Collect all the gold and silver coins and pearls before you run out of time. Eat fish to grow. If you have grown a lot, you can split yourself up in smaller stars, which increases your chances of finding all the coins. Watch out for the spiky fish, though, they will kill you. If you collect too many shells, you cannot collect other things. Shake off the shells if there are too many, but keep in mind that they will also add to your score at the end of the level.
Use the arrow keys to tilt the field. Press A to split yourself up into pieces, S to merge your pieces and D to shake off the shells.