Hello, have you eaten a shell fish before? If you have not it is exoskeleton-bearing aquatic invertebrates used as food.
I have eaten shell fish before. In fact, where I live the Native American population significantly relied on shellfish for food
Nice, harvested from saltwater environments, some kinds are found in freshwater.
What kinds are found in freshwater?
I'm not sure buddy but what I am 100% sure of is that shellfish are not a kind of fish, but are simply water-dwelling animals.
Are any modern shellfish descendants from prehistoric ones? 
Some category are eaten are shrimp, lobsters, crayfish, and crabs.
Out of all of those, I definitely prefer crabs.