Clam

Clams are giant bivalves that appear as rare enemies and obstacles throughout the Mario and Yoshi franchises. Their appearance changes slightly from title to title, but they retain their behavior of opening and closing their valves. A number of fictional species, including Clampies from New Super Mario Bros. Wii and Clambos from Donkey Kong Country, are derived from generic clams.

Super Mario 64 / Super Mario 64 DS
Clams are present in Super Mario 64 and its remake Super Mario 64 DS, appearing in Jolly Roger Bay and Dire, Dire Docks. Most clams in Jolly Roger Bay contain Red Coins, whilst the lone clam in Dire Dire Docks contains a Koopa Shell. If a clam is touched when it is opening or closing, Mario gets knocked away and takes two points of damage. Clams open up for a small period of time when Mario approaches them.

Yoshi's Story
In Yoshi's Story, Clams are generally harmless, but act much like underwater Warp Pipes from Super Mario Bros. 3, by blowing air up (nonetheless, these actually stop after a certain amount of time). Clams are usually found around Sea Cactus. They are rare, and even then they only appear in Stage 5-2: Lots O'Fish.

Paper Mario: Color Splash
In Paper Mario: Color Splash, Wendy O. Koopa the fourth boss waits for Mario inside a Clam before the battle with her. In battle, she attacks by summoning coins from the Clams and dunking them on Mario. She also summons Gold Shy Guys, Silver Shy Guys, and Ice Bros from them to aid her in battle if Mario fails to block the attack.