Shell Defense. You can withdraw into your shell as an action. Until you emerge, you gain a +4 bonus to AC, and you have advantage on Strength and Constitution saving throws. While in your shell, you are prone, your speed is 0 and can't increase, you have disadvantage on Dexterity saving throws, can't take reactions, and the only action you can take is a bonus action to emerge from your shell.
Nowhere in this description does it say what happens to items the tortle is holding. So if they are holding a shield and a weapon and go into Shell Defense what happens to their AC?
For a tortle wielding a shield, this question is going to have an impact on how useful this ability is. If the shield does not confer a benefit when in this mode, then they are effectively only getting a +2 AC which is a significant decrease in benefit given the costs.
Does a tortle still gain the benefit of wielding a shield while in Shell Defense?