I was out and about yesterday, so decided to satisfy my own (and perhaps others') curiousity about the clams. As I said they are saltwater clams from the Tonkin Gulf. All supermarkets and market stalls hold them in running water. As you can see, they are submerged.
The water is what I would describe as lightly salted - certainly not seawater salty.
(The sign requests that you don't use the water to wash your hands!)
The supermarket staff were, as usual, clueless, but I did enquire in the market and was told by different traders that the clams will survive happily in fresh water for two to three hours. This bears out my own experience. In fact, I've held them in fresh water for longer than that.
Information turned up by Google is very mixed, but a few sites concur that fresh water isn't going to do any damage in the short term
I hope that clarifies things, slightly.
