Prowling the dark depths of the world's oceans is an almost prehistoric predator. Larger than an average white shark with a luminous green eye and physiology that's practically primitive, the bluntnose sixgill is one of ocean's most widespread predators, but its deep-sea lifestyle and nocturnal habits make it difficult to study. Although researchers have gleaned what info they can from sixgills caught as bycatch or those that do sometimes stray from the briny depths into shallower water, there is still much to learn about these behemoths. So in an effort to expand our knowledge of the animals, a team from OceanX and the Cape Eleuthera Institute recently embarked on an ambitious quest to fit a satellite tag to a sixgill shark in its natural habitat. 

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Bluntnose sixgill snatching fish from a bait station attached to a research sub. Image © OceanX

Scientists have tagged these elusive creatures in the past but, in order to do it, they had to bring the animals to the surface. "Because bluntnose sixgills are a deep sea species, it’s hard on them physiologically to be tagged in this way," the OceanX team explain in a blog post. These sharks are unlikely to ever experience daylight in their normal life cycles, nor will they be exposed to the low pressure or warmer water temperatures found at the surface. "Typically, the data obtained after surface tagging of a sixgill is believed to be skewed, as the shark does not return to its natural behaviors for some time after the tagging."

The solution? Construct a specialised rig complete with bait station and a speargun loaded with a satellite tag; mount that sucker on a sub and head to the ocean floor. It was a bold plan and, perhaps unsurprisingly, it took a few attempts to get it right.

The first shot at making history came in August 2018 when a "gorgeous big sixgill" investigated the sub, but the team couldn't get a clean shot. Bluntnose sixgills can grow to about eight metres long and, an average, they are larger than great whites. So, given the size of the target, you'd think it would be relatively easy to shoot a satellite tag into a sixgill. However, the area suitable for safely applying a tag is actually only about the size of an iPad, the team explains, making an already logistically complex task that much trickier.

A second attempt was made in February of this year, but the tech failed and the speargun didn't fire. The team held hope that the third time would be a charm.

As their common name suggests, bluntnose sixgill sharks (Hexanchus griseus) have a distinctive rounded snout and six gill slits on either side of the head, compared to the five seen on most shark species. Juveniles of the species are known to frequent inland shallows, while larger specimens typically remain about 2,000 metres beneath the waves during daylight hours. At night, they swim closer to the surface to feed, so – in order to catch up with the sharks on their daily journey from the deep sea to shallower water – the team had to conduct their tagging operations at night.

After setting the sub down on the ocean floor, it was a waiting game. The sharks showed up en masse on the first night. "There were bluntnose sixgills everywhere," the team wrote. "We lined up the shot, fired the tag… and it bounced off the female sixgill’s skin." Take two. Adjustments were made and the rig was calibrated just right, but on night two no sharks turned up. Night three looked more promising until a grouper stole the show by managing to tag itself on the speargun. "We may have grouper tag info in a couple of months, unless a sixgill eats it," the crew joked.

By the fourth and final night the team had used up 300 pounds (136 kgs) of bait. Part of the tagging process involves a daily rigmarole of lashing baitfish to a long pole that extends from the front of the sub. It's designed to coax a sixgill into the perfect position for a speargun operator to fire a tag into the shark's flank. The team prepped the bait for the final time and the sub descended. "With nothing to do but wait, we made nervous jokes in mission control, which is where radio updates come up from the sub ... On previous dives, we’d noticed a trend of the sharks showing up around 9:50 pm – aka 'sixgill o’clock.' On this day, they were right on time."

An update came in that a tag had been launched, but a delay in communication left the topside crew hanging on tenterhooks. Twenty seconds later they received confirmation: "Tag went into a large male." Upon their return to the surface, the sub operators were given a hero's welcome.


 

The team's efforts have not only opened the doorway to unlocking secrets about sixgills, but they have shown that it's possible to tag a deep-sea denizen in its natural habitat. This technology and methodology have wide-reaching impacts for the future study of creatures that dwell in the ocean depths. For updates on the tagged sixgill, visit the OceanX Facebook page.

Here's some bonus footage from marine biologist Gavin Naylor of a female sixgill that the crew encountered on June 29:

Top header image: NOAA Ocean Explorer/Wikimedia Commons