Taxonomy & naming
Nemacheilus jaklesii was described by the Dutch ichthyologist Pieter Bleeker in 1852, one of the hundreds of Southeast Asian freshwater fishes he named during his long career in the Dutch East Indies. It is placed in the genus Nemacheilus (Bleeker, 1863) within the family Nemacheilidae, the true loaches — by far the largest loach family and one that has been repeatedly reshuffled as older, catch-all genera such as Nemacheilus and Noemacheilus have been split into narrower ones like Schistura, Barbatula and Triplophysa.
FishBase currently carries Nemacheilus jaklesii as a recognized species in the genus but flags it as species inquirenda — a species of uncertain identity or validity pending further taxonomic review. That uncertainty is worth stating plainly rather than glossing over: this is not one of the well-characterised, frequently redescribed loaches, and future revision could see it reassigned or synonymised. Pending that, this profile follows the current combination, Nemacheilus jaklesii.
Morphology
As a member of Nemacheilidae, Nemacheilus jaklesii can be expected to share the family's general build: an elongate, sub-cylindrical body, a flattened belly suited to hugging the streambed, and six barbels fringing the mouth — the origin of the family name Nemacheilidae, from the Greek for "thread" and "lip." Like many of its relatives it is likely near-scaleless or very finely scaled, a trait that makes loaches of this family sensitive to strong medications and to copper-based treatments in captivity.
No detailed morphometric description, colour pattern, or verified maximum size for this species was found in the available sources. Rather than invent measurements or markings, this account leaves those points open pending a dedicated redescription or a hobbyist record with photographic voucher material.
Habitat
The genus Nemacheilus is distributed across freshwater hillstreams and, in places, caves, from India across Southeast Asia and into East Asia. The type locality for Nemacheilus jaklesii lies in the Sunda Islands region of the former Dutch East Indies — present-day Indonesia — consistent with the genus's broader presence on Sumatra, Java and Borneo.
Stone and brook loaches in this family are typically found in clear, fast-flowing, well-oxygenated streams rather than still or brackish water, and there is no reason from the available record to place Nemacheilus jaklesii anywhere else. Beyond that regional placement, no specific river system, elevation or detailed habitat description for this species was located in the sources consulted, so this account does not speculate further.
Feeding
Stone loaches of the genus Nemacheilus are generally bold, bottom-oriented micro-predators, working the gravel and cobble of fast streams for small invertebrates, insect larvae and other tiny prey items rather than grazing algae or biofilm in the manner of hillstream loaches. There is no species-specific feeding study for Nemacheilus jaklesii in the sources reviewed, but this general pattern for the genus is well supported and is the best available guide.
In captivity, loaches following this pattern typically take small live and frozen foods (bloodworm, brine shrimp, chopped invertebrates) readily and sinking pellets or wafers less readily, often ignoring flake altogether. Any keeper working with this species should plan around a micro-predator diet rather than a herbivorous or generalist one.
Mating
No courtship, sexual dimorphism or spawning behaviour has been documented for Nemacheilus jaklesii in the available sources. Stone and brook loaches as a group show little externally obvious dimorphism between the sexes, and given how sparse the record is for this particular species, no dimorphic traits can be responsibly reported here.
For Nemacheilidae generally, reproduction in the wild is understood to follow the pattern typical of small stream loaches: ripe adults release eggs into flowing water over open gravel, without pairing behaviour elaborate enough to have been well documented for most species in the family, this one included.
Breeding
Nemacheilus jaklesii is not known to have been bred in the home aquarium, and no breeding account for this species was found in the sources consulted. Stone and brook loaches of this family are, as a rule, egg-scatterers: ripe fish shed eggs over gravel and among stones in fast-flowing water, with no parental care of eggs or fry afterward.
Aquarium accounts of Nemacheilus species more broadly describe them as difficult to breed in captivity, and this species should be assumed to follow that pattern until shown otherwise. A keeper should not expect to breed this fish at home; if it is bred, its mode should be treated as an unguarded egg-scatterer over gravel, not a cave-spawner or mouthbrooder.
In the aquarium
Given how little is documented specifically for Nemacheilus jaklesii, care advice here leans on what is well established for stone and brook loaches (Nemacheilidae) in general. These are active, bottom-dwelling fish of cool-to-moderate, fast, well-oxygenated water, and they do poorly in warm, still, low-oxygen tanks; strong water movement, high dissolved oxygen and a gravel or cobble substrate with hiding places among rocks and wood suit the family well.
As is typical for near-scaleless loaches, caution with medications is warranted — copper-based treatments and full-strength doses of many standard remedies can be poorly tolerated, so half-doses and gentler alternatives are the safer default when treating illness. Because so little is published on this particular species' adult size, temperament or social needs, a keeper taking it on should provide generous swimming space, structure to hide in, and companionship of its own kind as a precaution, and should not assume it will behave identically to better-known relatives such as the botiids.
Conservation
Nemacheilus jaklesii has not been assessed on the IUCN Red List, so it carries no formal conservation category; it is recorded here as Not Evaluated rather than assigned a guessed status. This is common among small, range-restricted or poorly studied stream loaches across Southeast Asia, many of which have never been the subject of a dedicated conservation review.
Without population data, threat information or a resolved taxonomic status, no meaningful conservation narrative can be offered beyond noting the gap itself. Given that regional freshwater habitats across the Sunda region face pressure from deforestation and river modification, a precautionary approach to any wild collection of this species would be prudent, but that is a general regional concern rather than a documented, species-specific finding.