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 13. Also when the crab's body is submerged in sand, it looks like an ordinary rock and takes a DC 18 Perception check to notice it.

Multi attack. The rock crab makes 2 attacks, one claw smash and 1 claw grab.

Claw Smash. Melee Attack Weapon. +6 to hit. Deals 2d8+4 points of bludgeoning damage. 

Claw Grab. Melee Attack Weapon +6 to hit. Deals 2d6+4 points of  piercing damage. Target makes a Strength Saving Throw (DC 15) or be grappled by the rock crab. The grappled target can repeat the saving throw at the end of their turn or on their action. Any grappled targets automatically take 2d6+4 points of 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.

Defensive Retreat. As a bonus action The crab retreats back into it's shell, and it's speed drops to 0. If the rock is still intact its AC is 23, if the rock is broken the AC is 18. The only action the crab can take is to get out of the shell as a bonus action. Any grappled creatures are let go.