Differentiating between species can be a convoluted process, but there's no question that this newly described ghost shark stands out from its deep-sea kin. The odd fish is one chunky monkey! 

Image: Walovich Et al./Zootaxa

Measuring in at about three feet (one metre) in length, the Robin's ghost shark (Hydrolagus erithacus) is the second-largest ghost shark ever discovered. It's also the 50th known to science (and clearly the long-lost relative of Jim Henson's Hoggle). 

Also known as chimaeras or ratfish, "ghost sharks" aren't actually sharks: they're shark relatives, fellow members of the class Chondrichthyes, a group of fishes lacking calcified bones. 

Interestingly, this isn't the first time the group's newest member has been spotted. Several of the specimens used to study the species were pulled up as bycatch by trawlers, and others have been sitting in museum collections for years. Because of the chimaeras' extraordinary appearance, the fishermen who first collected them suspected their haul could be something new, but scientists have only just confirmed that hunch.

"[Ghost sharks] in general have a pretty big head and their body tapers to a thinner tail," Pacific Shark Research Center graduate student Kristin Walovich, who described the species, told LiveScience. "This one was really chunky in the front, and just a big bulky specimen."

Looking at sex organs is a useful way to tell once chimaera species from the next. Image: Walovich Et. al/Zootaxa

Besides a fat frame and jet-black skin, Hydrolagus erithacus possesses prominent, flat teeth. The dentition certainly adds to the creep factor, but it's actually not unique to this species. 

"Unlike their shark relatives which have rows of teeth, all chimaeras have toothplates, which are used to eat a variety of prey," says deep-sea biologist Brit Finucci, who wasn't involved in the study.

Chimaeras in New Zealand, for example, typically munch hard-bottom dwellers like crustaceans, but will turn to other prey from time to time. "I found the remains of fish and squid in some of my chimaeras, while there are also old reports of cannibalism," Finucci says.

Walovich and her colleagues found what appeared to be fragmented crab parts in the bellies of their study animals, so it's possible they also regularly indulge in crustacean crudités. But because picky eating habits don't work well in the deep (where food is hard to come by), these animals likely have a pretty broad diet. "It looks like [the new] species may be opportunistic scavengers as well," notes Finucci.

The bucked position of the toothplates actually helped researchers place H. erithacus in its genus (the Latin name roughly translates to "water rabbit"). The seas around southern Africa where this ghost shark was discovered are also home to three other Hydrolagus species.

While describing "chimaera 50" is a significant achievement, the number also hints at how hard studying these animals can be. (For a bit of perspective, there are over 500 known shark species, and a new species of shark, skate or ray is described roughly every two weeks.)

It's thought that the Robin's ghost shark inhabits waters up to 2,000 metres deep, so it will likely be some time before we observe this species alive in its natural habitat, or find out more about its ecology. In fact, a closely related species – the pointy-nosed blue ratfish, Hydrolagus trolli was caught on camera for the first time just recently ... even though it was first discovered 15 years ago!