Amphibious. Acid Crab can breathe air and water.

Brittle Shell. When Acid Crab is reduced to 1/2 of its total HP, its shell cracks. It gains one use per turn of its Acid Spray attack and its AC is reduced by 3. 
This effect applies again when Acid Crab is reduced to 1/4 and 1/8 of its total HP.

Bludgeoning Susceptibility. If Acid Crab is hit with an attack that does bludgeoning damage, every hit on it until it takes its turn again is a critical hit. 

Final Expulsion. If Acid Crab dies, its organs are consumed by the acid inside it, leaving a 10 ft cubic hole with its shell inside. 

Multiattack. Acid Crab makes one Claw or one Crush Attack, with a variable number of Acid Spray attacks (see Brittle Shell).

Claw. Melee Weapon Attack: +8 to hit, reach 10 ft., one target. Hit: 12 (2d6 + 5) bludgeoning damage, and the target is grappled (escape DC 15). The crab has two claws, each of which can grapple only one target.

Crush. Acid Crab crushes attempts to a creature it is grappling. The creature must survive a DC 16 constitution saving throw or take 19 (4d6 + 5) bludgeoning damage. 

Acid Spray. Acid Crab bleeds out some of its acidic blood. Each creature in a 5 by 20 ft line in a random direction, causing each to make a DC 13 Dexterity Saving Throw or take 21 (6d6) acid damage on a failed save, or have on a success.

Summon Allies. (1/day) At initiative count 15, Acid Crab lets out a harsh war call to summon the crabs in the pack it leads. 1d4 Giant Crabs appear and will always attempt to be within 10 ft of Acid Crab.