Adult Rock Crab

Amphibious. The crab can breathe air and water.

Rock Shell. The rock crab's shell it made out of an actual rock. Dealing 20 points of bludgeoning damage destroys the rock and lowers the crab's armor class to 14. Also when the crab's body is submerged in sand, it looks like an ordinary rock and takes a DC 18 Wisdom (Perception) check to notice it.

Multi attack. The rock crab makes 2 attacks, two with its claw smash, or one with its claw smash and one with is claw grab.

Claw Smash. Melee Attack Weapon: +6 to hit, reach 5 ft., one target Hit: 13 (2d8 + 4) bludgeoning damage. 

Claw Grab. Melee Attack Weapon: +6 to hit, reach 5ft., one target Hit: 11 (2d6 + 4) piercing damage. Target makes an Acrobatics or Athletics check against the attack roll. Failure results them being grappled by the rock crab. The grappled target can repeat the saving throw as an action on their turn. Any grappled targets automatically take 2d6 + 4 piercing damage if the crab decides to use the Claw Grab on it again. The crab can only have 2 targets grappled at once, it then loses the ability to grapple until 1 of the 2 breaks free.