Mandible is a side-scrolling action game where the player controls an unknown sea creature called Mandible. The goal is to survive as long as possible by avoiding boats and eating fish while Mandible's abilities like ink spray and tentacle swipe are on cooldown. The game is set in the Pacific Ocean in the 1830s and has a film noir visual style in black and white. Difficulty increases over time as more boats appear and Mandible's health and abilities are balanced for challenge.