In my fictional universe, there lived a unique creature resembles today tortoises but with spherical shell. My question is what evolutionary advantage(s) does this shape have? How would said creature move on the ground and probably in water? This question is inspired by a bowling ball.
The problem would come if the creature cannot be loose inside the shell. Propulsion could be achieved by changing the centre of gravity - perhaps by moving water around in a special vesicle.
Spherical shells would be terrible on water. We've all seen those plastic balls with people inside going absolutely nowhere. If they are underwater however then there are definite possibilities.
The creature could be the perfect size to be a meatsnack for other creatures. Tortoise shells provide a great deal of protection from the crushing jaws of predators, but a really good spherical shell would be much stronger. It would be like a natural bear canister!
Unrelated, it would be really nifty if it had many holes to stick its head out of, and spun around inside the shell as needed. Why worry about getting flipped over on your back, if you can just turn around inside your shell and keep on trucking!