Crawfish and Shrimp are both scavengers or bottom feeders. Thats why you purge crawfish before you cook them and devein shrimp after they are cooked. They eat the nasty stuff on the bottom of the swamp or ocean or marsh.
I've never seen a toxin study on either one. I believe shrimp and crawfish should be labeled identifying their origins. I've seen reports and photos of how both are farmed in China. The waters they are farmed in are polluted cess pools. The fresh water fish fillets like talapia they export are about the same.
https://www.businessinsider.com/disg...ulture-2012-10