👉 Cetacea is a subfamily of marine mammals that includes whales, dolphins, porpoises, and sea lions. These animals are characterized by their long, streamlined bodies with a distinctive operculum, which allows them to breathe through their gills rather than through their lungs. Cetaceans also have the ability to swim at speeds up to 30 miles per hour (48 kilometers per hour) without assistance and have been known to dive as deep as 12 meters (