The Killer Blood of the Great White Shark.

Great White Sharks possess levels of arsenic and mercury in their blood that would be fatally toxic to most animals on the planet; however, the sharks do not appear to suffer any of the physical consequences.

Heavy metals such as arsenic and mercury are not toxic to most vertebrates until they have accumulated to a sufficient blood concentration and when they do build up to toxic levels, their effects can be devastating. In 1956, 900 people died and over 2000 were hospitalised in Japan after they consumed shellfish that had been contaminated with extremely high levels of mercury, which is now known as the Minamata disaster.

At toxic levels, heavy metals have been shown to increase the risk of developing neurodegenerative diseases and compromising immune systems. They have also been linked with "deregulating enzymatic and tissue function" as well as the deterioration of neurological function. These outcomes can be catastrophic for wild animals and especially for those individuals at the top of the food chain.

Generally, predators possess higher concentrations of arsenic and mercury than other animals because of bioaccumulation from dietary uptake, meaning that the potential threat of heavy metal poisoning increases with an ecosystem's trophic levels (the different stages of the food-web). In sharks specifically, an excessive quantity of arsenic and mercury in the body can alter their migratory and foraging behaviours, leaving them unable to hunt effectively.

Previous scientific research has found that sharks retain higher levels of these heavy metals than most species; however, the way that these toxic compounds affect the sharks is not well understood. This was the focus of a recent study by researchers from the University of Miami and the University of Cape Town who wanted to evaluate the impact of these heavy metals on the Great White Shark's body condition.

In one of the most exciting sampling procedures in all of science, the researcher team led by Dr Liza Merely, captured 43 different Great White Sharks at five different locations off the South African coastline in 2012. The scientists used bated barbless hooks before (very carefully) loading the sharks onto a hydraulic platform to collect their blood. The sharks were then given antibiotics, in addition to an electrolyte injection, to help speed up their recovery and the blood samples were sent to the PathCare Laboratory in Cape Town for chemical analysis.

Whilst they found that most of the sharks possessed high levels of arsenic and mercury in their blood, the levels were not correlated with either body condition or overall body size as the scientists had hypothesised. Furthermore, the concentrations of heavy metals were not only significantly higher than the levels previously recorded in other studies but are sufficient to be considered to be "toxic for humans."

Evidently, Great White Sharks have a level of arsenic and mercury in their blood that would be fatal for any other species, but why do the sharks not experience any of the negative effects associated with heavy metal poisoning? One theory is that Great White Sharks possess some form of natural defence which provides them with the ability to "manage the levels of heavy metals in the blood and possibly mitigate their effects."

This study is the first published recording of heavy metal concentrations in the blood of wild sharks and researchers suggest that future studies should build on this fascinating work. The next step in finding out why the Great White Shark is not affected by these toxic levels of arsenic and mercury. Unfortunately, it is not possible to conduct research on captive Great White Sharks as these animals cannot be kept in aquariums, but there is hope that one day scientists can study how these heavy metal concentrations change over time and whether environmental pollution is an influencing factor.

This is an enthralling area of research and future investigations will hopefully shed more light on how these predators are able to survive without serious neurological or immunological damage. Could it be a special type of cell in their body, a superior gene that regulates the levels in their blood, or is it that Great White Sharks are so hardcore that heavy metal poisoning is just not an issue of concern for them?

Author:
Thomas Llewellyn

Reference:
Merly, L., Lange, L., Meÿer, M., Hewitt, A.M., Koen, P., Fischer, C., Muller, J., Schilack, V., Wentzel, M. and Hammerschlag, N., 2019. Blood plasma levels of heavy metals and trace elements in white sharks (Carcharodon carcharias) and potential health consequences. Marine Pollution Bulletin, 142, pp.85-92.