Jellybeam

Jellybeams are a 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 along with Stone Spikes, Cheep-Cheeps, Deep-Cheeps, Bloopers, and Bulbers.

Despite helping Mario to see underwater, they can also hurt him if Mario touches them. Jellybeams can be defeated with fireballs or a Starman. Mario can also freeze them, causing them to stop moving, but they will continue to illuminate the water.

Trivia

 * Their name is a pun on jelly bean.
 * 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, and invincible.