0:00:02 > 0:00:05My name's Steve Backshall. Self-confessed shark-obsessed.
0:00:06 > 0:00:07This is Shark Bites.
0:00:10 > 0:00:13This time, we meet a shark that's positively prehistoric.
0:00:15 > 0:00:17The sixgill shark.
0:00:17 > 0:00:20It's changed little in 200 million years,
0:00:20 > 0:00:23and is the ultimate super survivor.
0:00:24 > 0:00:27True to their name, they've evolved an extra, sixth gill,
0:00:27 > 0:00:31and can live nearly 2km deep where there's less oxygen to breathe.
0:00:32 > 0:00:36For any shark, it's a tough place, but having changed little since
0:00:36 > 0:00:42the time of the dinosaurs, these sharks thrive in their hostile home.
0:00:42 > 0:00:44But what is their secret to survival?
0:00:44 > 0:00:47The waters of Vancouver Island, off Canada's mainland,
0:00:47 > 0:00:49are where they've been spotted.
0:00:50 > 0:00:53Living in deep water during the day and only entering
0:00:53 > 0:00:56the shallows at night, they're incredibly rare to see.
0:00:58 > 0:01:01To increase our chances, we rigged an underwater camera,
0:01:01 > 0:01:05and to attract them in, some smelly bait.
0:01:05 > 0:01:09This shark bait here... These crates are filled with old fish heads...
0:01:10 > 0:01:14..and lots of blood is floating up into the water.
0:01:14 > 0:01:17You can't see it, but the sharks can smell it.
0:01:17 > 0:01:22After a long wait back on the boat, we finally struck sixgill gold.
0:01:24 > 0:01:25What was that?
0:01:27 > 0:01:28That was definitely a shark tail.
0:01:31 > 0:01:32It's a sixgill!
0:01:32 > 0:01:34We've got a sixgill shark!
0:01:34 > 0:01:37Fantastic.
0:01:37 > 0:01:40This awesome animal that's been around since before the
0:01:40 > 0:01:44dinosaurs is, for me, one of the most exciting animals on Earth.
0:01:46 > 0:01:48And it's an ancient adaptation that enables them
0:01:48 > 0:01:50to survive their hostile home.
0:01:53 > 0:01:56Sharks use their gills to breathe underwater,
0:01:56 > 0:01:59and while most shark species have five pairs of gills,
0:01:59 > 0:02:03the sixgill has... Well, six.
0:02:03 > 0:02:08And as water passes through them, it goes through tiny gill filaments.
0:02:08 > 0:02:12These gill filaments have microscopic blood vessels which have
0:02:12 > 0:02:15a lower oxygen content than the water around them.
0:02:15 > 0:02:19This causes oxygen in the sea water to diffuse into the shark's
0:02:19 > 0:02:23bloodstream where it can be distributed throughout the body.
0:02:23 > 0:02:26It's thought that having an extra pair of gills allows this
0:02:26 > 0:02:30shark to survive in harsh, low-oxygen, deep waters.
0:02:32 > 0:02:34Using this extra set of gills,
0:02:34 > 0:02:37they can take advantage of deeper waters...
0:02:37 > 0:02:39ambushing prey in the darkness...
0:02:40 > 0:02:42..and scavenging from the sea floor.
0:02:44 > 0:02:48The sixgill shark. A colossal deep-sea giant,
0:02:48 > 0:02:54armed with an extra set of gills and barely changed in 200 million years.
0:02:54 > 0:02:58It is, without a doubt, positively prehistoric.