In seafood, clean meats are those that have fins and scales. Clean and unclean food is a study in itself. E.g., turkey and chicken are clean, ostrich and vulture are unclean. In mammals, unclean meats are the scavengers, or predators. I.e., you're eating second-hand garbage.
Pork is perhaps the worst.
This grease coats your digestive organs. The large molecules are kept out. Fish that have fins and scales have a filtering mechanism to keep impurities out of the meat. Those that don't have fins and scales don't have this filtering mechanism. Going one step further, lobster and shrimp are the cockroaches of the oceans.