There are probably close to 200 blind freshwater fish species worldwide. Amazingly, only one species is regularly available in the aquarium hobby: the blind Mexican cavefish (Astyanax mexicanus). There are several reasons for this. A good number of our blind fish are threatened or endangered, as they occur only in caves and are often limited to just one cave in their distribution. Cave systems around the planet are under threat from pollution and dropping water tables because of agricultural practices.
Many blind fish do not occur in caves (so not troglodytic but still found in cavelike conditions) but in the turbid, fast-moving deep-water environments of the Amazon and Congo rivers. In these extreme habitats, collecting fish is difficult and uncommon, given the limited popularity of blind species and the logistics to keep such fish alive.
One oddball example is the Indian subterranean catfish (Horaglanis krishnai), which is endemic to India and was first described in 1950 from specimens taken from a dugout well. The fish inhabit flooded subterranean tunnels, and several have turned up as hand-dug wells penetrate their tunnels. Not much is known about these fish, and they are listed under the “data insufficient” category on the IUCN Red List (iucnredlist.org). There is little chance that we will see a fish like this available in the aquarium hobby.
Becoming a Cavefish
Surface-dwelling fish are isolated from others of their species (floods washing individuals into cave systems is one way this can happen), and they develop cave-adapted traits (e.g., reduced eye size, loss of pigmentation); with enough time, speciation may and has occurred. This explains why cave species come from many different groups, the most common being Gobidae (gobies), Hepapteridae (naked catfishes), Balitoridae (loaches), Cyprinidae (barbs) and Bythitidae (brotulas).
I have observed blind cave brotulas (Typhliasina pearsei) in their natural habitat in Mexico’s Yucatan. The larger fish keep maximum distances from each other, meeting only occasionally in the open water column. They retreat to their territory to hunt for blind cave shrimp living along the bottom of the cave. They largely ignore the nonblind but common Rhamdia guatemalensis catfish, but the increasing number of these catfish is certainly a concern. The livebearing cave brotula has few babies at a time, and although the babies are born at 1 inch in size, they are still in danger of being eaten by the larger catfish. So little is known about the reproduction of the cave brotulas that it is hard to estimate what the size of the brotula population is and if the fish could be raised in numbers in captivity. Certainly it is the most primitive and strange-looking cave fish I have observed.
Among troglomorphic fishes (cave-dwelling fish that have sightlessness to some degree and are depigmented) and those from deep-water river channels, there is little difference. Both may or may not have completely absent eyes, pinhead-sized eyes or eyes covered by skin. It seems to make little difference in the fish’s ability to “see.” A cave fish’s other senses are more sensitive, and they often find food in their environment as quickly as related species that have eyes.
Check out our profile on the Mexican blind cavefish at FishChannel.com/CavefishPro.
A cave fish’s environment generally has fine sand substrates and smooth solid rock on the sides. By comparison, the blind fish living in the depths of the Congo and Amazon rivers also live over a fine sand substrate and among worn boulders and rocky caves in their respective river bottoms. There is not a big difference between the habitats, though most cave waters worldwide are clear, whereas the great turbid rivers have less than 3 inches of visibility on average.
World Tour of Cave Fish
Despite having many interesting endemic species, China is not known as an aquarium fish country. For one, not many Chinese fish make it to our aquariums, but also many species are threatened with extinction, including all of the known endemic cave-dwelling fish. Perhaps only two Chinese fish have been made popular in the aquarium trade: the gold barb (Puntius semifasciolatus) and white cloud (Tanichthys albonubes). As far as the amazing Chinese cave fish, almost nothing is known. China has many karst regions, and many of the caves do have their own species of the genus Sinocylocheilus, which are bizarrely shaped fish known as the Chinese cave dragonfish.
After China, the most cave fish species are found in Thailand and Mexico. But many countries in the world are home to some cave fish, including two blind cave-dwelling bristlenose plecos (Ancistrus spp.) in Brazil, gobies in Madagascar and several catfishes in the southern United States.
