Scientists using an ROV to map the sea floor around California's Channel Islands recently filmed two lumbering sixgill sharks. The heavy-bodied, solitary animals spend much of their adult lives in the deep ocean, so encounters like these are few and far between. 

As their common name suggests, bluntnose sixgill sharks (Hexanchus griseus) have a uniquely rounded snout and six gill slits on either side of the head, compared to the five seen on most shark species. While juveniles of the species tend to frequent inland shallows, larger specimens regularly drop up to 2,000 metres beneath the waves during daylight hours. 

These individuals – seen at 650 and 1000 feet (198/304m), respectively – were discovered by the crew aboard the exploration vessel Nautilus using the Hercules ROV.

"One of the larger sharks in the Eastern Pacific, bluntnose sixgills can stretch over 16ft long, bluntnose to tail!" writes the team. "This second shark [featured above] was spotted off Santa Cruz Island, and appears to be smaller in size than the first shark spotted."

Very little is known about sixgill social behaviour and ecology, but because the animals consume a wide range of mid-water prey (rays, crabs, seals etc.), scientists suspect they use their time in the depths to rest. 

Cruising slowly like this is a great way to conserve energy in extreme habitats, where food is scarce – but many of the sharks' known prey species, like marlin and swordfish, are apt swimmers. This suggests sixgills are capable of at least short bursts of speed when they need a boost. 

The sharks have also been known to scavenge carcasses from both the seafloor and longlines, which they sometimes follow into shallow water. 

What's particularly interesting here is that neither shark appears bothered by the ROV. Some past encounters suggest that sixgill sharks are extremely light-sensitive, as they tend to become highly agitated when exposed to submersible beams. Scientists are still working to understand that behaviour, which is rare for these relatively docile animals. 

Nautilus will continue to explore the Channel Islands National Marine Sanctuary (CINMS) in the coming weeks, so these sightings are likely just the tip of the iceberg. 

Less than 50 percent of the seafloor within the boundaries of the CINMS has been mapped by high-resolution sonar, and the team hopes gathered data will pave the way towards better protections for the area's delicate ecosystems.

The vessel will also be in communication with a concurrent expedition led by National Geographic Explorer Kenny Broad, who plans to focus on underwater cave systems in the region. 

"This dive team will test technologies that can be transitioned to remote vehicles in the future to map submerged caves that are below diver depth," explains Nautilus. "Developing maps of the seafloor using ROVs provides detailed information that will help guide CINMS resource protection issues including incident response and restoration, protected resource and fisheries management, navigational safety, and conservation." 

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Top header image: E/V Nautilus/Screengrab from YouTube