Evolution is a funny thing. Usually it acts on a species over hundreds of thousands of years, allowing it to eventually adapt to better suit it’s environment. However, in the case of the blind cave tetra (a type of fish), evolution seems to be in a bit of a hurry. This particular type of evolution is known as convergent evolution. The blind cave tetras have evolved to live in pitch black cave environments, whilst other tetras have not. Their cave dwelling traits are similar to those of other cave dwelling animals. However, these traits were not present in the last common ancestor of the tetra, meaning that, evolutionary speaking, they’re pretty new.
The blind cave tetra is, as it’s name suggests, blind. Because it spends it’s life living in complete darkness, it no longer has any need for eyes. Therefore, the eyes of this fish lack pigment and are covered by a layer of skin (in actual fact, their eyes are still able to detect a light source, they’re just not functional in the ‘seeing’ sense). Not only have these fish lost the pigment in their eyes, they have also lost all pigment in the rest of their bodies making them a ghostly, pale, white colour. Since they live in absolute darkness, they are not at risk of absorbing any harmful sun rays, and they do not need to be camouflaged since their predators are not able to see them in the dark.
As well as changes to their appearance, the blind cave tetra has evolved a great energy saving technique by forgoing the normal circadian rhythms their non-cave dwelling counterparts seem to have. Circadian rhythms act as internal clocks, helping organisms to coordinate their behaviour and physiology with the day-night cycle. In particular, circadian rhythms help with controlling metabolism. During the day, most organisms consume more oxygen and have an increased metabolism compared to during the night. However, the blind cave tetra seems to consume the same amount of oxygen throughout a 24-hour period, and does not rely on circadian rhythms to control it’s metabolism. This is because it can not distinguish day from night since it lives in total darkness. The blind cave tetra are, therefore, able to expend nearly 30 percent less energy in a 24-hour period than its surface-dwelling counterparts. Saving this energy is a useful adaptation for these cave fish because food can be hard to come by in such a hostile environment.
These fantastic fish are prime examples of the power of evolution. Living in perpetual darkness has eliminated the use of their eyes and messed up their metabolism. But if it works, why question it?