Most fish have the good sense to be opaque. Scales, pigment, and a solid-looking body are the basic template for a freshwater fish, reinforced by hundreds of millions of years of what works. Kryptopterus bicirrhis missed that memo entirely. The glass catfish — pla nuea on in Thai, which translates roughly as "soft-flesh fish" — has abandoned colour so completely that in clean water you can see straight through it: the spine, the fine ribs, the internal organs compressed into a small chamber just behind the head, all visible in a living, hovering animal.
It is one of the most visually arresting things in Thai freshwater. As a fishing target it is strictly a niche proposition. As a species that tells you something important about the rivers it inhabits, it is worth understanding.
The Transparent Body
The transparency of Kryptopterus bicirrhis is not a trick of the light or an exaggeration by aquarium marketers. The fish is genuinely see-through. In the aquarium trade it is sometimes called the ghost catfish, and in a well-lit tank the name is apt: a school of glass catfish hovering together in midwater appears almost as a shimmer of movement rather than a collection of solid animals.
The biology behind this is straightforward but unusual. The fish lacks the melanophores — pigment-bearing cells — present in virtually all other fish, making the body tissue itself colourless and largely transparent. The body cavity, which in most fish extends much of the body length, is in K. bicirrhis compressed to a small volume directly behind the head and gill covers, containing the essential organs. The remainder of the fish — the long tail section that constitutes most of the body length — is muscle, bone, and connective tissue with no concealing skin and no organ mass behind it, leaving it effectively transparent.
When light hits a glass catfish at certain angles, the transparent body produces a faint iridescent shimmer — a structural colour effect from the interaction of light with the internal tissues. In a school of hundreds of fish turning together in a sunlit river, this effect makes the school look like a slow, living reflection.
The dorsal fin is reduced to a single short spine, barely visible above the body. The primary locomotor fin is the anal fin, which runs the entire length of the underside of the body and undulates in smooth waves to produce the fish's characteristic hovering, slightly tail-down posture. Two long maxillary barbels extend forward from the upper jaw — these are the sensory organs that orient the fish in current and help locate food.
Taxonomy: A Complicated Family
The name Kryptopterus bicirrhis has been applied to glass catfish in the aquarium trade for decades, and it is the species native to Thailand. However, a 2013 taxonomic study by Heok Hee Ng and Ralf Britz formally described Kryptopterus vitreolus as a distinct species — smaller, with slightly different proportions — and suggested that much of what was exported from Thailand and sold in the aquarium trade as K. bicirrhis was actually K. vitreolus. The true K. bicirrhis is the larger, more robust species that reaches up to 15 cm and is the fish an angler in a Thai river is most likely to encounter.
This taxonomic distinction matters relatively little from a practical angling or observation perspective — both species are transparent, both school, and both occupy similar habitat — but it is a useful reminder that Thai freshwater biology is still being refined, and species you encounter may have changed names or been separated since older reference materials were written.
Habitat: Clean Water Required
Glass catfish are habitat specialists in a way that many Thai freshwater species are not. They require clean, well-oxygenated water with modest current. They are found in the slower sections of clear rivers, in the calm arms of reservoirs where water quality is high, and in streams with minimal sedimentation. They do not tolerate the turbid, polluted conditions that allow highly adaptable species like walking catfish and swamp eels to persist in degraded water.
This sensitivity makes glass catfish a useful environmental indicator. Their presence in a stretch of river tells you something positive about water quality and habitat condition. Their absence from areas where they were historically recorded suggests deterioration — increased turbidity, reduced oxygen levels, or the destruction of riparian vegetation that the fish depend on for shade and bank stability.
Glass catfish are habitat specialists. Their presence in a river tells you something positive about water quality. Their absence from historically occupied stretches suggests deterioration.
In Thailand, glass catfish are found across the river systems of the central and western regions — tributaries of the Chao Phraya system, the Kwai and Mae Klong watersheds in Kanchanaburi, and cleaner river sections in the central plains. In the north, suitable clear-water habitat exists in the smaller tributaries of the Ping and Nan river systems near Chiang Mai.
