Species of Glass Catfish
Author: Joshua Wiegert
The idea of an invisible fish is pretty bizarre, and people are usually shocked the first time they learn that such fish exist—transparent, and nearly imperceptible if not for their skeletons, like little X-rays moving through the water.
Transparency in fish occurs in a handful of species, such as the glass knifefish (Eigenmannia virescens) and the glassfish (Parambassis ranga), but it is extreme in the glass catfishes, as only their skeletons and eyes can be seen. Their truly bizarre, otherworldly appearance seems like science fiction, yet it is science fact.
The true glass catfishes are contained in the genus Kryptopterus, a group of 18 currently described species. However, it is suspected that many other species exist, and more are likely to be described in the near future. These catfishes are found in Southeast Asia and include not only several transparent species but also several that are only somewhat, or not at all, transparent.
The Glass Catfish
The glass catfish has a somewhat convoluted nomenclature history. Many sources have listed it as Kryptopterus bicirrhis, the traditionally assigned scientific name, and almost all literature published before 2013 will use this name. K. bicirrhis, which we’ll call the ghost catfish, has likely never made it in any large numbers into the aquarium trade. It’s distinguished from other members of the genus by having only a transparent head; the rest of the body is fairly opaque and mostly white. They’re a somewhat aggressive catfish and reach a larger size than the true glass cats, up to about 6 inches (15 cm).
It wasn’t until 2013 that two authors, Ng and Kottelat, began looking at the glass catfishes and the misidentification was determined. They discovered that the true glass catfishes represented two species, K. minor and a newly named species, K. vitreolus. Some sources published around the early 2000s will use K. minor to refer to all glass catfishes. K. minor, as the specific name suggests, is best known as the dwarf glass catfish. It is rarely encountered in the trade, though they are sometimes found mixed with true glass catfishes. Reaching only about 2½ inches (6 cm) in length, they’re a much smaller species than the actual glass cat.
Our actual glass catfish is K. vitreolus, which finally found a scientific name in the 2013 paper. This is a larger catfish than the dwarf glass catfish, but not by much, reaching just over 3 inches (7.5 cm) in length. It is likely that almost all of the glass catfishes in the hobby are K. vitreolus.
The body of the glass catfish is clear, with only the skeleton visible. Just behind the head is a silvery sack, which contains the organs. Sometimes these are slightly visible, especially the stomach after a large meal. Noticeably lacking is a prominent dorsal fin, to which the generic name of “Kryptopterus” refers, essentially translating to “hidden fin.”
Glass catfish are not particularly widespread, primarily entering the aquarium trade through Thailand. They’re found in small streams and bodies of water along the coast, where the water is soft and acidic. While many references list high temperatures for these fish, they’re actually found in waters that range from the low 70s to the low 80s F (low to mid 20s C) and will do best at a typical tropical aquarium temperature of about 76°F (24°C).
In the wild, they form massive schools of several hundred individuals, and while this is unlikely to be mimicked in the home aquarium, they certainly should be kept in fair-sized schools. I would never keep less than a half-dozen glass catfish and believe that larger numbers are even better. They spend much of their time in covered areas, often scattered, such as within plants, driftwood, and other decorations. They’ll form schools and move into more open areas, where they spend much of their time hunting.
Their natural diet is made up of small crustaceans, insects, fish fry, and the like, but they’ll thrive on a diet of prepared foods. They have small mouths and won’t bother other tankmates, though they may prey on small fish fry. More worrisome is that they can be picked on and harassed by tankmates. Large, boisterous fish should be avoided in favor of peaceful, quiet species. Ideal tankmates are small tetras, rasboras, and other catfishes, such as Corydoras spp. They also make ideal tankmates for some of the various anabantoids, such as smaller members of the genus Betta, as well as small gouramis, especially the Parosphromenus species, or even the various Badis and related species.
Breeding the glass catfish has been accomplished in the aquarium, but such reports are sporadic at best, and all accidental. Information on breeding them is sparse and seems to be primarily of the “I had 12, now I count 16” variety. The fry are quite small, and, like the adults, mostly transparent. As such, they can live in the aquarium for quite some time completely undetected. Glass catfish are egg scatters, and eggs are laid indiscriminately throughout the aquarium. Aside from this scant information, nothing is known, and I don’t expect many aquarists to succeed in breeding them.
The glass catfish is not bred commercially, and all of the fish in the trade are likely wild collected. Occasionally, a few albino fish make it into the trade. These highly unusual-looking fish have red eyes and a golden sheen around the head.
Like many catfishes, the glass cat is particularly susceptible to external parasitic infections, especially ich. They’re scaleless, so many of the typical treatments for these parasites can be lethal to the sensitive glass catfish, especially those containing malachite green. A strict quarantine regimen is recommended for these fish and any fish going into their tank. Elevated temperatures are tolerated well and should be the first line of defense against a potential ich infection. Simply increase the temperature to about 88°F (31°C). Copper-based medications seem to be effective without harming the catfish.
