Notable for its uncanny mouth of human-like teeth, the pacu is a popular pet store fish that has made its way into the lakes, ponds and creeks of at least 27 U.S. states.
While popular as juveniles, this South American native can grow aggressively, prompting owners to free them into local waterways. In the wild, the pacu can grow to more than 3 feet long and weigh up to 44 pounds. Their teeth, while humanoid in appearance, are used for grinding down tree nuts that fall into local waters.
While most pacu do not survive winter conditions in the U.S., there's a fear that a sizable population could take hold in warmer regions, leading to yet-another tip of the scales over native species for resources and habitat.
Editor's note: This slideshow has been updated with new information since it was originally published in August 2016.