The three major classes of aquatic mammals are:
1. Cetaceans
2. Pinnipeds
3. Sirenians

1. Cetaceans (order Cetacea)
Whales, Dolphins and Porpoises almost make up the order ‘Cetacea’
A) Whales
Whales are mammals not fish. They have skin not scales. They breathe through lungs and not from gills. The whales take enough oxygen to last up to a maximum of 45 minutes under the water, and in some species even upto two hours. The Sperm whale for instance remain submerged for more than two hours.How can aquatic mammals as whales survive under water for such a long time is because of a strange physiological phenomenon within their bodies which enables them to retain huhe quantity of oxygen in their muscles. The oxygen thus stored in a special protein, myoglobin, can be later released gradually during a dive. Some seals have ten times more myoglobin in their muscles than land mammals. But in any case, all aquatic or marine mammals must come to the surface regularly to obtain fresh supplies of oxygen.

Note: Sharks are one of the species of fish and hence breathe through their gills like all fishes do. The world’s biggest shark is the Plankton-feeding Whale Shark, while the largest marine mammal, and the biggest animal on planet earth is the C. Blue Whale (Baleen species). Its weight varies from 150 tons to naerly 200 tons.