Truth is: There are 10 pieces in the game in the current English WLO-version. However, some of the game data files reveal that there should actually be 12. Since updates are not coming anymore, we might never get the chance to complete the Ocean Heart Pieces quest or new regions, in which these last 2 pieces are to be found.

Most widely spread theory: With 12 of them, we should actually be able to trade them in for a heart-shaped gem, use the gem to open the door in MaKa Cave (South Island, door with the heart-shaped lock next to it) and enter a Cursed Arena, where a legendary spirit must be fought and beaten to obtain a secret treasure.

However, this seems to be just what was planned. I don't think even the Chinese have that quest in their version yet, so we might never see this happen.

Sad, because this quest was the greatest mystery for all players ever since the game opened, and wild theories were circling around over the years - but we might never find out what really lies behind this door.

Which means - throw them all away if you already threw one. It is not worth the space that it takes. =(