Starfish bubbles come from the bottom and rise up. They are a starfish, in a bubble (duh). Eat them and you get some points. Eat 1000 of them throughout the game and you get an achievement. No, they don't have to be in one playthrough. Level 39 is a bonus level that has only starfish bubbles if you go through adventure and timed mode and still do not have this achievement. When you beat the level just restart the game and play it again. (After you beat adventure for the first time you can choose whatever level you want, if you didn't realize this). I put this achievement here because I unlocked it after I beat adventure/before I beat time attack. If you don't have it by that time, don't panic, play through time attack first.