Why open the mouth?
They are threatening or funny to us. Their opening announces an attack on the victim, but it can also be just a way to scare off the enemy or cool down the body.

The victim must be caught quickly and efficiently. It cannot get away. The best invention of nature here are sharp teeth, which are found not only in mammals, but also, for example, in fish. Such a piranha prefers fresh meat in its diet. So it hunts in groups for fish…