It’s difficult to imagine a great white shark as anyone’s prey, but carcasses have been washing up on South African beaches this year, and they’re all missing one vital organ. The question, though, is who, or what, is repsonsible. Were this a story on rhinos or elephants, the first and most logical guess would be the human element, but this time the hunters are other ocean dwellers.

Terry Goss - Wikipedia Here’s the story from IFLScience: This year, five carcasses of the formidable hunters have been washed up, with the remains ranging in size from 2.7 to 4.9 meters (9 to 16 feet). All of them had a similar brutal wound inflicted on them, with a hole running down between their pectoral fins and the liver. As it turned out, the sharks were being hunted by a team of killer whales, recently spotted in the area when the sharks were found dead. It is not unusual for orca to target sharks as prey. Different populations of the marine mammals are known to specialize in different foods, with those off the north-western coast of North America focusing on fish, while some subpopulations in Antarctica specialize in minke whales, for example.

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Minette Layne Orca off the South African coast have been known to target sharks and rays before, plucking out the liver while leaving the rest of the carcass to drift away. And that’s the interesting thing about these carcasses… they’re not completely eaten. It seems that the orcas only hunt the sharks for their livers and pluck them out with an almost surgical precision, but, why? IFLScience explains:

It is thought that they target the liver of elasmobranchs (as sharks and rays are technically known), due to the organ’s high-energy content. Unlike most fish, sharks don’t have a swim bladder to help with buoyancy. Instead, they’ve evolved an oil-rich liver, which has the dual role of helping them move up and down the water column, while providing them with energy. It seems, however, that the orca have also figured this out. The Guardian has more on the subject: Shark livers are large, typically accounting for 5% or more of a shark’s total body weight. They are oil rich, with a principal component, squalene, serving as an energy store and providing buoyancy in the absence of the swim-bladder found in teleosts (bony fish).

_ _Wikipedia Analysis of white shark livers in particular shows an extremely high total lipid content, dominated by triacylglycerols (>93%). This results in an energy density that is higher than whale blubber. For the sharks this serves as an energy storage unit to fuel migrations, growth and reproduction (Pethybridge et al 2014). For the orcas this is like eating a deep fried Mars Bar with added vitamins. Generally speaking, livers contain vitamin C, vitamin B12, folate, vitamin B6, niacin, riboflavin, vitamin A, iron, sodium and of course fat, carbohydrate and protein energy sources. Even more interesting, though, is the way that the orcas hunt and subdue a shark so that they can extract their meal.

Wikimedia Commons It appears that they take advantage of a condition known as tonic immobility. This phenomenon occurs in both sharks and stingrays when the animals are turned on their backs in the water. They go into a trance and become unable to move. Researchers take advantage of this state to conduct measurements on the sharks, and now it appears that killer whales do the same, only for more lethal purposes. Check out the video below and see what this looks like as orcas take down a tiger shark: https://vimeo.com/105751008