I saw something about putting the mussels in a bowl of water with cornmeal sprinkled in it. It's supposed to make the mussels give up their sand.
What I learned about gumbo in southern Louisiana was that the cook used what what was on hand. If you've got clams and mussels on hand, go for it. I'd prefer that to the raccoon I had in one gumbo.
Actually, the gumbo was quite delicious. I noticed bits of really dark meat in it that I thought was beef, but there were lots of other goodies in it too, and I didn't pay that much attention.
It was only later, when I mentioned the "beef" in the gumbo to my in-laws, that I found out what it was. They all had a good laugh, then gently informed me that it was raccoon.
We get by with a little help from our friends