The simple answer - defence - as this superb National Geographic video shows.
It starts by giving a great insight into the natural habitat of Tetraodon mbu, the world's largest freshwater pufferfish.
Juveniles are shown inhabiting a shallow river, which drains into Lake Tanganyika. Their diet is crabs but they must also make sure that they don't end up on the menu of a rather ravenous otter.
Check out the video to see what happens...