What, Exactly, Is a Ghost Shark?

A video making the rounds shows first-ever footage of a strange creature deep in the ocean. In a paper recently published in Marine Biodiversity Record, researchers from Monterey Bay Aquarium Research Institute, the California Academy of Sciences, and Moss Landing Marine Laboratories identify it as Hydrolagus cf. trolli, also called a ghost shark

It isn’t, taxonomically speaking, a shark, but close. Class Chondrichthyes, fish with cartilage instead of bones, contains two subclasses, sharks, or Elasmobranchs, and Holocephans, which includes the order Chimaeriformes. It in turn branches into the family Chimaeridae, and two genera, Chimaera and Hydrolagus (which translates as water bunny). Members of the entire family are known as chimaeras, and, like their namesake from Greek mythology, with goat and lion heads and a serpent’s tail, look pretty strange.

The ghost shark moniker certainly fits Hydrolagus cf. trolli’s appearance. Bluish gray in color, it has large, featureless eyes and lines of dots, probably sensory organs, encircling the head that give it a Frankenstein look. Its narrow head ends in a pointed snout, which accounts for its common name, pointy nosed chimaera. The large, slender body has a whip-like tail, concave dorsal fin, broad pelvic fins and triangular pectoral fins, which it uses to swim, gliding effortlessly much like a ray. While actual sharks famously have rows of sharp teeth, Hydrolagus sports non-replaceable tooth plates.

Ghostly also describes the fish’s preference for very deep water and, therefore, infrequent appearances. Of 38 known chimaera species, 20 were only fairly recently described. Hydrolagus trolli was first named in 2002 and observed only in the southwestern Pacific off Australia, New Zealand, and New Caledonia.

Then, in 2007, researchers conducting deep-sea surveys by remotely operated vehicle (ROV) off California and the western Hawaiian Islands caught ghost sharks on video at depths between 5,300 and 6,700 feet. Scientists consider its physical characteristics a close match for Hydrolagus trolli, signified by the “cf.” part of the name, but in order to rule out the possibility that it is a new species of ghost shark, they need detailed measurements of the body or DNA samples.

That’s easier said than done. Ghost sharks are too big, fast, and agile to be caught by ROVs, and too deep and far from the coast to have much chance of ending up in fishing trawls.

“We’d probably never know they were there without the ROVs,” says California Academy of Sciences shark researcher David Ebert, an author on the paper. “We’ve talked about how we could get a specimen, but it lives at such depth that it would be hard to justify an expedition just to get one. Chances are slim, but you never know, one could turn up tomorrow. At least with all this attention, if someone does catch one, they know who to call.”

Either way, the find significantly extends the known range for ghost sharks. “It’s a cliché, but you always hear how we know more about the surface of the moon than the deep sea,” Ebert says. “And here we didn’t know what is living off the coast of California, which is so urban and high-tech.”