Here's why:
* Osmoregulation: Freshwater fish have a higher concentration of salts inside their bodies than the surrounding water. They constantly absorb water through their gills and excrete dilute urine to maintain this balance. Saltwater is the opposite, and fish living in saltwater need to conserve water and excrete excess salt. Piranhas lack the physiological adaptations to manage the high salt levels in saltwater.
* Habitat: Piranhas are found in the Amazon River basin and other freshwater ecosystems in South America. They are adapted to the specific conditions of these environments, including water temperature, pH levels, and availability of food.
While there are some fish species that can tolerate both freshwater and saltwater (diadromous species), piranhas are not among them.