Most fish must maintain a physical salinity percentage that is less than that of the ocean in which they live. To do so, they constantly drink seawater and filter out the salt with salt-secreting glands. Sharks, which are fish, process the salt of seawater differently. They have the ability to store additional metabolic wastes so that they have to drink less water to keep their body chemistry in balance. As a result of this waste storage, the salinity of a shark's body is higher than that of the water in which they swim.
By entering freshwater, a shark has to change the way its body filters salt and stores metabolic waste. Its body works overtime to process all of the freshwater and maintain the balance of salt necessary for survival. Subsequently, few sharks end up in freshwater environments. Those that do, do not stay there for long durations. Bull sharks and speartooth sharks are the only species known to stay in freshwater for extended periods of time. However, they are not native to those environments and ultimately return to saltwater. Therefore, all sharks are technically saltwater sharks, even those found in freshwater.
Sharks typically live alone and migrate constantly in search of food. Though most sharks live in open seawater, they are also known to live in swamps, lagoons and rivers. Some sharks can adapt briefly to freshwater environments for the sake of migration, which is how they spread into such diverse locations. Sharks thrive in both warm and cold climates.
There are over 360 species of sharks. Scientific research has yielded an estimate that sharks only occupy 30 percent of the world's water, leaving almost three-quarters of oceans shark-free.