Unlocking more sharks

To unlock more sharks in Hungry Shark Evolution, you first need to level up appropriately. You should try to complete all of the missions for a particular shark before moving onto the next one, which you should be able to purchase through the game store - the sharks don't actually evolve as such, you keep access to the previous one even once you have unlocked the next. Missions are unlocked by discovering shells dotted around the map.

Check your mission list by tapping on the shell icon to the top-left of the screen. The Megalodon, which is the rarest shark in the game, is unlocked once you have completely leveled up the Hammerhead shark and completed all of his missions. Leveling up sharks is useful for more than just progressing to the next evolution - in fact, they increase the statistics of your current shark and allow you to eat more food and travel around the map more easily.