Although they are natural products of the sea, the shells of mussels and clams (but also all the other shells) are mainly composed of calcium carbonate, a substance that makes them non-biodegradable and difficult to compostable.

Do mussel shells decompose?

The shells of mollusks such as oysters, mussels and clams are also rich in calcium. Those don't break down as rapidly as crustaceans do, but that's not a defect.

Are mussel shells compostable?

We'd like to take a moment to remind you that you can add shells from your everyday cooking to your compost! Seafood shells, egg shells, and nut shells.. Seafood Shells: Oysters, clams and mussels… ... Yes, all of these guys can be composted in your garden.