October 19, 2021

I, Science

The science magazine of Imperial College

The Great White Shark or Carcharodon carcharias if you want the Latin, otherwise known as Jaws. This infamous predator can grow to up to six metres in length, has three thousand razor-sharp teeth, a top speed of 40kmph and can detect a single drop of blood in 100 litres of water. A true apex predator, they are at the very top of the food chain and therefore utterly invulnerable.

Or are they? It seems that the Great White has an Achilles heel. It’s actually quite simple; all you have to do is turn them upside down.

The reason is tonic immobility, a reflex action that leads to a state of paralysis that sharks can enter into when they feel threatened. In the Great White the shark’s dorsal fin straightens, its breathing and muscle contractions relax and the shark remains immobile. It’s a survival strategy; looking dead to an observer is a handy trick for a whole range of animal species.

It can also be fatal. When held in an ‘unnatural’ position, the shark’s brain perceives it as a stressful situation and produces the same hormones that induce tonic immobility. The shark in other words can be put into a trance. We humans have been manipulating this for a while now. Shark researchers use tonic immobility to study their subjects in relative safety. New chemical shark repellents have even been developed using this technique as if a repellent is strong enough to wake a shark from its trance then you know that it works.

But we’re not the only species to have discovered this secret. In 1997 a Great White Shark was attacked and eaten by a Killer Whale in the Farallon Islands off the coast of California. It was the first ever recorded predation on a great white shark and the attack happened in full view of a whale watching boat. The onlookers later described the event. The shark had been taken completely by surprise as the orca rammed once into its flank. Dazed and confused the shark was quickly flipped upside down and held tight by the Orca. Holding it still for fifteen minutes the shark slowly suffocated to death without so much as a struggle.

Another attack happened in the same area in 2000. The shark killers, both times, were members of the LA Orca pod, also known as the “odd pod”. They’re unusual because they don’t fit into the three known orca culture groups in the area. There are the “resident” orcas, who hunt fish between Seattle and Alaska. The “transient” orcas that swim along the pacific coastline in search of marine mammal prey like Sea lions, and finally the “offshore” orcas, who hunt fish in the open ocean. The LA pod however are opportunistic hunters and have therefore developed hunting techniques for a huge range of prey animals. They have to make the most of every prey opportunity they encounter and the Great White Shark is as much on the menu as any other kind of fish.

The sharks it seems are very aware of the dangers of an encounter with these kinds of orcas. After both attacks the entire local population of around 100 great whites disappeared. One male had a satellite tag, recovered a few months later it showed that shortly after the second incident he dived to a depth of 500m and swam 3,200km to Hawaii. This information has opened up a new explanation for the huge Great Shark Migrations that occur every year. It seems that not only do they travel huge distances for food and to breed but for the Californian great whites at least it can be avoid an animal that even they are scared of. The greatest predator in the ocean is running away.