The pirahna is literally the worst thing for low levels. they have high damage and take a good bit to kill. They lock on as soon as you go under water.

It’s mega piranha now dododex
Hit the up arrow so others can see

These things will spawn in your water pen and kill your tames if they're on passive. Make sure you have Dino's to kill them or foundations all around your pen, they've killed so many of my Icthyosaurus' it's not even funny.

I placed a storage box in the water. Next day, it was gone . Piranhas can destroy storage boxes ?? ( I'm using the pc version )

Be very carful while crossing the smallest rivers, they might seem weak but usually around 3 of them comes. They also have a pretty big range.Spear can take one of them but for multiple you are going to need a decent armor and a pike.