Fresh seafood shouldn't be overly "fishy". It's generally older seafood that will get a stronger "fishy" smell and flavour.
That said, there are 3 approaches to a "less fishy" result:
- Absorb/reduce fishiness: you can always try something like soaking it in milk (which you can then save to use for a fishy bechamel when your sister isn't dining with you). You could then rinse it off and cook it however you want. This should help "absorb" some of the fishiness and make it milder.
- Disguise fishiness: The other angle is to "cover up" the flavour with lots of spicy glaze or garlic/lemon flavours instead.
- Pick mild seafood: The other thing to try is to pick a more mildly flavoured fish (e.g. a white fish instead of something like tuna or salmon). Avoid oily fish as they tend to have a stronger flavour. Choosing something really fresh also falls into this category.
Where I come from (Portugal) it is quite common to see in most restaurants and establishments shellfish being soaked several hours to even days long before consumption, especially sand dwelling shellfish, which is not the particular case of mussels.
The reason for this is that it keeps them alive and fresh for longer, while also purging any remaining sand or debris naturally found inside them, which is very common in burrowing shellfish, quite unpleasant to find when eating and can virtually ruin the dish.
These are however always soaked in either salted tap water at worse, or at preferably clean natural sea water from where they came from at best. Never just unsalted tap water, because this would obviously kill them quickly, and remove any natural salt that acts as flavor enhancer. Chlorine based disinfectants commonly present in tap water will also slowly affect quality of any living creatures (including aquarium fish) and may eventually be fatal them in the long run.
From your provided examples, most against soaking either state that tap water will kill them quickly, or considerably affect quality (which are both true); or is not needed for farmed shellfish.
I can't speak much for farmed shellfish, they are not as common here and quality may vary with providers and techniques, but even farmed one can some times be quite sandy.
So I'd say that soaking with either salted water or ideally sea water is at at worst redundant or not needed, but can be quite beneficial in some situations.
For farmed shellfish, if you find them clean and edible it is probably not needed, for "free range" ones you probably have more to gain by soaking than not.
Best Answer
This is definitely a rapa whelk. These are indigenous to the seas in the far East, but got somehow imported into the Black Sea and overtook the ecosystem.
First, people around the Black sea didn't have much use for them. The waves washed the shells of dead whelks ashore and these got crafted into souvenirs for tourists. Then, people started fishing them and selling them to the Japanese, who ate them. At last, around the summer season of 2000 or 2001, restaurants along the western shores of the Black sea started offering them as food to their guests. Still, I think that much more of the catch is exported than consumed locally (but have no hard numbers for this).
Since the only people who eat them seem to be Japanese and recently also Bulgarians and Romanians, I highly doubt that they have a kitchen-specific name in English, or that you can buy them at all outside of the above locations.
Biologically, they're not related to oysters. My speculation is that innovative restaurant managers who wanted to serve them had to come up with a name which sounded like something posh (sea oyster) as opposed to one which is both common and associated with kitschy ashtrays (rapane).