In Search of Borneo Suckers

Author: Avik De

“Could that be a sucker?” I asked myself when I noticed something stuck to the rock in the stream in which we were searching for fishes for the past couple of hours. “But this is such a stagnant part of the stream,” I murmured. The textbooks always say that the objects of this search stay in very fast-flowing water, but we had almost given up looking in those ideal sections of the stream that day. Now, if it were indeed a Borneo sucker, the next question would have been how to catch it and take a photograph. I was sure that as soon as we stepped into the water it would hide under the huge boulder it was attached to, but it was also impossible to simply scoop it from the rock.

We quickly came up with an idea: my friend Naseer would hold the big net on one side of the rock while I would use my bare hand to gently chase the fish into the net. It was not as easy as it sounds: I lay down on a halfway submerged rock and tried my best to slowly reach the fish… but, oops, it was too fast and promptly crawled over the rock and hid in a cave. We could have waited for its reappearance on the open part of the rock, but it might not have emerged for a long while. Desperate, I put my hand inside the cave, not even stopping to consider that there might be some potentially harmful creature inside. And as luck would have it, three Borneo suckers rushed out toward the opening where we had placed the net, and we grabbed all but one.

The Genus Gastromyzon

That was my second trip to Kuching in the Sarawak state of Malaysia and my second encounter with the Borneo sucker genus Gastromyzon, commonly known as hillstream loaches. The name for the genus Gastromyzon roughly derives from the Greek for “stomach” and “to suckle.” I collected at least two species groups of Gastromyzon on that trip, G. ocellatus and G. ctenocephalus.

Species Groups

It is worth noting that the genus Gastromyzon is, so far, divided into several species groups containing cryptic species that are morphologically similar, but reproductively isolated. Each group of fishes inhabits adjacent river basins and might occur sympatrically in some cases. Moreover, there are numerous intermediate forms of species in the same group, as follows:

  • The G. borneensis group, containing G. borneensis, G. monticola, G. ornaticauda, G. cranbrooki, G. cornusaccus, G. extrorsus, G. introrsus, and G. bario
  • The G. punctulatus group, containing G. aeroides, G. punctulatus, and G. katibasensis
  • The G. fasciatus group, containing G. fasciatus and G. praestans
  • The G. contractus group, containing G. contractus, G. megalepis, and G. umbrus
  • The G. ctenocephalus group, containing G. ctenocephalus and G. scitulus
  • The G. lepidogaster group, containing G. lepidogaster and G. psiloetron
  • The G. ridens group, containing G. ridens, G. crenastus, G. stellatus, and G. zebrinus
  • The G. danumensis group, containing G. danumensis, G. aequabilis, and G. ingeri
  • The G. pariclavis group, containing G. pariclavis, G. embalohensis, G. venustus, G. spectabilis, G. russulus, and G. viriosus
  • The G. ocellatus group, containing G. ocellatus and G. farragus
  • The G. auronigrus group, containing G. auronigrus

The G. ocellatus group specimens were collected from the Matang area, and the G. ctenocephalus group from the tributaries of the river Apar in the Lundu area. I was told months later by a local fish expert that the G. ocellatus were actually introduced to the Matang area by a fish collector for easy harvest later on.

Exploring Matang

Our expedition started early in the morning from the Kuching waterfront. I had to cross the Sarawak River by a longtail boat and then drove off for Matang, which was another 20 miles (30 km) and sparsely populated. Once we reached the location, we quickly trekked through the forest to get to the streams where Naseer had previously seen the fishes.


Locating them in that fast-flowing water was a difficult task. This particular habitat had huge boulders one cannot move, let alone take out of the water to check for any living being still stuck to them. So we went on and on, further upstream, but all we managed to see were some rasboras and barbs, which held little interest for us. This went on for several hours until we spotted the one hillstream loach in apparently still water confined between the boulders. Later, I hypothesized that those harshly sunlit spots must be their dining rooms. The boulders were quite slippery due to algal growth, and I almost fell a couple of times. There was hardly any open area to find substrate, gravel, or sand, and no submerged vegetation, which is common of higher-elevation streams.

Our Catch

The water was very cool and flowing swiftly. At that location, we found and caught several specimens from the G. ocellatus species group with superb red caudal fins and nice patterns all over the top body portion. While looking for them in the shallower parts of the streams, we accidentally caught a single mouthbrooding wild betta species, Betta ibanorum, named after the local Iban tribes.

Adventures in Lundu

That evening, I witnessed a beautiful opening celebration of the Chinese mooncake festival just behind the bed-and-breakfast where I was staying. The next morning, we went to explore another location in Southern Sarawak a bit further away and at a lower elevation, near the town of Lundu. It was completely different from the earlier one, more of a usual lowland stream with a sandy substrate and small- to medium-sized rocks everywhere.


