OK, I know oysters are tough to ID, but this is a cupped oyster, and only Crassostrea gigas are grown in the Ria de Aveiro lagoon. The last pure Portuguese oyster (Crassostrea angulata) populations in Portugal being restricted to the Sado and Mira estuaries, and I don't think it was ever found in northern Portugal. This isolated specimen was found attached to mussels (there's still one in the photo). It must be the result of the settlement of larvae produced by farmed oysters.
I found this oyster bank on the bottom of the pillar holding up the Santa Maria restaurant.
An oddly shaped shell often found in batches.