Taxonomy & naming
Gastromyzon cranbrooki was formally described by Heok Hui Tan and Zainal Abidin Sulaiman in 2006. The genus Gastromyzon belongs to the family Gastromyzontidae (sometimes still treated under Balitoridae sensu lato, but increasingly separated in modern classifications), a group confined to the rivers of Borneo. The genus name derives from the Greek gaster (belly) and myzon (sucker), naming the ventral adhesive disc formed by the modified pectoral and pelvic fins. All valid Gastromyzon species are Bornean endemics.
The species epithet cranbrooki honours Gathorne Gathorne-Hardy, 5th Earl of Cranbrook, a naturalist with a long association with Borneo and the biodiversity of the Belalong/Temburong system. There are no known synonyms; the name has been stable since its description. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the valid combination Gastromyzon cranbrooki Tan & Sulaiman, 2006.
Morphology
Gastromyzon cranbrooki is a small, dorsoventrally flattened loach reaching a maximum recorded standard length of 3 in, though most specimens encountered are smaller. The body is brown with nine to ten grey or dark crossbars across the dorsal surface, providing camouflage against the patterned rock and gravel substrate of its riffle habitats. The ventral surface lacks scales in the abdominal region — a characteristic of the genus — and the species possesses a complete postoral pouch, a fleshy fold behind the mouth used in adhesion.
The pectoral and pelvic fins are broad, horizontally orientated, and together form a powerful suction disc that allows the fish to hold station on smooth rock faces in strong current. This whole-body clinging adaptation is the defining feature of the hillstream loach body plan. Like other members of Balitoroidea, the skin is effectively scaleless or near-scaleless in texture, making the fish sensitive to chemical treatments. There is no erectile suborbital spine — that feature belongs to botiid and cobitid loaches, not to the torrent-adapted gastromyzontids.
Habitat
The species is endemic to the Temburong River basin in Brunei Darussalam on the island of Borneo. It is found in riffle and rapid zones of clear, shallow hill streams — the type locality is the Temburong River itself, and the best-documented population is from the Sungai Belalong stream system within the Ulu Temburong National Park, where it is reported as the most frequently encountered Gastromyzon in the riffle zones.
The habitat is fast-flowing, well-oxygenated, clear water over bare rock and coarse gravel, typically no more than 19.5 in deep in the areas where the fish aggregate. Recorded water parameters include a pH of approximately 6.6 or above. Temperatures in upland Bornean streams are cooler than lowland rivers, generally in the range of 68–75 °F. Dissolved oxygen is high due to constant turbulent mixing. This is not the warm, still or slow-moving lowland river environment of many popular aquarium fish, but a specialised, energetic habitat that imposes strict requirements on any fish attempting to replicate it artificially.
Feeding
Gastromyzon cranbrooki feeds at trophic level 2.0, placing it among the most herbivorous of fishes. In the wild it rasps biofilm, diatoms, algae, and associated micro-invertebrates from the surface of rocks in fast-flowing water — the aufwuchs community that coats every submerged stone in a mature stream.
In the aquarium this feeding mode is the central husbandry challenge. The fish needs a tank with densely colonised, biologically mature rock surfaces covered in biofilm and algal growth. A bare or newly set up aquarium offers nothing to graze on, and a fish unable to rasp adequate biofilm will slowly decline and starve — the most common cause of death in captive hillstream loaches. Supplementary feeding with algae wafers, blanched greens (spinach, courgette, cucumber), and occasional small frozen invertebrates (daphnia, bloodworm) can help, but these are a supplement to, not a substitute for, an established biofilm surface. The fish's digestive tract is adapted for a high-fibre, low-protein plant-based input and does not prosper on meaty or high-protein diets as a staple.
Mating
Sexual dimorphism in Gastromyzon cranbrooki has not been described in the literature, and information on mating behaviour in the genus is sparse. In related hillstream loaches, mature females are generally plumper across the abdomen when in breeding condition, and this is likely the most reliable external indicator in Gastromyzon as well. Males and females in a group are otherwise difficult to distinguish.
In the wild, spawning in hillstream loaches is presumably triggered by seasonal cues — changes in temperature, day length, flow rate, or water chemistry associated with wet and dry seasons in Borneo's equatorial climate. The Sungai Belalong habitat is within a system subject to rainfall seasonality, and it is plausible that spawning follows periods of increased flow. No detailed observations of mating behaviour for this species have been published.
Breeding
Gastromyzon cranbrooki is not reported as having been bred in home aquaria, and there is no documented captive spawning for the species in the available literature. As with the great majority of hillstream loach genera, captive breeding remains essentially unachieved outside specialist settings.
In the broader family, a handful of species — most notably Sewellia lineolata and some Pseudogastromyzon and Pseudohomaloptera species — have been induced to spawn in mature, strongly-flowing river-style tanks, depositing eggs among or beneath stones and gravel with no parental care thereafter. If Gastromyzon cranbrooki ever spawns in captivity it would likely follow this general hillstream pattern — egg-scattering with no parental investment, requiring mature tank conditions, a strong directed current, and a cooled temperature to mimic seasonal cues. The honest position is that this species is not bred in the aquarium and that the trade, to the extent any specimens circulate, depends on wild-caught fish from Brunei.
In the aquarium
Gastromyzon cranbrooki is a specialist fish with uncompromising requirements, and it is rarely encountered in the trade precisely because those requirements are difficult to meet and the fish performs poorly otherwise. A suitable setup is a river-biotope or hillstream-style tank: a bank of smooth and rough stones occupying most of the floor area, a powerful directed current from a wavemaker or powerhead aimed across the rockscape, and critically a biologically mature tank in which algae and biofilm have had months to colonise the stone surfaces. Temperature should be maintained in the range of 68–75 °F — cooler than is typical for most community aquaria — and the water must be clean, well-oxygenated, and moderately soft to neutral (pH around 6.5–7.5, GH 4–8 dGH).
The species has been observed in clusters in its natural riffle habitat and should be kept in groups of four or more. Being near-scaleless, it is sensitive to copper-based medications and should receive half-doses of any treatment with particular care. Fine sand in calmer zones and smooth stones for perching and grazing are both beneficial. This fish will not thrive in a generic community tank with warm still water, aggressive tank-mates competing for surface area, and no biofilm to graze. It is a reward for the keeper who builds the environment around the fish's real needs rather than fitting the fish into a pre-existing setup.
Conservation
Gastromyzon cranbrooki is assessed as Least Concern on the IUCN Red List (assessment date 30 December 2018). The assessment reflects the species' apparently stable presence in the Temburong River basin and the protected status of a significant portion of its known range within the Ulu Temburong National Park, one of Brunei's flagship forest reserves.
Nonetheless, the species' known range is narrow. It is endemic to the Temburong basin, a geographically small river system, and as with most Bornean stream endemics it faces long-term pressure from changes to forest cover that affect stream hydrology, sedimentation, and water temperature. Deforestation across Borneo — even adjacent to protected areas — raises water temperatures, increases sediment loads, and reduces the clean, fast, oxygen-rich conditions that torrent loaches require. The aquarium trade in this species appears minimal; most specimens in circulation, if any, will be wild-caught, and demand is low given the specialist care required.