Level 1587 requires 10 purple shells to pass. Despite playing for quite a few tries, never more than 7 have ever appeared. Seems like a great scam for eating up beans. How can you get 10 when there aren't that many available?
Its not a scam You won't get any more oysters fall until you have cleared those. I am going to give you a link for some tips and a demo video so you can see it done.