GeologyRocks

Also - I'm not even sure if shells count as rocks, so apologies if a geology forum is the wrong place to ask!!!

All kinds of things etch away and eat in to old shells. If I remember right, algae are one of the main culprits. If you look at sand from a tropical beach, under a microscope, sometimes you find that most of the grains are bit of shell that have been etched away just like this, almost to nothing.

Shells are almost rocks- just think of them as very young limestone

I think it is certain that we are seeing in your foto the remains of the shell of a snail. Look at the 3 lineae features spiraling around the central column. The large abrasion of the shell is likely due to wave action and interaction with the seafloor, rocks on the beach and animals.

The tiny holes can have several causes. Either the ones mentioned about but also sponges, other mollucs or other marine animals can cause these. It is a bit tricky to say.

Agree with all of the above - it looks like your shell has been attacked by boring algae. That's "boring" as in "tunnelling" not as in "dull"

Thank you for the fast replies - the reason I asked is because it really doesn't feel or look like a shell, but rather as if something else had formed around it, destroying the shell.

 Also- it's not the only one. There was an entire beach literally full of these - hardly any sand, just thousands of shells!