Definition of fish recognises vertebrate aquatic animals and, collectively, includes molluscs, crustaceans and similar aquatic organisms. The term fish is used both as a collective category including molluscs, crustaceans and other harvested aquatic animals and, in scientific usage, to denote cold blooded aquatic vertebrates with gills and commonly fins; popular usage often extends the term to include various non vertebrate aquatic organisms, while trade and regulatory contexts may treat organisms such as prawns and shrimp as crustaceans for classification purposes.