Well apparently the blood of crab fertilizes the sand, which sprouts shells underground making it more productive and also attracting other crabs... About 10-20 minutes please.
10-20 minutes you want hundreds of shells to be sat there? Sorry, very few games will allow you to do that. Just learn to pick shells up in a timely mannor and you'll be okay.
3 minutes is what the news said about 2 months after the game came out. I think it's still 3 minutes.
And there are plenty of non pk beaches, such as in start, by the shell shop, holiday island, and the hippie van. I completely agree with Ghetto, it is so annoying when people run away to hide on beaches.