In a hot pile, they can be composted. Of course, crushing first helps. They may act as a slow liming agent if pieces remain intact- taking quite a long time to work.

I have composted heavy clam shells, lobster shells, steamahs, shrimp shells, and a variety of other bivalves and oceanic insects. Heavy clam shells take longest, lobster, crab, and shrimp shells disintegrate quickly.

I was at the garden center today and saw that they had bags of lobster compost which contained quantities of lobster shells amongst other things.

In the bag of compost I mentioned earlier the shells were all broken up.

Mussels and oysters stay around a long time. But I think they're quite decorative around the garden.

I would not go to extra effort to add lots of shells to my home compost bin but I'm sure that compost comming from a facility that takes the waste off of a seafood processing would have lots of shells in it. Not a bad thing especially if you need some extra calcium or lime in your garden anyway.

Some kinds of shells will definitely take a long time to compost since they are not so much organic anymore but mineralized and instead of composting, they are really just slowly dissolving.

Here is a link that might be useful: My compost

Lots of things take a long time to decompose, but so what? You've got an eternity to make compost. I have tons of shells in my garden that came with my seaweed. If their whiteness offends me, and sometimes it does, I toss a trowel full of soil over them.

Lobster shells, though, get a bad rap. They do not take as long to break down as people suppose they might.

What I've found is that if they're right in the pile, crab shells don't take long (a couple seasons) to become crumbly, lobster shells next, oyster shells a long time after, and mussel shells are better for decorating the beds.