Chop Chop, called Piranha (ピラーナ) in the Japanese manual, is a Badnik from the 16-bit version of Sonic the Hedgehog 2 found in Aquatic Ruin Zone. Chop Chop is a blue mechanical fish found only underwater that swims faster when the player approaches. They don't have any fighting methods, so it's best to simply jump on their heads to take them out.
The enemy reappeared in Aquatic Relix Zone from Sonic Pocket Adventure. It also reappeared in Sylvania Castle Zone and White Park Zone from Sonic the Hedgehog 4 Episode II, this time with the addition of eyes.
||A Piranha inspired robot which was last seen in "AQUATIC RUIN ZONE" in "Sonic 2". It lurks underwater hunting for its next victim. Once they spot Sonic and his friends, nothing will stop their rapid approach and attempt to crush them with its powerful jaw.
— Sonic the Hedgehog 4: Episode II website