Taxonomy & naming
Paracanthocobitis nigrolineata was formally described by Singer, Pfeiffer & Page in 2017, in a revision of the Paracanthocobitis zonalternans species complex published in the journal Zootaxa (volume 4324, pages 85–107). That complex had accumulated a confusing mix of populations under a single name; the 2017 paper untangled it by describing three new species — P. nigrolineata, P. marmorata and P. triangula — while designating a neotype for the original P. zonalternans from Huai Mae Charno, Mae Romat, Thailand, since the original type locality (Tenasserim, Burma) could no longer be tied to verifiable material.
The genus Paracanthocobitis itself sits within Nemacheilidae, the stone or brook loaches — the largest family in the loach superfamily Cobitoidea, and one that has been repeatedly reshuffled as older catch-all genera such as Nemacheilus and Noemacheilus were split apart using more rigorous anatomical and molecular characters. The Catalog of Fishes (Eschmeyer, CAS) is the standing authority for the valid combination Paracanthocobitis nigrolineata; there is no synonymy to speak of, since the species was erected new rather than renamed from an older name.
Morphology
This is a small, elongated loach with the classic stone-loach body plan: a slim, roughly cylindrical trunk, a pointed snout, and a slightly flattened underside suited to hugging the streambed. FishBase records a maximum size of about 1.5 in standard length in females, making it one of the more diminutive members of its family. Its most distinctive field mark, and the source of both its scientific and common names, is a bold black lateral stripe running along the flank — nigrolineata is Latin for "black-lined," and the trade name "black-lined zipper loach" describes the same feature.
As in other nemacheilids, the skin carries only small, deeply embedded scales or none at all over much of the body, leaving it functionally naked and correspondingly sensitive to poor water quality and to strong medications. Many stone loaches in this family also bear a small erectile spine beneath the eye, a feature shared more prominently by their botiid relatives; where present in Paracanthocobitis it is far less developed than in the true spined loaches, but the same underlying caution about handling and netting still applies.
Habitat
Paracanthocobitis nigrolineata is known from the Irrawaddy and Sittang river drainages of Myanmar and the Mae Khlong drainage of Thailand — clear, flowing freshwater systems draining hill and lowland country in mainland Southeast Asia. FishBase classifies it as a demersal, tropical freshwater species, consistent with the rest of its genus: a bottom-dweller of streams with clean substrate rather than a fish of still, blackwater pools.
Specific published temperature and pH data for this species are not available; FishBase and the other cached sources record its distribution and morphology but not its water chemistry. Related Paracanthocobitis species, such as P. urophthalma, have been reported from cooler flowing water in the roughly 70–77 °F range, which is a reasonable proxy for what a fast, well-oxygenated tropical Southeast-Asian stream loach needs, but keepers should treat this species' precise parameters as data-sparse rather than firmly established.
Feeding
FishBase lists a trophic level of 2.9 for P. nigrolineata, placing it as an omnivore that leans toward animal prey — consistent with the micro-predatory habits typical of small nemacheilid loaches, which pick insect larvae, small crustaceans and other invertebrates off the streambed. Aquarium reports back this up: keepers note that the species readily accepts sinking foods along with live and frozen prey items such as Daphnia, Artemia and bloodworm.
In the aquarium this translates to straightforward feeding as long as food reaches the bottom. A varied diet of sinking pellets or wafers supplemented with small live or frozen invertebrate foods suits its natural foraging behaviour far better than flake, which a bottom-hugging fish like this will often miss entirely.
Mating
No detailed account of courtship or spawning behaviour for P. nigrolineata appears in the cached sources, and this is typical for a species described as recently as 2017 from a genus that has received little dedicated breeding study. What is documented is more encouraging than for most stone loaches: hobbyist reports indicate the species has been confirmed to spawn in aquaria without hormone induction, which is unusual for a group where captive reproduction is rare.
As with other nemacheilids, there is no evidence of pair formation, courtship display, or any parental investment beyond the act of spawning itself. Group maintenance is recommended by keepers, both because solitary individuals are prone to stress and because a shoal presumably improves the odds of natural spawning behaviour being observed.
Breeding
Paracanthocobitis nigrolineata is an egg-scattering species with no parental care, matching the pattern seen across Nemacheilidae: ripe fish release eggs over gravel or among stones in flowing water, and the adults take no further part in guarding eggs or fry. This is not a cave-spawning, male-guarding fish (that pattern belongs to gobies) and it is not a mouthbrooder.
What sets this species apart from most of its relatives is that home spawnings have actually been reported — one of the loachfanatics.com sources cataloguing loaches successfully bred in captivity lists P. nigrolineata as confirmed to spawn without hormone treatment, which is a genuine rarity among stone loaches. That said, published detail on the mechanics — egg number, incubation time, fry rearing — is not available in the sources consulted here, so keepers attempting it should expect to be working from general nemacheilid husbandry principles rather than a documented protocol.
In the aquarium
This is a small fish — around 1.5 in — that is best kept in a group rather than alone, following the general nemacheilid and loach pattern of stress-prone solitary individuals settling better as a shoal. Its near-scaleless skin means the same caution that applies across the loach family applies here: go gently with medications, avoid copper-based treatments, and dose conservatively if disease treatment becomes necessary, since scaleless fish absorb chemicals readily and are often among the first in a tank to show signs of parasitic infection.
Given its demersal, flowing-stream origin, a tank with a sand or fine gravel bottom, good water movement and strong oxygenation is likely to suit it far better than a still, heavily planted setup — though, absent published temperature data for this exact species, keepers should err toward the cooler-to-moderate tropical range reported for its relatives rather than assuming it wants warm, still water. It accepts sinking pellets, wafers and small live or frozen foods without difficulty, making feeding one of the more forgiving parts of its care.
Conservation
Paracanthocobitis nigrolineata has not been assessed on the IUCN Red List and is classified here as Not Evaluated. As a species described only in 2017 from a limited number of documented river drainages in Myanmar and Thailand, it fits the common pattern among recently split stone loaches: taxonomically resolved but not yet the subject of a dedicated conservation assessment.
No information on population trend, harvest pressure, or trade volume for this species is available in the cached sources. Given its restricted known range across the Irrawaddy, Sittang and Mae Khlong drainages, and the general vulnerability of small stream-endemic fishes to habitat degradation, an honest assessment is that its conservation status is simply unknown rather than presumed secure.