During a cage-diving trip off the coast of Australia, a group of tourists spotted some truly unusual behavior from a white shark. The creature was seen swimming on its back to the astonishment of the witnesses, including the captain, who said he has never seen such behavior before.

The footage was filmed by Wendy Bower-Leech on a boat belonging to the Calypso Star Charters. According to a spokesperson for the company, the shark was a female approximately 3.5 meters (about 12 feet) in length. It was not tagged.

"It swam on its back for several seconds as depicted in the video but the shark was in the vicinity of our dive cage for a couple of hours much to the delight of our guests," the spokesperson told Fox News over Facebook.

It is unclear why the shark was swimming in such a peculiar way, but there are some possibilities. 

"My guess is it was mouthing the boat to check it out and ended up unintentionally swimming on its back in the process, which can almost paralyze sharks," said Molly Zaleski, a marine scientist based in Alaska, to IFLScience. "Sharks (and some other animals) have this reaction to being upside down called 'tonic immobility' that 'causes a temporary state of inactivity.'"

During such a state, the animal's muscles relax and its breathing becomes deeper and more rhythmic, according to Shark Trust.

It is not clear what the function of this paralysis state is, if there is one. Some suggest the hypnosis-like state is advantageous during mating. Others say it could be a fear response that leads them into a catatonic state. In certain animals, such a state comes in handy when dealing with predators (ie., playing dead).

Apex predators like white sharks don’t seem to require such a defense mechanism. Still, it might be a vestigial behavior, leftover from when it was needed in ancient times. Nowadays, it might actually be used against them. SharkTrust reports that orcas have been spotted on a couple of occasions taking advantage (intentionally or unintentionally) of tonic immobility to suffocate white sharks, which need to move continuously to breathe.

[H/T: Fox News]