Sharks developed in salt water and are adapted to live there. While most species have to live in salt water, some sharks can live for long periods of time in freshwater.
Bull sharks travel far into freshwater rivers and lakes.
They have been found in the Mississippi River and in the freshwater Ganges River in India, the Zambezi River in Africa nd Lake Nicaragua in South America. Scientists often age an animal by looking at the enamel growth layers on the animal's teeth. This method does not work very well for sharks since most of them lose their teeth every seven to 21 days. Just recently, marine biologists began tagging or tracking some shark species. Early research shows that slow growing sharks, such as the tope shark and the piked dogfish, can live more than 40 years.