Adapta-Bull

Perhaps the ultimate aquatic survivors, sharks have remained unchanged for millions of years precisely because their sleek predatory forms are so perfectly adapted to their environment – but that doesn’t mean that they don’t have a few tricks up their proverbial sleeves.

One such tricky customer is the Bull Shark (Carcharhinus leucas), a large, stout predator that can reach lengths of 3.5m metres and has the remarkable ability to live in both salt and freshwater with ease. Thanks to its remarkably adaptable kidneys and to special glands near its tail, the shark is able to regulate the salinity of its blood, thus allowing it to move freely from the open ocean into brackish estuaries and onwards into rivers and lakes.

Indeed it’s possible for a Bull shark to live in freshwater its entire life, and there are populations living permanently in major rivers such as the Brisbane River in Australia, while individuals are frequently spotted as far as 2,500 miles inland, up the Amazon. They’ve even travelled up the Mississippi River as far north as Illinois and are regularly spotted in India's Ganges.

It’s thought that this amazing ability developed as a way of helping ensure survival by increasing the shark’s access to prey and allowing it to feed on freshwater as well as marine species. What’s unclear to scientists, though, is why the Bull shark is apparently completely alone amongst shark species in evolving what is obviously a very useful ability.

While there is obviously still much to learn about this most versatile predator, it’s clear that even if you’re swimming hundreds of miles inland, it’s best to keep an eye open, just in case...