Jellyfish

As you probably know, jellyfish are 98 percent water and they are almost invisible in the seas and oceans. Some species of jellyfish can glow in the dark because they have some bioluminescent organs that emit light. These lights are really useful because they can attract prey or scare off predators as well.  

Moreover, there is a well known fact that jellyfish can actually clone themselves if cut in two pieces. It seems that they can regenerate themselves and create two new separate organisms that can live without any problems. This is why they can be considered immortal. In addition, these transparent creatures do not have brains, but they have some nerve nets that can help them live.