Amphibious. The crab can breathe air and water.

Damage threshold: Any attacks that do less then 100 damage are ignored as they just glance off. Exception: Attacks from the inside do full damage and cause a reaction.

Claw. Melee Weapon Attack: +8 to hit, reach 20 ft., one target. Hit:  52 (4d20 + 10) bludgeoning damage, and the target is grappled (escape DC 30). The crab has two claws, each of which can grapple only one target.

When creatures under the shell attack the crab it will bash its shell doing 52 (4d20 + 10) Bludgeoning damage, and the targets are prone.