From Pikipedia, the Pikmin wiki
Collecting the treasure
To get it, the clog on top of the Shower Room needs to be destroyed. Multiple Yellow Wollywogs block the path from the treasure's locations to the Hocotate ship. White Pikmin are needed to dig up the treasure near one of the corners in the tiled area.
“I wonder what's become of the creature this shell was defending. Enigmas like this always make me curious. Was it tasty? Maybe I should find one and give it a taste? All in the name of science and exploration, of course...”
“A twisted shell whose surface is covered in spikes. The durable housing protects a meal that is said to define deliciousness. (Where the meal went is known only to Captain Olimar's crew.)”
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