The species is typically found in the midwater column rather than on the bottom — hovering in loose schools, facing into gentle current, and moving as a unit. Look for them in shaded areas near the bank in calm river sections, particularly under overhanging vegetation or in the lee of submerged structure where current is reduced. In reservoirs they use the littoral margins and the inflowing stream arms where water quality is highest.
Observing Versus Fishing
Most visitors to Thailand who encounter glass catfish do so in one of two settings: in an aquarium or in a river while doing something else. The species is not a fishing target in any mainstream Thai angling context. It is small (reaching perhaps 12–15 cm in the wild), delicate, and not considered a food fish in the way that larger catfish species are.
That said, glass catfish can be caught on very light tackle, and there is a modest tradition of catching them specifically — primarily by anglers interested in observing the fish closely and in the context of broader light-tackle fishing on clear rivers. The method is simple: a 1 lb class ultralight spinning rod or fly outfit, fine line (1–2 lb monofilament or the equivalent), a tiny hook (size 14–18), and a small bait such as a fragment of prawn, a small worm, or a piece of bread.
Glass catfish are mid-water feeders and will not typically take a bait on the bottom. Present the bait suspended at the depth where the school is hovering — usually 30–60 cm below the surface in calm river sections — and allow it to drift naturally with the current rather than retrieving actively.
Fly fishing for glass catfish is possible with very small wet flies or nymphs drifted through the water column at the appropriate depth. This is a patience exercise rather than an active hunting approach; the fish will take a well-presented small fly but are not aggressive in the way that snakeheads are. A catch-and-release session with glass catfish on a 2-weight fly rod in a clear river is an unusual and genuinely pleasant experience, though not one that fits the conventional image of Thai freshwater fishing.
Schooling Behaviour
Glass catfish are obligate schoolers — they do not naturally exist as isolated individuals in the wild. Schools range from small groups of a dozen fish to large aggregations of several hundred, all oriented in the same direction and maintaining consistent spacing with one another. The school moves and responds to disturbance as a coordinated unit, a behaviour driven by visual and lateral line communication between individuals.
In the aquarium trade, keeping glass catfish in groups of at least six is standard advice, and single fish rapidly decline in health in isolation. This behaviour in the wild means that if you find one glass catfish, there are almost certainly many more nearby. A school hovering in a shaded river pool, all facing upstream with their bodies slightly angled down, catching the light as they turn — this is a sight that most visitors to Thailand will never see and that rewards the angler willing to seek out the right kind of water.
Broader Significance
The glass catfish is primarily a biodiversity story rather than a fishing story. It represents the kind of species that a healthy Thai river system holds — specialised, habitat-sensitive, visually remarkable — and that disappears quietly when water quality declines, riparian vegetation is stripped away, or sedimentation increases to the point where the clear water the fish requires becomes unavailable.
The decline of wild Thailand fishing addresses the broader context of how freshwater ecosystems have changed over recent decades. Species like the glass catfish are among the first indicators of that change — less visible than the disappearance of large trophy fish, but significant as markers of ecosystem health.
For those interested in protected and rare species, the protected and endangered species guide for Thailand provides a framework for understanding which species require specific protection under Thai law, and how anglers should respond to encountering them.
The Case for Clear Rivers
Fishing for glass catfish, or simply observing them, requires finding clear water. In much of central Thailand's agricultural landscape, clear water in natural rivers is not the default condition — it must be sought out, often in upstream reaches, forested tributaries, or reservoir margins shielded from direct agricultural runoff.
The search for glass catfish — with light tackle in hand, working upstream along a clear-water tributary of a larger river — is an exercise in finding what remains of wild Thailand's freshwater ecosystem at its best. Other species will be present: small cyprinids, loaches, perhaps dwarf snakeheads in the margins, climbing perch in weedy corners. Glass catfish in midwater are the indicator that the water is clean enough to support specialists.
That is, perhaps, the most useful thing the glass catfish teaches the angler. Not how to catch it — that is simple — but where to find the kind of water worth protecting.
