Jellybeam

Jellybeams are a new type of jellyfish enemies that appear in New Super Mario Bros. Wii. They are only found in World 8-4 and serve to illuminate the dark waters. They are found in flocks along with Stone Spikes, Cheep-Cheeps, Bloopers and Bulbers.

Despite helping Mario to see underwater, they can also hurt him if Mario touches them. Mario can also freeze them, causing them to stop moving but they will continue to illuminate the water.

Trivia

 * A very similar enemy named Jelectro appears in Super Mario Bros. 3. They have the same appearance as a Jellybeam, although they're smaller immobile.