I found it surprising that some shellfish are found in freshwater.
I have heard of freshwater pearls, so I'm not so surprised.  How many kinds are there?
I am not sure as to the number but I do know that some crustaceans that are enjoyed by diners are lobster, shrimp, and crabs.
Are crawfish from fresh water?
Yes, crawfish are found in freshwater and they resemble small lobsters.
In the South I believe they are called crawdads.
Crawdads is a term that is used to refer to crayfish although I'm not sure if it's limited to the south.