or long-term care for this species in captivity — any such information would be speculation rather than grounded fact, and is deliberately omitted here.
What can be said with reasonable confidence, by extension from its family, is that any nemacheilid this small and stream-adapted would need clean, well-oxygenated, flowing water and a fine gravel or sand substrate, and — being a near-scaleless loach — would likely share the family's sensitivity to copper-based medications and full-strength treatments. A prospective keeper encountering this species, most plausibly through specialist or scientific channels rather than the general trade, should treat it as an experimental subject requiring cautious, conservative care rather than a fish with an established protocol.
Conservation
Paracanthocobitis triangula has not been evaluated by the IUCN Red List and carries no formal conservation status. Its known range is restricted to two stream systems in the Sylhet region of northeastern Bangladesh, a pattern that in many other narrowly endemic nemacheilid loaches has drawn conservation concern once formally assessed, given the vulnerability of single-watershed endemics to pollution, damming, and habitat degradation.
No assessment currently exists to confirm or rule out such concerns for this species, and no population trend data has been published. Its absence from the aquarium trade means collection pressure is not a documented threat; the more likely risks, as with many small-range Bangladeshi and northeastern Indian stream fishes, would be habitat-related rather than trade-related, though this remains unconfirmed for this specific species.