Identify this seafood from the Black Sea known in Romanian as "rapane"?
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).