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 Blind Cave Tetra

The "Blind Cave Tetra", "Astyanax fasciatus mexicanus", is a very unusual fish. It is completely devoid of skin pigments and has a pink skin due to the haemoglobin in its blood. The most distinctive part is that it has no eyes as an adult. For the first two weeks of life it does have eyes and certainly appears to be able to see.

There are several other common names for the Blind Cave Tetra, these include: the "Blind Cave Fish", the "Mexican Tetra", and the "Silvery Tetra".
They can find their way around an aquarium without much trouble. It is not completely clear how they do this. They have a good sense of smell which helps them locate food, but this does not fully explain how they navigate around an aquarium. There are several theories and they are being studied.

Length and Longevity

It grows to about three and a half inches long (9cm). The life span is about 5 years.

A Recent Example of Evolution?

For some people, "Evolution" is a dirty word so I hesitated about even using it in a descriptive article about fish. If you prefer other explanations for the formation of this sub species, I am quite happy with that.

What appears to have happened is that some fish of the species Astyanax fasciatus found their way into an underground cave system in Mexico.  These fish had eyes and could see as most fish can.  In the darkness, their eyes were of little use, and eyes use up energy as well as a substantial amount of brain power to interpret images.  The fish that did not use so much energy and brain power for their eyes had an advantage and bred more.  Over many generations the fish without eyes replaced the fish with eyes and the new sub species was formed.

I should explain that the phase "Sub species" does not suggest any form of inferiority, but is simply a taxonomic group below the level of species, but above that of race or variety.
The Blind Cave tetra is not considered a separate species from the fish that stayed on the surface and kept their eyes. (In light, vision is an advantage.) The blind cave tetra will still breed freely with their sighted cousins, so this is not an example of the formation of a new species.


The Blind Cave Tetra is found in caves in Mexico and Texas while its sighted cousin is found a little further south in Central and northern South America.

Water Conditions

The Blind Cave Tetra is a very tough fish, so although its ideal temperature range may be between 20 and 30 degrees C (68 to 86 degrees F), they will survive water a little bit hotter than this as well as much colder, so they are suitable for either a tropical or an unheated tank.
They can take some acidity (down to about 6) or some alkalinity (up to about 8), so as long as you avoid extremes this should not be a problem.
Blind Cave Tetras can take quite high levels of hardness in the water.


The Blind Cave tetra is a very easily fed omnivore. They will eat all normal types of fish food and have the ability to eat much faster than most fish, and a lot at one time. It is a good idea to give them a variety of foods.


Some sources describe the Blind Cave Tetra as being peaceful.  This is not my experience.  I would definitely avoid putting them with small fish like Neon Tetras and Cardinal Tetras as well as slow moving long finned fish like Siamese Fighting Fish, Guppies and Endlers Guppies.

Fish more suited to be companions for Blind Cave Tetras are: Red Eye Tetras and other medium sized tetras, barbs, Danios of similar aggression.  as well as the Corydoras catfish like the Peppered Catfish.

Some people keep Blind Cave Tetras in an unheated aquarium with Goldfish, Rosy Barbs and other suitable fish. This can work all right, but I would definitely avoid the fancy goldfish with long fins or big eyes. With comets or shubunkins a bit bigger than the Blind Cave Tetras in the tank they should be fine, but always keep an eye on your fish, and avoid huge size differences between the different fish.

Do not put the Blind Cave Tetra with large or predatory fish like the larger cichlids and Murray Cod.


The females become much plumper than the males when they are loaded with eggs.


The Blind Cave Tetra is an egg scatterer. They are easy to condition for breeding because they eat so readily and so much at each meal. They are stimulated to breed by a drop in temperature. 18 - 20 degrees C (64-68 degrees F) seems a suitable spawning temperature. The parents should be removed after spawning, but the eggs should not be disturbed.

The females only lay about 100 eggs each, which is less than the great majority of tetras. The eggs are also much larger than with most tetras and should hatch in between 1 and 3 days, and the fry should be free swimming in 6-7 days from spawning.

Because of their large size, the babies will eat larger things earlier than small fish. The babies can see for about two weeks. They will eat protozoa (infusoria) like most baby fish, but can also eat things like the finest screened daphnia. Commercial fry foods are also suitable. The babies are vigorous and should grow well.


Pest Fish


Ensure your pet fish cannot get out into ecosystems they are not native to. The potential for ecological damage by the Blind Cave Tetra is considerable.

Sources and Picture Credits

I am grateful to the following sources of information about the Blind Cave Tetra: Hardy's Aquarien- und- Volelspinnenpage , , , Gee Fish, and particularly Tim's Tropicals .