Of the sixty shells in the water, forty-nine now carry five resting spots, three carry four, and two carry two — all inside the same stretch of water the shell has always held, so a shell you know is still where you remember it being, just not always on the same rock. Six shells stay exactly put: a couple of neighbourhoods genuinely have no second safe spot, and a few shells hold their ground for reasons a diver can discover. Nothing was forced into a marginal position for the sake of variety.
The guarantees matter as much as the movement. A shell you have never prised never moves. A shell mid-countdown never moves — it settles only at the instant it reopens. Closing the game, reloading, or switching devices reshuffles nothing: a save comes back showing exactly the layout you last saw. And every candidate spot was proved safe before it shipped — no possible layout can bury a pickup, block a passage, or land a shell on top of anything that matters. Full mechanics in the guide: Pearl Shells.