A bull shark can swim in fresh water as well as in the salty sea, and the explanation lies in how its body manages salt, because it can adjust the salt and water balance in its body, so it can handle sea water, brackish water, which is a mix of the two, and fresh water. A bull shark's gills, rectal gland, blood, hormones and kidneys all play a part in managing salt as it moves between salt and fresh water. It copes with fresh water by lowering the urea in its tissues, and in fresh water its kidneys work hard to keep salt in its body. Each of these parts does a small job, and together they let one animal live in waters that differ enormously from one another.

Scientists have investigated this in more than one way, and each separate investigation describes a different part of the story. In a laboratory study, young bull sharks moved from fresh water into sea water tolerated rapid and big increases in salinity by quickly raising the concentration of their blood. Separately, bull sharks caught in fresh water had lower salt levels in their blood than bull sharks caught in salt water. Bull sharks can also live in water as salty as 53 parts per thousand, which is saltier than ordinary sea water. They have a semi-permanent population in Lake Nicaragua. Each of these findings comes from its own report, and each one is a separate piece of evidence about the way that bull sharks live with salt, so no single result should be read as the whole explanation.

Young bull sharks often grow up in river mouths and bays, where the water is not very salty, and they enter estuaries and lagoons with low salt as easily as adults do. A bull shark mother carries her pups for 10 to 11 months before they are born, and in the Gulf of Mexico the pups are born from April to June. A bull shark pup grows inside its mother, fed through a yolk-sac placenta. A newborn pup is about 55 to 80 centimetres long, and bull shark pups are well developed and can swim, and even hunt, soon after birth. In these shallow, less salty waters near the mouth of a river, the youngest bull sharks spend their earliest years, growing up in a place that is very different from the open sea.

Some of the river journeys are very long. Bull sharks have been reported more than 2,000 miles up the Amazon River, in Peru, and as far up the Mississippi River as Illinois. They have been recorded repeatedly in the Potomac River, they have a semi-permanent population in Lake Nicaragua, and they have been found hundreds of kilometres from the sea. Bull sharks, especially young ones, can travel far up rivers. A shark that can manage salt, as the first pages explained, is a shark that can make these long river journeys, which is the answer to how a bull shark can live in rivers.

Words to know

• brackish: slightly salty
• salinity: saltiness
• urea: body waste