The native ostrica comune (ostrea edulis, flattish and round, the same as a Colchester oyster or French belon) isn't as common as it used to be because of a nasty parasite. Most of the oysters raised now are the elongated Japanese or Portuguese species.
Italians learned to raise oysters in Roman times, but they don't do many now; most come from France. Most people prefer them raw, with lemon and pepper, though there are some gratinée recipes, such as ostriche alla tarantina.
Casanova is said to have eaten four dozen every day for breakfast.
On the half shell: valva di conchiglia
Image by Michele M.F.