Fish
Glass Catfish
Kryptopterus vitreus · also called glass catfish, ghost catfish, phantom catfish
The glass catfish is a small fish that has taken invisibility about as far as a backboned animal can. Its body is so clear that you can see its spine as a thin line and its organs bundled into a little silvery sac behind the head. And when a school of them hovers in a shaft of light, their aligned muscle fibres split it into rippling colours.
Overview
The glass catfish lives in slow, clear forest streams of mainland Southeast Asia, especially Thailand. It is 6–8 cm long, slender, and almost entirely transparent, with only the eyes, the small cluster of organs, and the swim bladder opaque. It feeds on drifting invertebrates and zooplankton in the water column — unusual for a catfish.
Transparency as camouflage
With almost no pigment and clear muscle and skin, the fish scatters very little light, so a predator scanning open water for a fish-shaped target sees very little.
The shimmer
A 2023 study found that the glass catfish’s regularly aligned muscle fibres act like a diffraction grating, splitting light into colours as the fish swims — so a school can produce coordinated flashes of iridescence.
It needs a crowd
The glass catfish hovers rather than rests, its long anal fin rippling continuously, barbels held forward. Kept alone it becomes stressed, stops eating, turns milky, and usually dies; it needs a shoal.
Status
The glass catfish is Least Concern, common in suitable habitat but traded in large numbers, mostly wild-caught, and vulnerable locally to river pollution and damming.
Glass Catfish: common questions
Why is the glass catfish transparent?
It is a form of camouflage suited to life in open water. A fish drifting in a clear or lightly stained forest stream has nowhere to hide, so instead of matching a background it minimises how much light it reflects or blocks. The glass catfish has almost no pigment, its muscle and connective tissue are unusually clear, and only the eyes, the small cluster of organs behind the head, and the swim bladder are opaque. To a predator scanning for the outline of a fish, a transparent one is close to invisible, especially against the shifting light of moving water.
Do glass catfish really flash colours?
Yes, and it is not pigment. A 2023 study found that the muscle fibres running along a glass catfish's body are lined up so regularly that they behave like a diffraction grating, an optical structure that splits white light into a spectrum. As the fish swims and the light hits the body at changing angles, this produces rippling iridescent colours. Because a school swims in a coordinated way, the flashes can appear across many fish at once, which may confuse predators or help the fish signal to one another. The effect is most obvious in bright, directional light.
Can you keep a glass catfish in an aquarium?
It is a popular aquarium fish but a demanding one, and it is not for beginners. It needs a group, ideally of a dozen or more, in a mature, stable tank with soft, slightly acidic water, gentle current, dim lighting, plenty of open swimming space in the middle of the tank, and small live or frozen foods. A lone glass catfish, or one kept with rough or fast tankmates, will hide, refuse food, turn milky, and usually die. Because most stock is wild-caught, buying them also has a conservation cost, and captive breeding is difficult.
Is the glass catfish a true catfish?
Yes, it belongs to the catfish order, in a family of Asian sheatfishes, and it has the group's defining features, including a pair of long sensory barbels near the mouth, though it has lost most of the others such as a prominent dorsal fin and bony head armour. What makes it unusual for a catfish is its lifestyle. Most catfish are bottom-dwelling, often nocturnal scavengers with strong pigment and a heavy build. The glass catfish is a slender, mid-water, daytime shoaling fish that feeds on drifting plankton, so it has evolved away from the typical catfish body and habits.