Taxonomy & naming
Protomyzon aphelocheilus was described by Robert F. Inger and Phui Kong Chin in 1962 from specimens collected in northern Borneo. Inger and Chin were cataloguing the freshwater fishes of Sabah (then British North Borneo), and the species appears in their foundational 1962 monograph on the group. The genus Protomyzon was established to hold a small cluster of flattened Bornean hillstream fishes that did not fit cleanly into the older and larger genus Gastromyzon; the two genera are close relatives and share the same torrent-rock niche.
The family placement has shifted with modern systematic revisions. FishBase and much of the older literature place the Bornean hillstream loaches in Balitoridae, but a number of recent classifications — including those underpinning the Catalog of Fishes — recognise the clade as a distinct family, Gastromyzontidae, separated from the Balitoridae of India and mainland Asia. Either placement is found in reputable sources; in this article the family is treated as Gastromyzontidae following current FishBase usage for the Bornean group.
A related species, Protomyzon pachychilus, has been moved to a distinct genus — hobbyist sources sometimes note this fish under the synonym Yaoshania pachychyla — and the broader genus Protomyzon remains small, largely understudied, and essentially unrevised since Inger and Chin's original work.
Morphology
Protomyzon aphelocheilus is a tiny, strongly flattened loach reaching a maximum of 1.5 in standard length. The body is depressed — essentially disc-like in cross-section — with broad, paired pectoral and pelvic fins spread horizontally to form a sucker-like planing surface against rock. The ventral mouth is similarly adapted for life in fast water: the lips are modified into a rasp for scraping biofilm and diatoms from stone, and the jaw lacks the pronounced fleshy rostrum of some related genera, which is the character the species name commemorates (see Etymology).
Published lateral-view images document the flattened head and characteristic body compression shared across the Gastromyzontidae — morphology refined for clinging to torrent substrate in strong current. The skin is only lightly scaled on the body, increasing sensitivity to dissolved chemicals and reinforcing the need for pristine, well-managed water. Like other hillstream loaches, the species lacks the suborbital spine found in botiid loaches; its defence is cryptic colouration and adhesion to rock, not a locking spine.
Colour in life is reported as a typical hillstream pattern of mottled brown and cream, providing camouflage against the rock-and-gravel substrate of its native riffles. Specific colouration details for this species are sparse in the available literature.
Habitat
Protomyzon aphelocheilus is a small-range endemic of northern Borneo — the island of Borneo broadly, with records centred on Sabah and the streams of the northern and north-western highlands. Its habitat is the fast, clear, well-oxygenated riffles and runs of hill streams: shallow water over exposed bedrock, cobbles and boulders, with strong current and high oxygen saturation. These streams are typically cool relative to Borneo's lowland rivers, fed by elevation and shaded forest canopy, and carry very low loads of organic matter.
The substrate in its home riffles is carpeted in biofilm — the thin, living mat of algae, diatoms, cyanobacteria, protozoans and detritus that forms wherever light, clean water and stable rock surface coincide. This biofilm is the foundation of the species' diet and, by extension, the minimum habitat requirement it cannot do without. The water is typically soft and slightly acidic to neutral, in keeping with the granite and sandstone geology of interior Borneo. Lowland and slow-water habitats are not part of its range.
Feeding
Protomyzon aphelocheilus is an aufwuchs grazer — a biofilm specialist that rasps algae, diatoms, and the associated organic matter from the surface of hard substrate. In its native riffles this material is constantly renewed by nutrient-laden water washing over exposed rock, and the fish moves steadily across the stone surface in search of it, pressing flat against the current with its ventral surface and cropping the biofilm layer by layer.
In the aquarium this feeding strategy creates a strict dependency on a mature, well-established, biofilm-covered rockscape. A fish placed in a bare or newly set-up tank, where bare glass and sterile gravel offer nothing to graze, will starve even when supplementary food is offered — particularly if it is not yet acclimatised to accepting food from the water column. Supplementary feeding should focus on wafer or tablet foods that sink to the bottom and rest on hard surfaces (spirulina wafers, algae tabs, blanched courgette/zucchini pressed against glass), and small frozen or live invertebrate foods such as baby brine shrimp and microworms to supply protein. This species is not a generic algae eater and should not be expected to clean a tank; it is a dietary specialist that needs real biofilm as its staple.
