Aquatic animal are those, which live in water. Example of these are dolphins, whales, porpoises etc. In such animals, forelimbs are like flipper and hindlimbs are absent. Dolphins and fresh water porpoises are quite intelligent. Seals, sea lions and walruses are also aquatic mammals which come to the land in order to rest and breed.
Whale have highly elastic lungs in order to take in maximum air before they go under water. They have a fatty layer called blubber which reduces the lose of heat and also specific gravity of the body. There is a special humpback whale which changes it tune according to the season and also in response with the change in sea temperature.
Source: https://www.youtube.com/watch?v=V8ODVqnd9ts
