While filming for their new series Blue Planet II, a BBC camera crew encountered a group of sixgill sharks feasting on a decaying whale carcass in the shadowy depths of the Atlantic Ocean. Several of the hungry scavengers took interest in the submersible, but contrary to recent sensational headlines, the BBC team was in no real danger during filming.

The rare encounter was caught on camera near the Azores archipelago at approximately 700 metres (2,300ft). The crew had been exploring the area when they happened upon the decomposing remains of a dead sperm whale. Intrigued, the filmmakers went in for a closer look – and discovered that the carcass had also attracted seven bluntnose sixgill sharks (Hexanchus griseus). 

If you go by some media reports, the BBC team "feared for their lives" during the "terrifying attack" that followed – but while we certainly imagine this was an adrenaline-pumping experience, the idea that the "sixgill beasts tried to smash" the underwater vehicle is more than a bit far-fetched.

For starters, these hardy submersibles are designed for deep-sea exploration. Their windows are made of thick acrylic or specially tempered borosilicate glass, both of which are capable of handling the extreme pressures found at these depths. During filming, the BBC submersible would have been exposed to about 1,000 psi. As you can see, the vehicle remained intact despite these forces, and a sixgill shark is certainly not capable of inflicting any serious damage to such durable materials. 

(If you're looking for an accomplished oceanic "smasher", try the bowhead whale.)

The audio added during post-production (or foley) in the BBC footage does make the sharks' "rams" sound forceful. But if you play the clip with the sound off, the gradual nature of the animals' approaches becomes clearer. The sharks become progressively emboldened, and their resulting nudges seem consistent with investigative behaviour. 

While humans have hands to explore their environment, sharks do their investigating with their mouths, prodding foreign objects to determine what's food and who's friend or foe. A 2016 study on sixgill feeding habits in Washington State's Puget Sound documented very similar interactions: many of the animals nudged or nibbled the deployed camera gear. The researchers noted that these bumps may also be accidental collisions during close passes. 

Several media reports have suggested that the sixgill sharks became aggressive towards the BBC vehicle because they viewed it as a competitor: another hungry carnivore moving in on their scavenging score. Such territorial behaviour is often seen in wild sharks (check out this tiger shark encounter), and it's a plausible explanation in this case, too. We suspect, however, that the sixgills were actually drawn in by the unfamiliar electrical signals, lights and other stimuli emitted by the submersible. 

You may have noticed the sharks' eyes rolling back during most of these interactions. This is a protective measure commonly employed during aggressive strikes, but shark peepers will also shift into "safety mode" in response to abrupt physical contact. Scientists who study bluntnose sixgill sharks often see this when fitting tracking tags on their subjects, for example. An accidental collision with the submersible's window, or an intentional nudge, would be enough to prompt the eye-rolling response.

According to former ReefQuest Centre for Shark Research director R. Aidan Martin, adult sharks of this species are also extremely light-sensitive. 

"In the deep-sea, ambient light levels are very dim to nonexistent," he noted in a blog post. "As an adaptation to this darkness, the pupils of the bluntnose sixgill are permanently dilated — the muscles that formerly contracted the irises have atrophied from eons of disuse." 

It's possible, then, that the submersible caused some stress or disorientation during the encounter, which then prompted the shark bumps and nudges.

So, the BBC camera crew certainly didn't capture a "bloodthirsty attack" off the Azores – their footage, however, does offer an amazing look at a shark that's seldom seen by humans.

Bluntnose sixgills are among the world's most wide-ranging sharks, but these nocturnal hunters spend the daylight hours resting on the seabed, as deep as 2,000 metres (6,000ft) down. When the sun dips below the horizon, the animals ascend to hunt. Closer to the surface, they're more accessible to curious scientists, but there's a problem: all that daytime inactivity means the sharks can endure lengthy stints without eating, which makes predicting the timing of their feeding excursions incredibly tricky.

In fact, many of our best observations have come from a population of so-called "shallow water" sixgills that frequent the British Columbia coastline each fall. The group appears to use the cover of light-blocking seasonal algal blooms to shelter them while they feed in the shallows (up to 30m/90ft) during the day.

Prey like cephalopods, bony fishes, crustaceans and even other sharks make up the bulk of sixgill diets. Mammalian fare like this sperm whale carcass becomes a more prominent menu item as the sharks mature. But when it comes to people, the species poses little threat: just one – provoked – sixgill attack on a human has been documented in the past 500 years.  

"Sixgills often indicate their discomfort by holding their pectoral fins stiffly downward," explains Martin. "If a diver persists or touches a sixgill, it will often snap aggressively then dash away."



Top header image: NOAA Ocean Explorer/Wikimedia Commons