Mating
Almost nothing is documented about the reproductive biology or sexual dimorphism of Protomyzon aphelocheilus specifically. In the closely related Bornean hillstream loaches, the sexes are generally very difficult to tell apart externally; mature females are often somewhat plumper and wider-bodied when in spawning condition, but this is a subtle and condition-dependent difference rarely useful in practice. There is no documented colour polymorphism or secondary sexual structure for this species.
Hillstream loaches as a group reproduce in fast-flowing, well-oxygenated water — conditions that in nature are also tied to seasonal cues such as temperature variation, rainfall and water-level changes. Spawning in a few well-documented hillstream species (notably Sewellia lineolata) has occurred in mature, strongly-flowing aquarium setups, suggesting that the right conditions can be replicated to a degree, but Protomyzon specifically has no published accounts of captive reproduction and should be considered essentially unbred in the hobby.
Breeding
Protomyzon aphelocheilus has not, to the knowledge available from the literature and hobbyist record, been bred in the aquarium. Like other hillstream loaches it is presumed to be an egg-scatterer, depositing eggs among stones and gravel in fast water with no subsequent parental care — a pattern consistent with what is documented for Gastromyzontidae more broadly and for the few hillstream species for which spawning accounts exist.
The trade in this species is very limited, and it is almost certainly wild-collected when it does appear. No hormone-induced or captive farm production is documented. For keepers who obtain specimens, the honest expectation should be that breeding is not a realistic goal with current knowledge; providing mature, high-flow, high-oxygen conditions with a well-established biofilm substrate is the prerequisite for any attempt, but documented success in this genus remains absent from the literature.
In the aquarium
Protomyzon aphelocheilus is an expert-level fish that demands conditions matched tightly to its natural habitat. The non-negotiable requirements are: strong, continuous water flow (a river manifold, powerhead or wavemaker directed across a bank of smooth rocks); high dissolved oxygen at all times (power-failure with no aeration is a rapid death sentence for a fish at this oxygen dependency); cool water in the range 68–75 °F (68–75 °F), significantly cooler than the usual tropical fish room; and a mature tank with a well-established algae and biofilm cover on all hard surfaces — tiles, smooth river stones or slate are ideal. The minimum tank footprint recommended for hillstream loaches in general is around 20 US gal, with more length and more rock surface being more important than raw volume.
Being lightly scaled, this species — like all hillstream loaches — is sensitive to copper-based medications and should receive half-doses or copper-free alternatives when treatment is necessary. Ich and other skin parasites are a real risk in a fish under stress, particularly at temperatures outside its preferred range. It is not an obligate schooling fish in the way that botiid loaches are, but keeping a small group of two or three individuals in a correctly set-up tank allows natural behaviour and reduces stress. Do not combine with warm-water tropical fish that require temperatures of 79 °F or above; the temperature mismatch will compromise one species or the other. This is not a beginner's fish, and it should not be purchased as a tank-cleaning convenience — it is a specialist grazer that will simply perish in an unprepared setup.
Conservation
Protomyzon aphelocheilus is assessed as Least Concern on the IUCN Red List (assessment date 25 January 2019). The assessment reflects a species with an apparently stable population within its restricted northern Borneo range, currently not subject to a trade or collection pressure sufficient to threaten its numbers. However, the Least Concern category should be read in context: this is a small-range endemic of hill streams in a region subject to extensive deforestation, agricultural runoff, sedimentation and water abstraction — all pressures that can degrade or eliminate the fast, clear, biofilm-rich riffles the species requires.
Its appearance in the aquarium trade is sporadic at best, and any fish sold as Protomyzon sp. or from northern Borneo hill streams is almost certainly wild-collected. The species has no documented captive production. Given the sensitivity of hillstream fish to habitat deterioration and the ongoing land-use change across Sabah and the broader northern Borneo region, the conservation situation merits monitoring even for a currently Least Concern species.