As this was Sarawak, known for its man-eating crocodiles that often venture into connected freshwater streams, we had to be very careful exploring that waterbody. To make things worse, it started raining mildly after we had already started our trek into the stream. A bit worried both about the reptiles and a possible flash flood, which is very common in this part of the world, we started with the shallow water in the nearest bank in only a little more than ankle-deep water.

I rapidly took the submerged river rocks one by one out of the water while Naseer held the net underneath with the hope of catching any fish detaching themselves from the rocks in an attempt to slip away. We saw hundreds of greenstripe barbs (Puntius vittatus) grazing and swimming crazily among the rocks, and it was in following those little beauties that we subconsciously, and fortunately, arrived in the deeper—yet still only knee-deep—water where we finally caught our first G. ctenocephalus-group specimen.

Fish Findings

In the middle of the stream we caught many more specimens, but we found none in the shallower water along both banks. These shallows with slower water currents only held rasboras, barbs, and some Nemacheilus loaches. We also caught some Macrobrachium shrimp, as well as the catfishes Glyptothorax sp. and Leiocassis micropogon. The weather conditions were deteriorating, so we had to call it a day much earlier than we had planned and departed for the hotel. We released most of the catch, and I brought back only a few specimens to keep in my home tank.

Keeping Borneo Suckers

To keep these highly evolved loaches in home aquaria, a sophisticated setup is needed. These fishes are not meant for ordinary community tanks and will not accept dry or frozen foods readily; neither are their digestive tracts prepared for meaty or high-protein foods. Gastromyzon loaches have a specialized morphology adapted to life in fast-flowing water whereby the paired fins are orientated horizontally, head and body flattened, and pelvic fins fused together. An ideal hillstream setup must have a strong water current, with unidirectional flow if possible, to mimic the fast-flowing mountain streams where they naturally occur.

Water Requirements

Gastromyzon spp. and sympatric fishes need high oxygen saturation in their water, so surface agitation in the form of filter outlets kept just above the water surface and an additional air pump or two is vital. Filter turnover of 10 to 15 times that of the tank volume are recommended, with high-quality porous filter media to hold a large colony of beneficial bacteria, along with a regular water change regimen. Their water must be pristine. As they originate from clean mountain streams, these fishes cannot handle excessive nitrates, much less ammonia or nitrite, in the water column.


They are algae eaters, and as such their long digestive tracts are made to slowly digest vegetable matter. Meaty foods can be provided as an occasional treat, but never as a staple diet. To keep them fed, one must install intense enough lighting, and dose enough fertilizers if needed, to grow prolific algae on the rocks in their tank. Flat, round, and smooth river rocks are ideal for this purpose. If there are many algae-eating fishes in the tank, a separate container can be maintained in a sunlit spot with some rocks and a regular dose of fertilizers to grow algae, with the algae-covered rocks circulating between the container and the main tank.

Their natural habitat lacks submerged vegetation, but some broadleaf plants like Anubias can be kept in their aquarium to provide more surface area for algal growth and grazing. Since they need stable water conditions and feed on biofilm, the hillstream loaches should never be added to a biologically immature setup, and other than the front glass, none of the other walls of the tank should be cleaned.


Cool water of around 77°F (25°C) is loved by these fishes, so cooling fans can be installed to blow across the water surface if required. Possible tankmates include other nonaggressive algae-eating fishes; shrimps; small, surface-dwelling rasboras, barbs, and danios; and perhaps a few nonaggressive dwarf catfishes as well. For more details on maintenance, research general care and keeping notes for hillstream loaches.

A Note on Conservation

Many of the exciting tiny fishes that are appropriate for home aquaria are not game or food fish, so they go largely unnoticed or may even be completely unknown to many of the people living near their habitats. As humans are not usually involved in conserving species of which they are unaware, the unique habitats of these fish are being destroyed every day without any monitoring or protection.

Gastromyzon species, in general, are not listed as threatened by the IUCN (in most cases because of a lack of data regarding their distribution, taxonomy, habitat, and ecology), but they are still affected by habitat loss and, to a lesser extent, by uncontrolled harvest from the wild for the international aquarium hobby market. Exacerbating this latter pressure is the fact that this genus does not do well in the ornamental fish supply chain due to lack of oxygen and proper feeding, and they cannot cope well with the rigors of lengthy trips. Many suffocating, malnourished specimens reach hobbyists’ tanks with dismal chances of survival.

It is my sincere hope that, in the future, more captive-bred Gastromyzon specimens will flourish in the hobby scene, and that their native habitats will be better protected from human encroachment through suitable protection policies.