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. Without them, there'd be miles of muck on the ocean floor. It's in one of the most published books on earth.
Most people have a copy, or easy access to one.
Few ever read or apply it. It's the Bible. Try it for 60 days and see for yourself. I can train someone who smokes. My 11 lbs.