Jellyfish are weird looking animals that can be found in all of the oceans of the world in thousands of species, and even some in fresh water. They have been around for the last 500 million years, making them the oldest animals.
Many of those animals are transparent and with an elaborate lighting they can be seen in many different colors at aquariums. But some, the ones commonly referred to as crystal jellyfish, are bioluminescent. The beautiful color you see is because of a combination of two molecules: aequorin and green fluorescent protein. This means that they actually produce light!