Bull shark

The bull shark, known scientifically as Carcharhinus leucas, is a powerful predator found in warm coastal waters worldwide.

This species is a member of the requiem shark family. It is famous for its aggressive behavior and its ability to thrive in diverse environments. Because they often live near beaches, they are sometimes subject to culling efforts to protect humans. These efforts have contributed to declining populations, and the species is now listed as vulnerable on the IUCN Red List.

What makes the bull shark truly unique is that it is euryhaline. This term means the shark can adapt to a wide range of salinities, or salt levels. While most cartilaginous fishes stay in the ocean, bull sharks can move between salt and fresh water with ease. This ability makes them diadromous, allowing them to navigate estuaries and even travel far up rivers. For example, a bull shark was recorded traveling 1,100 kilometers up the Mississippi River to Alton, Illinois.

To survive in different waters, the bull shark uses a complex process called osmoregulation. In the ocean, sharks maintain high levels of urea and trimethylamine oxide in their blood to match the saltiness of the sea. When they enter fresh water, they must change this balance to prevent too much water from entering their bodies through osmosis. The shark uses several organs to manage this. The rectal gland reduces its salt-excreting activity to conserve sodium and chloride. The kidneys produce large amounts of dilute urine to remove excess water. Meanwhile, the liver adjusts urea production, and the gills help take up necessary salts from the environment.

Physical appearance and size also vary significantly between individuals. Bull sharks are stout and heavy compared to other requiem sharks of similar length. They typically feature a grey top and a white underside. Adult females are generally larger than males. An average female measures 2.4 meters and weighs 130 kg, while an average male is 2.25 meters and weighs 95 kg.

However, exceptional specimens exist. A single record shows a female reaching 4.0 meters in length. Such a large female could weigh over 600 kg.

They also possess an incredibly strong bite force of up to 5,914 newtons. Weight for weight, this is the highest bite force investigated among all cartilaginous fishes.

Evolutionary history shows that bull sharks have been around for a very long time. The earliest fossil teeth found in Egypt and Peru date back to the Early Miocene. From the Middle Miocene onwards, they became much more widespread in geologic formations globally.

Interestingly, bull sharks share similar freshwater abilities with river sharks of the genus Glyphis. However, they are not closely related. This suggests they experienced convergent evolution, where different species develop similar traits independently. Some scientists believe a population bottleneck during the last ice age may have favored the genes needed for osmoregulation.

Their movement between water types is closely tied to their life stages. Young bull sharks are often found in freshwater or brackish areas. This is likely a strategy to improve juvenile survival. As they age, their tolerance for different salinities increases, and they move toward the saltwater ocean. While they can live in fresh water for their entire lives, research suggests they may struggle to thrive there long-term. In one experiment, sharks kept in fresh water died within four years, possibly due to a lack of saltwater food sources.

Bull sharks inhabit many different regions across the globe. In the Atlantic, they range from Massachusetts to southern Brazil and from Morocco to Angola. They are also found in the Pacific from Baja California to Ecuador. In South America, they have traveled 4,000 km up the Amazon River.

They have been spotted in many specific locations, including the Brisbane River in Australia and the Tigris River in Iraq. Even in Lake Nicaragua, they were once thought to be a separate species until taxonomists realized they were actually bull sharks in 1961.