The Red Coin Fish
From the Super Mario Wiki
Upon entering the level, the Noki Elder's grandson will provide Mario with the helmet, then tell him to talk to his grandfather at the top of the mountain. The elder will say that he and the other Nokis want to reward Mario for helping to clean their bay. Diving into the waterfall once again will bring Mario to the underwater cavern.
The Red Coin Fish is composed of all eight of the Red Coins and several Yellow Coins. The player has to collect all eight Red Coins while collecting the Yellow Coins to avoid running out of air. Occasionally, the Red Coin Fish will split apart, sending its remaining coins in several directions before coming back together a few seconds later. Once all the Red Coins are collected, the Shine Sprite will appear at the bottom of the cavern.
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