From the Super Mario Wiki
Mario encounters three Jellybeams
Jellybeams are a type of jellyfish enemies that appear in New Super Mario Bros. Wii. Their name is a pun on jelly bean. 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.
Jellybeam reappear in New Super Mario Bros. U acting in the same manner as in New Super Mario Bros. Wii appear only in Deepsea Ruins of Soda Jungle with Bulbers, Fish Bones and Circling Boo Buddies.
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. Their movements can be compared to a Thwomp, although rather than slamming down on the ground when Mario approaches, automatically moves up and down slowly.
There is a very similar enemy named Jelectro that appears in Super Mario Bros. 3. The differences between the enemies is that the Jelectro is smaller, immobile and invincible.
Jellybeams are also seen in the underwater sections of the GCN Sherbet Land and Dolphin Shoals track in Mario Kart 8.
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