Taxonomy & naming
The species was first described by German ichthyologist Wolfgang Klausewitz in 1957 as Noemacheilus phuketensis, from a holotype (SMF 3966) collected on Phuket Island, Thailand. It was reassigned to Acanthocobitis by Bănărescu & Nalbant in 1964, then treated as Noemacheilus (Acanthocobitis) phuketensis in 1968 — the usual churn of names that has run through this family for decades as genera were split, merged and re-split.
For a long stretch the name was submerged entirely: P. phuketensis was treated as a synonym of the wide-ranging Paracanthocobitis zonalternans (itself often seen in older hobby literature as Acanthocobitis zonalternans). A 2017 taxonomic revision of the P. zonalternans species complex, which also described three new species, re-examined the type material and restored P. phuketensis as a valid, distinct species, distinguishing it by an incomplete lateral line ending near the dorsal-fin insertion, 9½–10½ branched dorsal-fin rays, a usually absent (or only rudimentary) axillary pelvic lobe, and a circular black spot on the upper margin of the caudal-fin base. The Catalog of Fishes is the authority for the current combination, Paracanthocobitis phuketensis, and that is the name used here.
Morphology
This is a small, elongate, cylindrical loach typical of the genus — the informally named "zipper loaches," for the broken, zipper-like row of dark blotches that runs along the flank in many populations, sometimes overlaid with a faint dusky stripe. A conspicuous circular black spot rimmed in white — an ocellus — sits at the upper base of the caudal fin, a field mark shared across the genus and useful for distinguishing it from superficially similar nemacheilids. The mouth is subterminal with a papillated lower lip carrying a medial notch, and the upper lip bears two to five rows of papillae — fine sensory and gripping structures suited to picking food off stones in current.
As in the wider zonalternans complex, adult size is modest: close relatives top out around 3 in standard length, and P. phuketensis is expected to fall in a similar small-loach range, though a size figure specific to this species was not found in the sources consulted. Like other nemacheilids it lacks the erectile suborbital spine of the botiid loaches and carries only minute, deeply embedded scales — effectively naked skin that matters for how the fish should be handled and medicated in captivity.
Habitat
Paracanthocobitis phuketensis is known from Phuket Island and the adjoining Thai-Malay Peninsula, with type-locality records spanning the Panang Tak drainage in Peninsular Thailand south to the Perlis, Padang Sarai and Ketil drainages of Peninsular Malaysia. Nemacheilids of this genus are characteristically fish of clear, flowing streams — cool-to-warm, well-oxygenated water running over gravel, sand and stone, exactly the kind of habitat where a bottom-hugging micro-predator can find cover among current-swept substrate.
Specific water-chemistry data for P. phuketensis was not located in the cache, but the closely related and better-documented P. zonalternans is reported from moderate temperatures (around 68–77 °F) and near-neutral pH (roughly 6.0–7.5) with strong water flow — a reasonable proxy for the conditions this species is likely to encounter given the shared peninsular range and stream-dwelling ecology. This is not a warm, still blackwater fish; it belongs in flowing, oxygen-rich water.
Feeding
As a nemacheilid stream loach, P. phuketensis is best understood as a small benthic micro-predator, working the streambed for small invertebrates — insect larvae, small crustaceans and other invertebrate prey dislodged or found among stones and gravel in current. This matches the documented diet of its close relative P. zonalternans, which forages for small invertebrates in the wild.
In the aquarium, fish of this genus generally accept sinking dried foods along with small live or frozen items such as bloodworm, Artemia and Daphnia. Flake that sits on the surface is often ignored by a fish built to feed head-down against the current, so sinking foods matched to its small mouth are the more reliable choice.
Mating
No species-specific account of courtship or spawning behaviour for P. phuketensis was found in the sources consulted. Nemacheilid stone and brook loaches as a group show no elaborate courtship display and no pair-bonding; reproduction in this family is generally opportunistic egg-scattering triggered by suitable flow and temperature conditions, without the seasonal mass migrations seen in the botiid loaches. Given the sparse data specific to this species, this section is necessarily brief rather than speculative.
Breeding
Paracanthocobitis phuketensis is not documented as bred in the home aquarium, and data on its reproduction is sparse — consistent with the pattern across Nemacheilidae, the largest loach family and also the family about which the least is generally known. What is understood for the wider genus, drawn from the better-studied P. zonalternans, is that these are egg-scatterers with no parental care: eggs are released over gravel or stones in flowing water and left to develop unattended, hatching in roughly 24–36 hours, with fry becoming free-swimming around day three and needing infusoria-type foods (Paramecium) before graduating to Artemia nauplii.
That account comes from a related species rather than from P. phuketensis itself, so it should be read as a plausible model rather than a confirmed record for this fish. Home spawnings of small nemacheilid stream loaches of this kind remain uncommon, and this species should be assumed rarely or never bred in captivity until better documentation emerges.
In the aquarium
Kept as a small stream loach, P. phuketensis should be given a tank that mimics flowing, well-oxygenated water — good filtration with real current, a substrate of sand and smooth gravel, and scattered stones or wood that let it forage and shelter without needing to fight the flow constantly. Like other nemacheilids it is essentially scaleless, which makes it sensitive to copper-based medications and to full-strength doses of many standard treatments; gentler, reduced-dose approaches are the safer default whenever treatment is needed.
Because detailed sourced information on this exact species' aquarium behaviour is limited, keepers should extend general stone-loach practice cautiously: provide company of its own kind where possible, avoid warm, still, poorly oxygenated setups, and match tankmates to its small, unassuming size rather than to boisterous community fish that could outcompete it at feeding time.
Conservation
Paracanthocobitis phuketensis has not been formally assessed on the IUCN Red List and is recorded here as Not Evaluated (NE). This is a common situation for small-range stream loaches in this genus, many of which are known only from a handful of peninsular drainages and have never been the subject of a dedicated conservation assessment.
No specific threat information for this species was found in the sources consulted. In general, peninsular Thai and Malaysian stream systems face pressure from land-use change, deforestation and water abstraction, and a narrow-range endemic stream fish such as this one would be expected to be vulnerable to degradation of its home drainages — but without a formal assessment or population data, that remains an inference rather than a documented status.