From the Super Mario Wiki, the Mario encyclopedia
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Spiked Shells are obstacles that appear in Super Mario Odyssey. They behave similar to the Spiked Balls seen in other games, but are made out of shells instead of metal as the name suggests. They are found in the Seaside Kingdom rolling inside a cave area near the Rolling Canyon Checkpoint Flag on the way to the The Seal Above the Canyon Power Moon. If the player touches one of them, the Spiked Shell will break and they will lose a piece of their Health Meter.
The boss of the Seaside Kingdom, Mollusque-Lanceur spits out Spiked Shells during his battle that will float on the surface of the water for a few seconds before flashing yellow and exploding. The explosion can cause the player to lose a piece of their Health Meter. They will not float if he spits them out onto an area with shallow water and they will instead explode when they land.