Blue Lum
A Blue Lum is a type of Lum that is found underwater in all versions of Rayman 2 except for Rayman Revolution, in which they are replaced with air bubbles. They contain oxygen, which Rayman needs when he swims in bodies of water he cannot resurface to, otherwise his oxygen meter will run out and he will drown. Some stay to provide all the oxygen he needs, other will disappear, but those ones may reappear.
History
Blue Lums in Rayman 2 are said to represent the origin of all life, and the very first breath of a creature's life. When a creature is born, it is given a Blue Lum to keep until the day it dies. As he is the only being Polokus didn't dream about, Rayman may or may not carry his own Blue Lums to breathe.
Usage in other games
In Rayman 3 for the Gameboy Advance, Rayman: Hoodlums' Revenge, and Rayman Raving Rabbids (GBA) the Blue Lums are used by Rayman to power his Helicopter and Throttle Copter and fly around until they wear off.