Behemoth Bivalve

Water Breathing. The behemoth bivalve can breath underwater indefinitely.

Multilimbed. The behemoth bivalve has 8 tendrils. Each tendril has 10 HP and and 12 AC independent of the main body. The tendrils use the main bodies statistics for everything else. When a tendril is reduced to 0 HP it falls off and the behemoth bivalve and it uses its Snap Shut reaction. If all tendrils are destroyed the behemoth bivalve will attempt to flee.

Multiattack. The behemoth bivalve makes up to 4 Tendril attacks.

Tendril. Melee Weapon Attack: +3 to hit, reach 30 ft., one target. Hit: 4 (1d4 + 2) bludgeoning damage. If the target is a Medium or smaller creature, it has the Grappled condition (escape DC 13) from all eight tendrils. While Grappled, the target has the Restrained condition.

Tongue (Recharge 6). The behemoth bivalve tries to use its prehensile tongue to snare one Medium or smaller creature it can see within 10 feet of itself. The target must make a DC 12 Dexterity saving throw. On a failed save, the target is grappled by the tongue and Engulfed (escape DC 10). The tongue can grapple one creature at a time. While Engulfed the target will suffer (2d4 + 2). 

Snap Shut. The behemoth bivalve withdraws all of its tendrils, releasing any grapples, and slams it's shell shut emitting a wave of sonic energy. All creatures within 60 ft must make a DC 12 Constitution saving throw. On a failed save, the target is knocked off kilter and suffers (2d6 + 2). On a successful save, the target takes half the damage and is not knocked off kilter. A creature without a swim speed that is off kilter must use it's action to right itself before moving or acting. If the creature has a swim speed it may right itself with a move action.