I add one every so often, but mine are alive. Ones I either pick up at the beach or while snorkeling in the Florida Keys. But I do it because they are filter feeders and I like to see how long I can keep them alive.
I do a giant cockle (juvenile) when I find a live one along the beaches here. And some basic small clams I collect in the Keys. The cockles seem to survive 3 to 6 months and I'd like to get them to make it longer. The clams I've been able to keep for several years now.