If you want to lose weight, there’s a big difference between eating 20 calorie per ounce shellfish and eating 80 calorie per ounce prime rib. If you’re trying to eat only 500-600 calories per day, you can eat a lot more shrimp or lobster than you can prime rib. Protein (and carbohydrate) contain 4 calories per gram. Fat contains 9 calories per gram. It’s the fat in animal protein that drives up the calories. The most successful weight loss is almost always the fastest weight loss, so you can’t eat much. But eating 500-600 calories per day, you can eat a lot more low calorie protein than high calorie protein. Your call.