From Monterey Bay Aquarium:

Brilliant and seemingly glowing, the bloodybelly comb jelly comes in different shades of red but always has a blood-red stomach. The sparkling display on the outside comes from light diffracting from tiny transparent, hair-like cilia. These beat continuously, propelling the jelly through the water.

This species has only recently come to the attention of scientists, thanks to images like this, supplied by the