Bengkayang, a region in West Kalimantan, Indonesia, hosts freshwater habitats that remain exceptionally well preserved, and in the fast-flowing upper stretches of these rivers, a wonderful family of highly adapted fish species can be found. Much like lichens can be a biological indicator of air quality, or earthworms an indicator of soil health, the Gastromyzontidae family of fishes found in Bengkayang comprise species that are often used as bioindicators of river health due to their sensitivity to subtle changes in their underwater biotopes. Species from the Gastromyzon and Hypergastromyzon genera known as the Borneo suckers or hill stream loaches are perfect examples of freshwater bioindicator species in Borneo, as these fish can only thrive in high-quality, unpolluted water. In addition, they rely on periphytic biofilm growing undisturbed on the surface of riverbed rocks, which only thrives in clear water where sunlight can penetrate right down to the riverbed. This means that human activity and the associated impacts on these habitats can pose a significant threat to the existence of these quirky, specialised fishes.
The Hypergastromyzon species found in this region is Hypergastromyzon sambas (above). It was only identified as a distinct species in 2021, and is an obligate dweller of swift-flowing torrents with very high oxygen content.
Meanwhile, the Gastromyzon species found here is Gastromyzon viriosus (above), characterized by a distinct morphology with a vivid yellow tail; in the aquarium hobby, they are typically known as the yellow-finned hill stream loach. Until now, the consensus has been that Gastromyzon viriosus is endemic to the Tatau River basin in Sarawak, Malaysia, approximately 450km northeast of the population in Bengkayang. Due to its restricted geographic distribution and multiple anthropogenic threats including agro-industry farming, wood and pulp plantations, logging and wood harvesting, and the effluents of these industries, when last assessed for the IUCN Redlist in 2019 the species was listed as Near Threatened. The occurrence of a second population at Bengkayang could be indicative that the species is more widespread than previously thought, perhaps due to a lack of research in the region, but it could also be an indication that the species was once more widespread and populations have been gradually extirpated. Further research would be required to determine this for certain.
The Malaysian population of Gastromyzon viriosus is known from Sarawak (right), whereas the Indonesian population (left) is found over 450km SW of the Tatau River in Bengkayang.
Gastromyzon viriosus (above) from dorsal and lateral perspectives showing its distinctive yellow dorsal an caudal fins
Borneo Suckers exhibit a social lifestyle, this is particularly evident in their tendency to aggregate in large numbers within a single microhabitat. They also cohabit well with several other species such as the frecklefin eel Macrognathus maculatus, Paracrossocheilus spp, and Barbodes everetti, commonly found in this location (see below).
Macrognathus maculatus (above) is commonly known as the frecklefin eel and can usually be found in shaded forest streams with dense growths of aquatic vegetation and areas with thick leaf litter. This spiny eel tends to hide among vegetation and feeds predominantly on benthic (bottom-dwelling) insect larvae and crustaceans, allochthonous (terrestrial) insects, and worms.
Paracrossocheilus sp. from Bengkayang observed in syntopy alongside Gastromyzon viriosus (above), and in small species-only groups (below) grazing on periphyton on rocks
An intriguing observation is that Borneo Suckers and their algae-eating friends are consistently found in open microhabitats with water surfaces not shaded by overhanging trees, allowing direct sunlight penetration to the waterbed. This contributes to a composition of gravel, rocks, and bedrock carpet rich in periphyton and biofilm. Periphytic biofilms are structured microbial communities either attached to surfaces or in suspended aggregates. They consist of microbial cells (bacteria and/or fungi) embedded in a self-produced extracellular matrix made of polysaccharides, extracellular DNA, and other components; this biofilm serves as the primary food source for Borneo Suckers.
Barbodes everettii (above) is often called the clown barb in the aquarium hobby, however this does lead to a great deal of confusion between this species and the quite similar-looking Barbodes dunckeri, which is frequently given the same common name. With B. everettii named after zoologist Alfred Hart Everett, and B. dunckeri named in honour of ichthyologist Paul Georg Egmont Duncker, a nice way to differentiate and fully appreciate each species is to use the common names Everett’s barb, and Duncker’s barb.
The habitat for both species mainly comprises swift, shallow waters with substrates made up of large flat rocks, gravel, and some sandy areas.
A question that arises when coming across two very similarly adapted species that share the same biotope like Gastromyzon viriosus and Hypergastromyzon sambas is, why have they evolved to become two different species if they are so similar? Morphologically similar species like G. viriosus and H. sambas often have to evolve similar adaptations to endure their environmental conditions, the obvious example is their suction-pad style fins to control their movement in rapidly flowing rivers. Such effective adaptations allow species to proliferate rapidly and eventually, this leads to intraspecific competition, the competition between members of the same species for resources. It is at this point that one species can begin to branch off into two or more. Gastromyzon viriosus prefer to attach themselves amidst medium-sized sunlit rocks during the day, and contrastingly, Hypergastromyzon tends to cling to flat rock surfaces, possibly for camouflage due to their harmonizing color with the substrate and their flatter bodies, facilitating more efficient movement across even substrates in areas of high water velocity.
Hypergastromyzon sambas (above); dorsal, lateral, and ventral views, showing the highly-adapted morphology of the species.
These species adapt well to fast-flowing rivers due to these morphological features. Their horizontally oriented paired fins, flattened head and body, and fused pelvic fins form a powerful sucking cup (above image), enabling them to cling tightly to solid surfaces. Their ability to swim a mid-level in open water is greatly reduced, and they maneuver by ‘crawling’ over and under rocks.
Although the habitat in this area remains relatively intact, human activities are encroaching upon these waters, posing a threat to the species’ existence. Observed declines in fish populations in this location could likely be attributed to various factors such as detergent waste, domestic refuse, and changes in the environment like deforestation and irrigation practices. Protection measures are crucial as these fish are endemic to Borneo and nowhere else on Earth; their extinction would result in the loss of an irreplaceable genetic pool.
