Quirks & Quarks

Sharks cope with levels of heavy metals in their blood that would kill other animals

Researchers studying Great White sharks have found lethal levels of mercury and arsenic

Researchers studying Great White sharks have found lethal levels of mercury and arsenic

Great white shark in the water off South Africa. (Neil Hammerschlag)

Sharks have figured out a few things about how to survive in the over 400 million years they've been around. But researchers never suspected one of those things was an ability to live with levels of toxic heavy metals that would kill other animals. 

Liza Merly, from the University of Miami Rosenstiel School of Marine and Atmospheric Science, and her colleagues made the discovery while gathering general health data from blood samples extracted from Great White sharks in the waters off South Africa.

Heavy metal exposure can cause a wide range of devastating neurological and physiological problems in humans and other animals. But Merly told Quirks & Quarks host Bob McDonald that these sharks suffer no apparent negative consequences from having high levels of mercury and arsenic in their blood.

"We could not find any relationship at all between the level of heavy metals and any negative impacts on the shark. So, as far as we could tell, they're handling the presence of those metals relatively well."

Great white sharks do not seem to suffer from high levels of heavy metals in their blood. (Neil Hammerschlag)

Getting blood from a shark

Merly's colleagues collected blood samples from 43 sharks as part of a larger project to develop physiological baselines for sharks. The animals were captured and carefully raised out of the water onto specially designed platforms.

"It's not a trivial thing," said Merly. "These are very large animals, and so you have to be careful. And once they are secured on a platform, we can then take a blood sample from the caudal vein, which is near the tail and that usually yields a good amount of blood where we can do all kinds of tests."

Pieter Koen from The South African State Veterinary Service draws blood from a great white shark raised out of the water onto a special holding platform. (OCEASRCH)

After the blood samples and body measurements were taken, the sharks were tagged and released. The blood was then screened. Part of the testing involved looking for blood levels of 12 trace elements and 14 heavy metals, to better understand the impact heavy metals, in particular, are having on great white sharks which are a vulnerable species. 

Mercury, arsenic and lead

All 14 of the heavy metals that were screened for were present in the great white shark blood. The team found alarmingly high concentrations of three metals: mercury, arsenic and lead. In fact the levels of mercury and arsenic were so high that they would be lethal to most other species.

The great surprise was that despite this, the sharks seem to show no ill effects. This suggest that great white sharks may have a protective mechanism that mitigates the harmful effects of exposure to heavy metals.

Merly and her colleagues can only guess what that mechanism is and how it works."There's a possibility that sharks have potentially important protein components, like metal binding proteins, that may mitigate the impacts of metals, and may also be part of how they metabolise the metals in the body. It could be how and when these metals are moved from the blood to the tissues. We really don't know."

A tagged great white shark swims away from the research platform. (Ocearch)

Understanding shark physiology may help us

One question they could at least partially answer is where these toxins came from. Heavy metals bio-accumulate in the food chain and as sharks are top predators, they will tend to build up levels by absorbing these toxins from the animals they eat. The team can't say just how much of this is from natural sources and how much is due to human pollution.

Measuring these heavy metals in shark blood, though, may be a useful way to determine the health of the ecosystem in which they live, and the levels of heavy metals present in the animals they eat on down the food chain.

This could also have implications for humans, as we are also top predators. We eat many of the same fish as sharks, and we also consume shark.

But the real benefit of this knowledge might be that figuring out how sharks deal with these heavy metals in their blood could help us do the same.

"The best thing that sharks are doing for humans has nothing to do with shark consumption", said Merly. "It has to do with sharks being used as a model to study some important aspects of their physiology. This is why losing sharks at the rate that we're losing them is really problematic because they could actually serve really important biomedical role."

Dorsal fin of a great white shark in the study. (Neil Hammerschlag)


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