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
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
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.
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
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.
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:
Aquarien- und- Volelspinnenpage
Gee Fish, and particularly