The Other Glass Catfishes
While the name “glass catfish” almost universally applies to the members of the genus Kryptopterus, two other groups of catfish are also sometimes called glass catfishes, most applicably, the rarely encountered members of the genus Parailia, especially Parailia pellucida. Known as the African glass catfish, it looks every bit like a glass catfish, except for several dark lines down the body that kind of spoil the effect.
While the body of P. pellucida is fairly transparent, the fish can also appear somewhat opaque, depending on the viewing angle, light, and the fish’s mood. At times, they’re off-white to silvery. You’ll generally see a blurry image of what’s behind the fish rather than the clear appearance of a glass catfish, something like looking through a foggy windshield. On close inspection, you’ll note that the main dark line of the fish is not actually a stripe, but rather its black spine.
The African glass catfish is larger than its Asian namesakes, reaching a size of nearly 6 inches (15 cm). While larger, they tend to be quite a bit more shy and will spend much of their time hiding in the aquarium. They must be maintained in a shoal; single fish will generally just waste away. They’re typically found just above the gravel line and will forage there for preferred foods. Ideally, their diet should include live foods, such as blackworms and similar fare, but they’ll often adapt well to sinking aquarium foods.
Similarly to the true glass catfishes, their tankmates should be relatively shy and unassuming, though not too small to be eaten. Anything that will prevent the glass catfish from eating is definitely off the list. Instead, species like loricariid catfishes or moderately sized, slow-swimming tetras should be considered.
They’re found in the Nile River system and typically in fairly deep, dark waters, though they adapt well enough to aquarium waters. They’ll appreciate roughly neutral water and, provided extremes are avoided, they’re unfussy. Though rarely seen in the trade, these African glass catfishes make an interesting addition to the aquarium.
Also from Africa, there is a second small genus of catfishes that are known as glass catfishes. Members of the genus Pareutropius frequently make it into the trade, with the most common member being known as the Debauwi catfish, African glass catfish, and sometimes African grass catfish. Pareutropius debauwi, like the ghost catfish K. bicirrhis, has likely never been seen in the aquarium trade. Instead, the fish we know as the Debauwi catfish is P. buffei, which is better known as the three-striped glass catfish. The two species are relatively easy to tell apart. P. debauwi has a single stripe along the body, while P. buffei has, unsurprisingly, three.
Like the true glass catfishes, the body of P. buffei is mostly clear, though these fish are quite a bit more opaque. They’re a midwater, torpedo-shaped catfish and can often be found hiding in small groups in aquarium plants. They’re found in plant-heavy, flowing water throughout much of western Africa, including Nigeria, Benin, and Guinea.
As smaller catfish, these guys reach only about 3 inches (7.5 cm) in total length. As a midwater fish, they’re fairly active and should be kept in small schools. They’re a bit boisterous and can easily be mixed with small cichlids, such as members of the Pelvicachromis genus, as well as the usual mixture of barbs, tetras, and the like.
Let’s Be Clear
The clear bodies of these fish and especially the glass fishes in the genus Parambassis are one of the most intriguing things about them. Yet, for some reason, the practice remains of injecting these unique, clear fishes with dyes, often fluorescent ones. While this practice has mostly vanished in the past few years, especially after the introduction of brightly colored genetically modified fish that are not dyed or injected, it still continues, and dyed fish of several species are often seen in the trade.
Dyed fish are not simply dyed; instead they’re injected with a fluorescent dye, which fills the body cavities and gives the fish a bright stripe of color. To do this, the fish are removed from the water, held out of it for a minute, and jabbed with a large needle filled with dye. The process is not sterile, and many of the fish are left with a large wound that soon becomes infected. Even when the wound itself doesn’t become infected or provide an avenue for pathogens to enter the fish’s body, the fish is severely stressed by the process, and these animals are far more susceptible to future disease outbreaks. The entire process has an incredibly high rate of mortality.
Ironically, even when the fish do survive, the dye is temporary, and customers who purchase such fish are soon disappointed to discover that they’ve lost their color. Not only is it potentially harmful to the fish, but it’s harmful to the hobbyists who expect their fish to last. Indeed, when I was in retail, I more than once had to explain to customers that this is not the norm and that the bright colors on, say, a cardinal tetra (Paracheirodon axelrodi) would not fade after a couple of weeks.
While the practice is slowly vanishing, such fish remain in the trade. Avoid purchasing them, and hopefully the practice will entirely vanish.
Ghosts in the Aquarium
There are certain definite, obvious advantages to being a clear fish. They’re literally invisible to many predators, and we can often overlook them in the home aquarium. Once your eyes adjust and you can see the “invisible fish,” they’re definitely worth giving room to in the aquarium.