The Mollusks are very slimy giant green slugs without any eyes. Their size varies, as they are ever growing, a trait that causes them to seek a new shell constantly. The first shell was indigo with bits of a pillar stuck in it and the second shell was constructed of school supplies. The first mollusk also had an appendage attached to a claw-like rock, but it never got the chance to use it.
The first mollusk was first seen being hunted at night by Steven and the Crystal Gems. While Steven is walking, the ground suddenly moves underneath him, and it is revealed to be the Mollusk, whose shell was made out of the pillar pieces Steven had stepped on. The Crystal Gems quickly poof her, and Steven grabs another shell from the rubble to bring to class for Show and Tell.
When it is time for Show and Tell, Steven realizes that the small shell is missing along with various school supplies. Following a green trail of slime and supplies, another Mollusk is soon found in the gymnasium. Connie leads her classmates in a fight against it with dodgeballs being thrown at her weak spot, the shell.
Eventually, when the Mollusk breaks through the walls into the cafeteria, high sodium lunch is thrown at her causing her to shrink due to her slug characteristics. Although she is now small and slippery, the Crystal Gems arrive in time to poof her, but at the expense of the school which subsequently collapses from battle damage.
Snail/Slug Physiology: The Mollusk has similar appearance, capabilities, and vulnerabilities found in snails and/or slugs.
Shell Formation: The Mollusk is capable of forming a hardened shell used for defensive purposes from debris and materials around her. Ironically, what she uses as her defense also doubles as her weak spot.
Projectile Launching: The Mollusk can launch projectiles from the materials acquired to make up her shell, such as pencils and chairs.
Slime Mimicry: The Mollusk's body is primarily composed of slime and is able to slip away from attacks easily due to the amorphic nature of slime.
Slime Generation: As her body is primarily made of slime, the Mollusk is also capable of leaving behind slime and is often used as defense, usually causing her enemies slip on her slime.
Constant Growth: The Mollusk is constantly growing, something that requires her to seek a new shell often. When bigger, she is harder to defeat and, while smaller, she is harder to hit.
The first gem was located in the small castle on the shell.
The second gem is in the small purple shell.
The Mollusk's biggest weakness is salt, as it was used to shrink her and make her easier to poof.