Seafood includes any marine animal caught or raised for food, whether saltwater or freshwater. It is categorized into shellfish like mollusks such as bivalves, univalves, and cephalopods, as well as crustaceans like shrimp and crabs, and finfish including flatfish, roundfish, sardines, and milkfish. The anatomy of a fish includes both external features like fins, scales, eyes, and mouth, and internal organs like the heart, gills, and digestive system.