For millions of years, great white sharks have maintained the reputation of being as tough as nails (although they're not quite as bad as Jaws made them out to be). As if these kings of the sea couldn’t get any more badass, it turns out their blood contains high levels of poisonous heavy metals, yet they remain in perfectly good health.

A new study, published in the journal Marine Pollution Bulletin, has analyzed blood samples from 43 wild great white sharks collected off the coast of South Africa and discovered their veins are coursing with a cocktail of mercury, arsenic, and lead. The concentrations were so high, in fact, they would be considered toxic to both humans and large fish.

Nevertheless, the great whites appeared to be totally indifferent to the toxic metals. The team also checked the blood samples for levels of leukocytes, white blood cells, which are deployed by the immune system to fight off infections and deal with toxins. The sharks' leukocyte counts remained relatively normal, suggesting their bodies weren't under much strain from the heavy metals.

"The results suggest that sharks may have an inherent physiological protective mechanism that mitigates the harmful effects of heavy metal exposure," Liza Merly, study lead author and senior lecturer at the University of Miami's Rosenstiel School of Marine and Atmospheric Science, said in a statement.

In case you were wondering how the blood was collected, the sharks were caught using barbless hooks and then carefully raised onto a specialized platform where blood samples and body measurements were taken by a team of South African biologists and University of Miami researchers. Before being tagged and released, the sharks were given antibiotics and electrolyte injections to enhance their recovery time.

As you probably guessed, finding high levels of heavy metals in an animal is not terribly good news. Although the discovery is a testament to the toughness of this species, it also points to the wider issue of pollution in the marine environment.

Great white sharks are apex predators at the top of the food chain. This means they are prone to bio-accumulating toxins in their tissues from the prey they eat, which can include anything from seals and dolphins to tuna and turtles. In turn, these creatures have also feasted on smaller prey and other marine life. Therefore, if a great white has toxic metals in its blood, it’s a sure sign these pollutants exist throughout the ecosystem.

The team of researchers even suggest that great white sharks could be used as “ecosystem indicators”, allowing scientists to study the wider state of the marine environment. Considering we are also an inseparable part of this ecosystem, and even eat some of the same things as great whites, this could also have implications for humans.