Unknown to most, there is a genus of sharks that strictly live in freshwater. They are the spear tooth shark (glyphis glyphis), the northern river shark (glyphis garrecki), and the ganges river shark (glyphis gangeticus). These sharks are under the threat of overfishing due to their already small population numbers. While they primarily live in freshwater, they sometimes journey to brackish water. It is thought that the glyphis genus is relatively new compared to other sharks. The theory of how they came to be is that an ancestor of the lemon shark that could tolerate fresh water permanently moved into a river. The reason could range from plentiful prey and protection, to being trapped in the river by a natural disaster. Eventually, this group of sharks began to have separate genetics than their oceanic counterparts. As they reproduced and formed a population, a new genus was born.