Taxonomy & naming
Acanthocobitis longipinnis was described by the German naturalist Wilhelm Peters in 1861, based on material from the Ganges river basin in India — the holotype, a fish of about 6 in standard length, was collected near 25.4°N, 86.4°E. It is, in fact, the type species of the genus Acanthocobitis, which Peters erected in the same paper, making it historically important to the genus even though its status as a distinct species is now in question.
A taxonomic revision of the zipper loaches (Singer et al., Copeia 2014) examined Acanthocobitis and the related genus Paracanthocobitis in detail and concluded that A. longipinnis is a junior synonym of Acanthocobitis pavonacea, the peacock loach. Under that treatment the genus Acanthocobitis becomes monotypic, with A. pavonacea as its only valid member. The name A. longipinnis nonetheless persists in older literature, museum records and some hobby references, and the family placement — Nemacheilidae, the stone and brook loaches — is not in question.
Morphology
Like its close relatives, Acanthocobitis longipinnis is a slender-bodied stone loach with a pointed, forked caudal fin and an elongated dorsal fin — the long-finned appearance behind its species epithet, longipinnis. The body carries a greenish ground colour typical of the genus, and the head bears a small, vertically oriented flap of skin below the eye rather than the true erectile suborbital spine of the botiid loaches; this fleshy suborbital structure is a hallmark of the Nemacheilidae. The anus sits noticeably closer to the anal-fin origin than to the base of the pelvic fins, a detail used to separate Acanthocobitis from similar genera.
The skin is essentially scaleless to the touch, embedded with minute scales rather than carrying the coarse scaling of many other freshwater fishes — a trait shared across nearly all loaches and one that matters for how the fish should be handled and medicated. As with many small stream-endemic nemacheilids, detailed morphometric data for this specific nominal form is sparse in the literature, most of it folded into work on the wider Acanthocobitis/Paracanthocobitis complex.
Habitat
The species is recorded from the Ganges and Brahmaputra river systems of northern India and Bangladesh — fast-flowing, clear, well-oxygenated streams and rivers typical of the genus. Acanthocobitis and its relatives are bottom-dwelling fish of hill and foothill streams with a stony or gravelly substrate and continuous current, not the still, warm, blackwater habitats associated with many other tropical aquarium fish.
Water in this part of its range runs cool to moderately warm depending on season and elevation, and is well aerated by the current itself. As with many single-basin nemacheilid loaches, precise population-level habitat data — depth, exact substrate composition, seasonal flow variation — has not been separately documented for this nominal form, and much of what is known is inferred from the wider distribution and habits of Acanthocobitis and closely related genera.
Feeding
Acanthocobitis longipinnis is a benthic micro-predator, foraging along the streambed for small invertebrates — insect larvae, small crustaceans and other minute prey items dislodged by the current or hiding among gravel and stones. This matches the feeding style of stone and brook loaches generally: active, close-to-the-bottom hunters rather than grazers or scavengers.
In the aquarium, related Acanthocobitis species take readily to small live and frozen foods such as bloodworm and brine shrimp, and to sinking pellets or wafers; they can be reluctant to compete for food at the surface and may largely ignore flake. A keeper aiming to house fish under this name should offer a steady supply of small sinking foods rather than relying on flake alone.
Mating
No specific account of courtship or spawning behaviour for Acanthocobitis longipinnis was found in the sources consulted. Stone loaches in this genus are not known to pair-bond or exhibit elaborate courtship; spawning in related Nemacheilidae is typically a matter of ripe fish releasing eggs over gravel in flowing water with no extended pairing behaviour beforehand. External sexual dimorphism has not been documented for this species in the available literature, so nothing further can be said with confidence here.
Breeding
As with the great majority of stone and brook loaches, Acanthocobitis longipinnis (and its close relative A. pavonacea) is an egg-scatterer: ripe fish release eggs over gravel and among stones in fast-flowing water, with no parental care afterward. There is no cave-guarding or mouthbrooding behaviour in this group — that belongs to entirely different fish families.
Verified captive breeding of this species has not been documented in the sources reviewed, and Nemacheilidae as a whole are rarely bred in home aquaria. The honest picture is that this is a wild-collected or scientifically-collected fish rather than a hobby-bred one, and any keeper should expect to enjoy it as an adult specimen rather than plan on spawning it.
In the aquarium
Fish sold or labelled under this name should be kept as a small, fast-water stone loach: strong filtration and current, a gravel or cobble substrate, and consistently high dissolved oxygen are more important than any particular decor scheme. Being near-scaleless, like all loaches it is sensitive to copper-based medications and to full-strength doses of many standard treatments — half-doses and gentle products are the safer default — while at the same time loaches are frequently among the first fish in a tank to show signs of ich, so close observation matters.
Given the taxonomic uncertainty around this name, a prospective keeper should treat any fish sold as "Acanthocobitis longipinnis" with caution regarding identification, and provide the same husbandry recommended for its close relative A. pavonacea and other small Acanthocobitis/Paracanthocobitis stone loaches: a modestly sized, well-oxygenated, current-swept tank with sinking foods and no strong medications without care.
Conservation
Acanthocobitis longipinnis has not been separately assessed on the IUCN Red List, and no numeric assessment record was located for it. Its likely synonymy with Acanthocobitis pavonacea, a species with a wide historical distribution across the Ganges and Brahmaputra basins and generally considered secure, suggests no acute conservation concern under either name, but this is an inference, not a formal assessment.
As a small stream fish of the Ganges-Brahmaputra system, any population going by this name would face the same general pressures common to South Asian hill-stream fishes — water abstraction, siltation and habitat alteration in its native rivers — though no species-specific data on population trend or threat level was found in the sources reviewed.