Clam

Clams are basically giant clams found in the seas surrounding Yoshi's Island. They 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.

In Yoshi's Story they are rare, and even then they only appear in Stage 5-2: Lots o' Fish.

In other games like Super Mario 64 clams appear however they don't have life and are just objects that have items in them. However in Super Mario World 2: Yoshi's Island, little clam enemies appear.