Mexican Tetra (Astyanax mexicanus)

The vast variety of environments on Earth gives rise to an incredibly diverse array of species, all which have adapted to live in specific ecosystems. Today’s animal is an excellent example of how environments influence species’ characteristics, as different forms of the Mexican tetra are radically different, depending on where they live.

Mexican tetras are found in Mexico (what a surprise!), but also occur in Texas. They live in the Rio Grande and the Neueces and Pecos Rivers, as well as in caves in northeastern Mexico. They are freshwater fish that like warm waters with temperatures of 20 to 25 degrees Celsius. In winter these fish migrate, moving to find warmer waters.

Mexican tetras can get as large as twelve centimeters in length (terrifying, I know). They are fairly normal-looking fish, and are somewhat compressed laterally. They don’t come in any particularly flashy colours, being silver with reddish fins.


Mexican tetras, looking pretty normal and boring. Image credit: haplochromis via Wikipedia

So what is so exciting about this little fish? Why am I blogging about it? So far the most exciting thing about it is that lives in Mexico, which is a pretty awesome place, or so I’ve heard. Well, Mexican tetras have two distinct forms, a normal form and a blind cave form. Both of them are members of the same species, but they have one very important difference.

You see, it’s not very helpful to be able to see in caves, because there isn’t much light underground. So Mexican tetras that live in caves have lost their sight. Some populations do retain some sight, while other cave tetras are completely blind, and have even lost their eyes.


The blind form of the Mexican tetra – doesn’t it look creepy? Image credit: JohnstonDJ via Wikipedia

There are other differences between cave tetras and normal tetras. Cave dwellers have taste buds on their heads, which lets them smell better, and they can store four times as much fat in their bodies. As food sources in caves aren’t particularly reliable, extra storage helps these fish survive long term. Cave tetras also are albino, having lost all the pigmentation in their skin.

The result is a two very different animals: one fish that is perfectly normal and perfectly bland, and one fish that looks like it is some kind of freak from a horror movie. Don’t worry though, both forms of the species are still the same species and so they can breed and produce fertile offspring.

Because of the weird differences between surface-dwelling and cave-dwelling tetras, scientists use these guys as a model to study different kinds of evolution. They are also popular aquarium fish, especially in their blind form. I don’t know if I’d want a blind cave tetra in my house, I think they look really creepy. But that’s just me!

Cover image source