Taxonomy & naming
Nemacheilus cacao was described in 2022 by Peter Bohlen, Maurice Kottelat and Vendula Šlechtová in the Raffles Bulletin of Zoology, from material collected in the middle Mekong Basin in Laos. It belongs to Nemacheilidae, the stone or brook loaches — by species count the largest family in the loach superfamily Cobitoidea, and one still being actively revised as researchers work through the enormous, poorly resolved radiation of small Asian stream fishes once lumped under the old genus name Noemacheilus.
The describing paper places N. cacao as phylogenetically sister to Nemacheilus platiceps within the so-called Selangoricus species group, and separates the two on colour pattern: N. cacao is a nearly uniform dark brown, where N. platiceps carries 12–16 narrow bars across the flank. As with much of this family, generic placement in Nemacheilus should be read as current rather than final — the Catalog of Fishes is the authority for the valid combination, and stream-level loach taxonomy in this region continues to shift as more material and molecular data come in.
Morphology
Nemacheilus cacao is a small, slender, cylindrical loach in the typical Nemacheilidae mould — an elongate body, a flattened underside adapted to holding station on stream substrate, and paired fins used to brace against current rather than to swim in open water. The original description notes an incomplete lateral line, a detail used alongside colour pattern to separate it from its closest relatives.
The species' most distinctive feature is developmental rather than structural: large, presumably mature males develop extensive tuberculation across the flank, dorsal and pectoral fin bases, head and nape, and turn a plain chocolate to dark grey-brown overall — the colour that gave the fish its name. This tuberculate, dark-brown male livery contrasts with the barred pattern of its sister species N. platiceps. Like other members of the family it is expected to carry the erectile suborbital spine typical of this side of Cobitoidea and the fine, deeply embedded or absent scalation that leaves nemacheilid skin close to naked — though the source material for this species does not spell out spine or scale detail beyond the family-level pattern, and is otherwise data-sparse on fine morphology.
Habitat
The species is known from the middle Mekong Basin in Laos, the stream system that also gives the fish its type locality. As a member of Nemacheilidae it fits the family's general profile: a benthic specialist of clear, flowing, well-oxygenated streams, typically over sand, gravel or stony substrate rather than still or heavily vegetated water.
Beyond the type-locality record, detailed habitat data — exact stream gradient, substrate composition, seasonal flow and measured water chemistry — has not been captured from the literature available for this profile. Many Nemacheilidae are known from a single drainage or even a single stream, and N. cacao, described only in 2022, sits squarely in that data-sparse category; this profile reports what is documented rather than extrapolating beyond it.
Feeding
No diet-specific data for Nemacheilus cacao has been captured from the sources used for this profile. Based on the well-established pattern across Nemacheilidae, it is reasonable to expect the species is a benthic micro-predator, picking small invertebrates — aquatic insect larvae, tiny crustaceans and other stream-bottom fauna — off gravel and sand in fast-flowing water, in the manner typical of the family.
In the absence of a hobby-keeping record for this species, there is no aquarium feeding experience to report. Any care should follow the general stone-loach pattern of small live, frozen or sinking foods rather than flake, but this is inference from the family rather than a documented fact about N. cacao itself.
Mating
No courtship or spawning observations for Nemacheilus cacao have been documented in the sources available for this profile — expected for a species described only in 2022 and never reported in aquarium culture. Nothing is known here about sexual dimorphism, breeding season or spawning behaviour beyond what can be inferred from the wider family.
Across Nemacheilidae generally, reproduction is not tied to elaborate pairing rituals or nest-building; species scatter eggs over the substrate in flowing water rather than courting at a fixed site. There is no basis in the available material to say more than that for this particular species.
Breeding
Nemacheilus cacao has no known aquarium-breeding history — it is a 2022 taxonomic description of a wild population, not a hobby fish. Following the well-documented pattern for stone and brook loaches generally, the expected mode is an egg-scatterer: eggs released over gravel or among stones in fast-flowing water, with no parental care of eggs or fry.
Stone and brook loaches as a family are rarely bred in captivity, and most remain entirely unknown in this respect — spawning is presumed to occur in the wild in response to seasonal flow and temperature cues that are difficult to replicate at home. For N. cacao specifically, the honest position is that breeding data is sparse to nonexistent; nothing here should be read as a confirmed captive spawning.
In the aquarium
Nemacheilus cacao is not known to be established in the aquarium trade. It was described from scientific collecting in the Mekong Basin in Laos, and the sources available for this profile do not record any hobbyist keeping experience, import history or care reports.
Were it ever to appear in the trade, the general expectations for a small Nemacheilidae stone loach would apply: cool-to-moderate, clear, fast-flowing, well-oxygenated water over a hard substrate, and the caution that near-scaleless loaches in this family tend to be sensitive to copper-based medications and full-strength treatments, while also being quick to show the first signs of ich in a tank. But this is general family guidance, not species-specific experience, and anyone encountering this fish should treat it as an unknown quantity requiring conservative, closely observed care rather than following a established template.
Conservation
Nemacheilus cacao has not been assessed on the IUCN Red List. Described only in 2022, it is simply too recent to have gone through the Red List assessment process, so it is recorded here as Not Evaluated rather than assigned a guessed status.
No population trend, threat assessment or trade data is available for this species in the sources used for this profile. As a narrow-range Mekong Basin endemic — the typical profile for a newly described Nemacheilidae species — it would be reasonable to expect the same pressures facing much of mainland Southeast Asia's stream fauna, such as damming, sedimentation and land-use change in its home drainage, but no documented assessment of those pressures exists yet for this particular fish.