The Acid Worm (ムーア Mūa ) is a miniboss Samus fights in Metroid: Zero Mission. Once Samus has activated the rail system in Kraid and attempts to move onwards, she will encounter this creature. The Acid Worm resides in a considerably large pool of acid in the second door down from Kraid's main shaft.
The Acid Worm attacks by lunging out of the acid at Samus, spitting globs of some sort of substance (presumably more acid) which contain Energy Capsules or Missile Ammo when destroyed, and raising the acid level (not present in Easy Mode). The worm will flick its teeth in and out just before it lunges at Samus. To avoid this attack, Samus must use the zip line on the ceiling to travel to the other side of the room. When this happens, the Acid Worm will ram into a block hard enough to accidentally lodge its teeth in the block, making it unable to move for a few seconds. When it raises the acid level, Samus should jump to one of the two higher platforms in the chamber; however, if Samus has performed a Sequence Break to obtain the Varia Suit early, the acid is nothing to worry about.
To cause injury, Samus must fire at what appear to be the creature's eyes just after it has stuck itself in a platform after lunging. She needs to fire from directly opposite the creature to be able to cause injury. The most effective weapon to use is a Missile, but Samus can still use the Ice Beam if she has run out of Missiles, which is most likely to happen on Hard Mode. If Samus happens to have the Wave Beam or does not have the Long Beam, she won't be able to cause any damage, either because her shots are too large or because the shots won't reach.
As the Acid Worm takes more damage, it will strike more quickly, forcing Samus to act faster. Once it has been defeated, the acid will drain, allowing her access to the floor. A path leading to a Missile Expansion lies under the spot where the worm was resting.
- The Acid Worm cannot be battled unless Samus arrives after activating the ancient Chozo machine in Kraid's Lair. If she arrives without doing this, the room will be flooded with acid and several Reos will be present instead. It can be assumed that the Reos are eaten by the Acid Worm when they're no longer found in the room.
- The battle with the Acid Worm is somewhat similar to that of Serris, as both battles involve some sort of locomotion on the ceiling to avoid the enemy's attacks.
- The Acid Worm bears a great deal of similarity to a Yapping Maw in appearance, habitat and behavior.
- It is possible to keep the Acid Worm from raising the acid. This can be made possible if Samus jumps onto one of the higher blocks just before the Acid Worm goes underneath the acid.
- It is even possible to prevent the Acid Worm from appearing by getting off of the zip line before passing over the platform the creature emerges through. However, the creature must still be killed to unlock the hatches.
- When the Acid Worm lunges, the acid level lowers slightly, as is the case when an object is moved within a pool of liquid.