Blind fish have also evolved in the turbid, deep waters of the Amazon and the Congo rivers. The latter is unique in that no other habitat has as many different species of blind fish. While the Democratic Republic of the Congo has many karst caves and also has a true hypogean (subterranean) species (Caecobarbus geertsi), a large number of blind fish have evolved in the main stream of the river. Some have pin eyes typical for such an environment, but several others are depigmented and completely sightless. At least the blind eels Mastacembelus crassus and M. brichardi are occasionally available in the hobby because in the dry season, fishermen can trap them in shallow waters. The other blind fish from the Congo River have so far not been available in the hobby, including the tiny Gymnallabes nops catfish and Lamprologus lethops, the world’s only blind cichlid.
The Blind Fish Aquarium
The aquarium. Set up a large tank where your fish can live in a group. When kept in groups, blind fish are much more active than one would expect and are constantly searching for food. For most species, a tank of 40 to 65 gallons should be large enough.
Lighting. In order to create a natural-looking cave-dweller aquarium, do not provide bright lights. The animals themselves do not seem to care, as long as there are no sudden changes, but it seems unnatural to keep cave animals in a brightly lit or colorful environment. Plants are also not found in their habitats, so avoid plants in this aquarium as well. Ideally, a cavefish aquarium will have a fine sand substrate and some rocks with deep crevices.
Blue LEDs generate nice cavelike lighting. The subdued lighting is more about giving our eyes the feeling of observing a cave than about giving the cavefish a more natural environment. I have been unable to discern a difference in the animals’ behavior based on the intensity of the light.
Water flow. Many caves are not still-standing waters but instead part of underground streams and river systems, and surface rainfall can generate fast water flow in such cave habitats. With this in mind, any cavefish aquarium should have some current and well-oxygenated water.
Water parameters. For most blind fish species, the water should be around neutral. Temperature requirements vary greatly, but caves in general are cooler than the surrounding regions. The Congo River, however, is very warm, and species from here should be kept in water higher than 80 degrees Fahrenheit at all times.
Caves are special habitats that usually do not have a lot of food, and in most cases they have very stable water parameters. Changes of any kind are not good for cave fish, so stability is an important factor when keeping them in the aquarium.
Feeding. Cave fish are easy to feed in general. The few species that have been kept in the aquarium accept any food offered, with frozen bloodworms and krill as favorites. Flake food seems to be accepted by all cave fish species. Also, they are able to find any food quickly, even in an aquarium with many crevices and strong current.
Tankmates. Most cave fish worldwide do not get very big, usually about 4 to 6 inches, so it should be possible to keep several species in the same aquarium (if only they would be available). In a community tank, blind fish seem to do well as long as their tankmates are not overly aggressive. Anyone who has observed Mexican blind cavefish in a mixed tank of other tetras can see that this mix is really no problem. Likewise, the blind eels of the Congo do well with the species they would normally occur with: buffalohead cichlids, Synodontis catfish and Congo tetras. The blind loaches and Garra species will show some aggression toward conspecifics but never to the point where they need to be separated.
Purchasing blind fish
Besides the blind Mexican cavefish, few blind fish are available. Some of the blind eels may be sold online in the summer months when the water of the Congo River is low and fishermen can trap some of them for export. Certainly, blind fish are intriguing fish well worth some space in the aquarium. Aquarists should be aware of conservation issues and remember that most species are threatened with extinction and should not be for sale in aquarium stores under normal circumstances. Maybe in the future we will see some captive-raised specimens of some of the most interesting species, such as the blind bristlenose catfish (Ancistrus formoso) from Brazil, the large-finned Cryptotora thamicola loach from Thailand or the strange livebearing cave brotula (Typhliasina pearsei) from Mexico. FAMA
Oliver Lucanus has collected, filmed and photographed fish in many countries. He has been a tropical fish wholesaler for more than 15 years, and has published several books and more than 100 articles on fish husbandry and habitats.