Hold Breath. While out of water, the giant barnacle can hold its breath for 24 hours by closing it's shell and trapping water inside of it's shell.  It does this during low tide.  If it's shell is opened during low tide it can only hold it's breath for 2 minutes before suffocating.

Water Breathing. The giant barnacle can breathe only underwater.

Multiattack. The giant barnacle makes one attack with it's tentacles.  When a creature it grappled it makes a second attack by pulling the creature in to it's shell.

Close shell. The giant barnacle can pull it's shell closed as an action giving it a +4 bonus to it's AC.  While it's shell is closed the only action the giant barnacle can take is opening it's shell.

Tentacles. Melee Weapon Attack: +6 to hit, reach 15 ft., one target. Hit: 7 (1d6 + 3) bludgeoning damage, and the target must make a DC 16 Constitution saving throw, taking 6 (2d6) poison damage on a failed save, or half as much damage on a successful one and is poisoned for one minute.  The creature can make poisoned condition at the end of each of it's turns. If the target is a creature, it is grappled (escape DC 16). Until this grapple ends, the target is restrained, and the giant barnacle can't use its tentacles on another target.

Shell. Melee Weapon Attack: +6 to hit, reach 15 ft., one target. Hit: 7 (2d6 + 3) piercing damage. While a creature is grappled by the giant barnacle the giant barnacle pulls the creature in against it's shell for 2d6 piercing damage.