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.

I'm around lvl 30 and ever since I started the game I can honestly say "I hate these creatures!" I've died by them more then anything else lost two parasaurs, a raptor, and who knows what else. Advise keep glancing under water to see if any are around, avoide rivers crossing on foot. I like rock hopping and dino swimming they can take more damage

Piranhas are a huge pain if you have a shark set to passive they will kill it they once killed my Rex so if you see piranhas run or kill them

Tip: just stay away from them, they are fast and furious!