Amphibious. The crab can breathe air and water.

Regeneration. The crab regains 1d6 hit points at the start of its turn. If the crab moves this trait doesn't function at the start of the crab's next turn.

Grappler. The crab has advantage on Attack rolls against a creature it is Grappling.

Vulnerable state. The crab has disadvantage on all saves against being Frightened.

Multiattack. The crab makes three attacks: one with its big claw and two with its small claw.

Big Claw. Melee Weapon Attack: +5 to hit, reach 10 ft., one target. Hit: 7 (1d8 + 3) [bludgeoning] damage.  The target is grappled (escape dc 13). If the crab has a creature grappled in its big claw at the start of its turn, it can choose to automatically deal 1d4 bludgeoning damage to the creature. The crab cannot attack with its big claw while it has a creature grappled in this way.

Small Claw. Melee Weapon Attack: +1 to hit, reach 5 ft., one target. Hit: 2 (1d4) [slashing] damage. 

Big Claw Shield. The crab raises its big claw to protect itself. Until the start of its next turn, it has a +4 bonus to AC. This includes the triggering attack. During its next turn it can only attack twice with its little claw. The crab cannot use this reaction while it has a creature grappled in its big claw.