Amphibious. The crab can breathe air and water.

Multiattack. The crab makes two claw attacks. It can replace one of these attacks with a whirlpool.

Claw. Melee Weapon Attack: +3 to hit, reach 5 ft., one target. Hit: 4 (1d6 + 1) bludgeoning damage, and the target is grappled (escape DC 11). The crab has two claws, each of which can grapple only one target.

Whirlpool. This ability requires a body of water at least 10 feet square and 5 feet deep. The crab causes a whirlpool to form in the center of the area. The whirlpool forms a vortex that is 5 feet wide at the base, up to 20 feet wide at the top, and 25 feet tall. Any creature or object in the water and within 25 feet of the vortex is pulled 10 feet toward it. A creature can swim away from the vortex by succeeding a DC 16 Strength (Athletics) check. When a creature enters the vortex for the first time on a turn or starts its turn there, it must make a DC 16 Strength saving throw. On a failed save, the creature takes 2d8 bludgeoning damage and is caught in the vortex until the spell ends. On a successful save, the creature takes half damage, and isn't caught in the vortex. A creature caught in the vortex can use its action to try to swim away from the vortex as described above, but has disadvantage on the Strength (Athletics) check to do so. The first time each turn that an object enters the vortex, the object takes 2d8 bludgeoning damage, this damage occurs each round it remains in the vortex. The whirlpool remains for up to 10 turns.