Down in the deep ocean, where the Sun's rays don't penetrate, there dwells a beast so perfectly efficient it has remained practically unchanged for 200 million years. It's called the bluntnose sixgill shark (Hexanchus griseus), and, like many deep-sea creatures, its lifestyle remains something of a mystery.
Scientists have managed to bring them up to the surface to tag them for tracking in the past. But under normal circumstances, they prefer the darker waters of the meso- and bathypelagic zones (up to 2,500 metres or 8,200 feet deep), coming into shallower waters only under the cover of night to feed.
Since being brought up to the surface can disorient and discombobulate the sharks, the data collected afterwards may not be a true representation of their normal movements. So a team of scientists sought to do something that's never been accomplished before: tag a sixgill shark in its natural habitat.
A team of marine biologists led by Florida State University took a trip out to the Bahamas aboard the OceanX research vessel Alucia, to dive in a submersible to meet the bluntnose sixgill on its own turf.
A blow-by-blow of our evening as we took our (literal) last shot at tagging an animal from a submersible for the first time ever. First, the setup... pic.twitter.com/uiAeSWqEnP— OceanX (@oceanx) July 1, 2019
The shark is actually something of a marvel. Unlike its more evolved relatives, which have five gills, the sixgill's, er, six gills are more primitive - a relic of the Early Jurassic, when the sharks ancestors first evolved.
And, based on the fossils of those early sharks, the bluntnose sixgill has changed surprisingly little.
It's a large shark, growing up to 8 metres (26 feet) in length, with a wide body and luminous green eyes, dwelling in the depths of temperate and tropical oceans. Like most sharks, it survives both by hunting live prey and scavenging, feasting on fallen carcasses on the seafloor, using its serrated teeth to tear off chunks of flesh.
This lifestyle works well; it's thought that individuals can live up to 80 years.
With boxes of bait, the team took the submersible on nightly dives hundreds of metres below the ocean off the coast of Cape Eleuthera, hoping to snare a shark with their GPS tagger.
"The first night there were bluntnose sixgills everywhere. We lined up the shot, fired the tag… and it bounced off the female sixgill's skin," wrote OceanX in a blog post.
"On night two, we made the relevant adjustments… but no sharks showed up. Night three: The sharks were back, and we were very excited to deploy the tag, but unfortunately a large grouper came and tagged itself (exactly in the correct tagging position. So we may have grouper tag info in a couple of months, unless a sixgill eats it)."
Finally, on the fourth night, they succeeded, tagging a large male.
More footage of six-gill at 528 meters from inside the sub last saturday. This sequence was taken by Lee Frey, our multi-talented sub pilot/engineer/inventor who designed the solenoid triggered spear guns for sub-based tagging. Thanks again to the entire OceanX team. Amazing.. pic.twitter.com/hnW4hQLhm7— Gavin Naylor (@gavinnaylor) July 2, 2019
The video posted to Twitter by marine biologist Gavin Naylor of the University of Florida isn't the tagged shark, but a huge female the team encountered on Saturday, June 29.
Attracted by the bait, she is clearly visible in the lights of the submersible as she circles, stirring up silt on the seafloor and nosing the vessel.
It's a sight few have ever seen.
The team, understandably, are elated - not just for the footage, which is magnificent, but for finally tagging a shark from a submarine.
"This is historic for a variety of reasons," they wrote. "Now that we've proven this method can work for the sixgill, we can unlock the world of leviathan deep-sea dwellers and gain important insights into their movement and behaviour."