Sharks are notorious sea predators and could present more of a problem for the dolphin populations if not for their built-in defense against such threats. The long nose they possess provides them the ability to kill sharks which threaten them. They also can communicate with each other to defend each other from predators.
Dolphins always have access to oxygen. They are able to do this because as they dive their heartbeats slow down. This causes blood to flow from other areas in their body to fill their lungs and brain as well as their heart. Dolphins also utilize their muscle tissue to store oxygen. They can do this because of the myoglobin in their muscles, a protein capable of storing oxygen. Before diving underwater dolphins gather air through their blowholes and close them up just before their descent to keep any water out.
With echolocation, the dolphins locate food sources by bouncing sound waves off of their prey underwater. When these waves bounce back to the dolphins, they know what they have found. Then they use their teeth to catch the prey. The echolocation is not able to be heard by human beings, but it is useful underwater where it is dark and hard for the mammals to see.
Blubber works as a protective coat for dolphins. It enables these mammals to maintain warmth even in very cold water temperatures. It also works to streamline the physique of the creatures. This makes it possible for them to swim at very fast speeds as well as very smoothly through the ocean.