What happened to this seashell?
The holes are probably caused by one of a family of parasitic sponges.  The holes, as you'd expect, kill the oyster, and the sponge then takes up residence in the shell. A number of species of this family live in the Mediterranean, with Cliona apparently being the most common genus (e.g. Cliona celata and C. viridis).  Images of oyster shells after a Cliona infestation can be seen here and here; these images are more dramatic than the one you found, but show the same general pattern.