An animal known for its ability to swim in both freshwater and saltwater is the bull shark. Bull sharks are unique among shark species for their adaptability to varying salinities; they can thrive in oceans as well as make their way into rivers and lakes. This adaptability is due to their specialized kidneys which help regulate the salt concentration in their bodies. Bull sharks are often found in coastal waters but have been known to travel great distances upstream in rivers, such as the Mississippi. Their robust bodies and aggressive nature make them formidable predators, contributing to their reputation among shark species.