When a two-year-old clip of the strange-looking Greenland shark resurfaced this week, it quickly made the rounds on social media. These deep-sea fish are among the internet's most beloved oceanic oddities ... but there's just one problem here: this probably isn't a Greenland shark. 

"It was at a depth of 2,200 metres," the team wrote on Facebook. "The discovery of the Greenland shark was a big milestone for [us], and to the natural history of the world." 

The footage was captured by Brazilian shipping operator Sapura back in 2014. The shark came cruising past one of the company's remotely operated vehicles (ROVs) during a routine run (full video here), and until then, they'd never seen one like it in the region. 

Curious about the animal's scars, we reached out to Julius Nielsen, a Phd student with the University of Copenhagen and the Greenland Institute of Natural Resources, who has done extensive work on Greenland sharks. His reply came with a surprisng twist.

"I don't think the shark in the footage is necessarily a Greenland shark (Somniosus microcephalus)," he said. "It is much more likely that the shark is a Pacific sleeper shark (Somniosus pacificus) – which is a close relative." Because both species belong to the family Somniosidae, their names are often used interchangeably. But these creatures are quite different.

Also known as "mud sharks", Pacific sleepers are known to inhabit cold, deep waters across the Pacific, most commonly from Japan to Baja California. Greenland sharks, on the other hand, tend to stick to the Atlantic and Arctic oceans.  

Their prey species differ as well. Although most Somniosid sharks have a reputation for being generalist food-hoovers, analysis of Pacific sleeper shark stomachs reveals that they have a taste for squid, octopus and other invertebrates. Their Atlantic-dwelling kin prefer fish, mammals and carrion. 

In both species, the skin often appears scarred, its battered condition sometimes compared to the tumbled surface of weather-beaten stone. Those markings, Nielsen explains, are the result of a hard-knock life.

The scars along the animals fins are normal wear and tear. "They come from ploughing through the ocean floor hunting for bottom-resting fish and cephalopods," he says. "Such white 'wear brands' are also found in many specimens of Greenland shark, and are very common around the snout and on the lower side of the pectoral, anal and caudal fins."

The body scars could also be the result of contact with other sharks, during mating (if this animal is sexually mature), fights over territory or during deep-sea scavenging. 

"The shark's mouth is not super big compared to its body size," says Nielsen, adding that specialised, interlocking teeth in the lower jaw help these animals tear off morsels of flesh. "During this procedure, I expect them to roll (similar to a crocodile) and they can potentially get scars from rocks during such activity."

But there is one more possible explanation, and it's got to do with the sharks' propensity for pilfering. Because the animals inhabit deep waters, they often come in contact with long-line fisheries – and in the deep sea, where food is often scarce, a dangling free meal is hard to resist.

"Greenland sharks are famous for stealing catch," says Nielsen. "During this, they easily become entangled and destroy the equipment to get free." Nielsen and his colleagues regularly encounter hooks in the skin of their study animals (though superficial injuries like these don't seem to bother the sharks). 

"Potentially, these sharks are extremely old," he adds. "And it is likely that they have been in contact with human fishing gear many times during their lifetimes." 

How old is old? That's just one of the many questions about these animals that we have yet to answer. Upper estimates suggest that some sleeper sharks don't reach sexual maturity for some 40 to 70 years, and can reach 200 before making a final descent to Davey Jones's locker. 

Much of our knowledge of these animals comes from museum specimens, which, from my own experience, smell like a savoury mingling of sea and smoky scotch. Alive, these animals have a slightly less appealing aroma: urine.

The smell comes from high levels of urea in their tissues, a feature that has inspired many an Inuit legend. One story suggests a village elder washed her hair in urine and dried it with a cloth, which blew into the ocean to become "Skalugsuak", the Greenland shark. In another myth, the sleeper shark's origins lie in the chamber pot of Sedna, Inuit goddess of the sea, who later brought the animal to earth where it would serve as a spirit guide. 

Nielsen and his team hope that tagging and tracking these creatures of lore will help us to fill in our knowledge gaps: Where do they go? How do they mate? And most importantly, how are we affecting them? 

Somniosid shark meat is toxic, and produces symptoms akin to extreme drunkenness (lack of balance, stiff legs, hyper-salivation, disorientation, vomiting and explosive diarrhoea) in those who consume it. The meat can be safely ingested only when fermented through a lengthy rotting process, so it's easy to see why it's eaten only by a hardy few. Being unappealing to our palates protects these animals from fishing pressure – which seriously affects other species and threatens the lives of roughly 100 million sharks per year.

Still, more than 1,000 Somniosids are taken annually as bycatch in Greenland commercial fisheries alone – and unravelling the ins and outs of their ecology and lifestyle is the first step towards protecting them. 

For more on Greenland sharks and expedition updates, follow Nielsen on Instagram.


Top header image: NOAA Photo Library, Flickr