We found this seashell on a Mediterranean beach in the south of France. While most of the shells we found were quite intact, this one caught our attention. It is light and littered with tiny holes, and thicker than other shells we found. Is this due to erosion in the water? How did the shell look like before the ocean changed it? Was the shell simply exposed to the water for a longer time?
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.