Never seen these huge shell creatures before? Allow me to introduce you to Pinna nobilis, also known as noble pen shell or fan musse. In my home languange we call them "Πίννα" (singular) or "Πίννες" (plural).
This is large species of Mediterranean clam that can get up to two meters long! The one you see in the photo is probably half a meter or something. You can use the standard 330 ml soda can I placed next to them, if you are autistic enough and want to calculate the exact length.
The sea in my village is full of them and as a child and up to few years ago I used to remove lots of them from the ocean bottom once every year that I visit the place for holidays. I guess I liked to do that because its a bit challenging.
You see they usually are at 2-3 meters deep (or more) and they are quite hard to remove with bear hands because their surface is quite raw and it's quite easy to get yourself full of cuts if you don't know how to handle them. I remember as a kid, the first time I tried to remove one my thumb was so deeply penetrated by a spike or something from the shell that I had two options. Get drown or pull it hard and get myself a nice, deep cut. I am writting these lines, so yeah, I went for the later option.
However, a few years back I learned that these things are actually threatened with extinction, which was really surprising to me since as I mentioned before there are tons of them in the sea next to my village. So I stopped removing them. The two specimens you see in the photo above were actually long dead, with the insides already eaten by whatever... Probably crabs or something!
My plan now is to first leave the shells in the sun for a couple days. Then I will put them in a 80/20 water, clorine solution for a few days, to get them all clean and use them for decoration or something. I use the same procedure to clean and whiten other sea stuff, like the skeletons of urchins. Will update you with the final results!
I will now close with a video about the sudden mass deaths of these creatures in the Mediterranean thanks to a stupid protozoan for which you can read more at https://www.iucn.org/news/mediterranean/201805/pinna-nobilis-mass-mortality-outbreak-mediterranean-%E2%80%93-call-action-central-and-western-mediterranean-countries
It's possible the two in my picture died from the same cause...References