A 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 NEW Super Mario Bros., by blowing air up (nonetheless, these actually stop after a certain amount of time). Clams are usually found around Sea Cactus.
Clams appear only in Yoshi's Story, and even then only in Stage 5-2: Lots o' Fish.