I've never seen raw, fresh oysters for sale anywhere here. The only mussels are farmed mussels and I don't know what they're fed. I'm skeptical of any claims that farmed seafood is "at least as good as wild," just as I'm skeptical that conventional beef is at least as good as 100% grass-fed or that cooked meats are just as good as raw--all claims that are frequently made but which my experience and research doesn't seem to support. I also can't see myself paying high prices (much of the weight you're paying for with shellfish is the shells--so the cost of the actual meat is much more than the listed price/lb) for seafood that is fed crap. If I'm going to pay a lot, I want to get top quality. Plus, some types of fish farming are damaging to the environment (though I haven't heard that re" mussels).
Cherimoya is correct, farmed bivalves merely filter nutrients from the surroundind ocean-water so it's probably not much different between farmed and wild. That said, I'm sure location is everything, and wildcaught shellfish in far-out(less polluted?) waters would be more nutritious than any farmed shellfish within caged, polluted areas or whatever.
My own personal experience has been mixed. The raw shellfish, including oysters and mussels, I get from 1 specific fishmonger's at a farmers' market are always of incredibly high quality whereas the mussels I get from supermarkets and other fishmongers are often much smaller and less tasty|(and the shells of the latter mussels was always very thin and broke easily). The fishmonger I mainly buy from said that this was because he practised ethical fishing(ie he refused to fish outside the correct harvesting season for those creatures, so that that was why the size of his oysters etc. was far larger).
Does anyone know the vitamin D levels of farmed mussels or wild scallops? Nutritiondata just has a "missing or incomplete" listing for that nutrient for both. If farmed mussels have significant vitamin D and I get ambitious maybe I'll ask the market what the source of the mussels is so I can try to contact them and find out what they feed them and whether it's an environmentally-friendly operation.
Have you checked the usda nutrient database, it's great as, unlike other sites,they have checked the nutrient-profiles of some raw wild game as well as the usual grainfed meat crap.
I'm not fond of the taste of scallops, but if wild ones have significant vitamin D I'll try them in case they taste better than farmed.
The raw scallops I've gotten in the past from numerous sources have always been free of chemicals(well, outside the supermarkets, that is). I'm particularly sensitive to any chemicals found in raw seafood, so I'm sure I would notice if there had been.
Interesting to hear about this steaming issue with bivalves. I'd never heard of it before, I suppose because my family didn't usually cook shellfish at home and would routinely enthuse about the superior benefits of raw oysters over cooked - the only rule I ever heard of was the one about never eating raw shellfish during months with an "r" in them.