What could be more terrifying than ‘ghosts,’ ‘sharks,’ and ‘the deep ocean’ during Halloween for some people? Not the movie but the little we know about ghost sharks who live in the deep, also known as chimaeras (in the Chimaeriformes order). This International Ghost Shark Day (October 30), scientists who study them want to introduce you to them!
Chimaeras are a small group of cartilaginous fishes (collectively called Chondrichthyes or Chondrichthyans) that form the subclass Holocephali and are the sister group to sharks, skates, and rays. They were separated from the rest of Chondrichthyes around 360 million years ago. They aren’t true sharks, even though nicknamed “ghost sharks.” Other common names include “ratfish,” “rabbitfish,” “elephant fish,” or “spookfish” and these mysterious animals belong to three families even though they are often lumped into one. The Callorhinchidae family is made up of the ploughnose chimaeras, the Chimaeridae family consists of the shortnose chimaeras, and the Rhinochimaeridae family of the longnose chimaeras.
Worldwide, these animals spend most of their time at depths between 400-2,000 meters deep, with some species coming to shallower depths (as little as 3 meters) to mate and lay eggs. Most chimaeras as listed as “Least Concern” on the IUCN Red List, however, they are all largely understudied. Because of their largely inaccessible habitat, the life history and reproductive biology of chimaeras are poorly known.
“The deep sea is vast and remote, and also a place of extremes – there are near-freezing temperatures, crushing pressures and a complete absence of light. It’s a place we cannot venture easily and this makes it an incredibly difficult place to work,” says Dr. Diva Amon of the Natural History Museum in London, UK.
In the spirit of Halloween, one should know that there are fantastic creatures in mythology and folklore that also go by the name of “chimera,” but spelled differently. Not to mention one is real and one is… well, not. According to Greek mythology, chimeras are a monstrous fire-breathing hybrid creature composed of the parts of many animals. Chimeras are usually depicted as a lion, with the head of a goat arising from its back, and a snake’s head tail.
Deep sea chimaeras are a little bit less odd-looking. It’s not too hard to distinguish between the ghost shark and true sharks. For one, chimaeras lack those rows of razor-sharp teeth like their shark cousins! Unlike many true sharks, they have tooth plates instead of individual teeth; the upper jaw plates are sharp, while lower jaw plates are flat and used to crush the prey held in place by the upper jaw. Another key dentition different is their teeth don’t fall out like a shark’s and instead continue to grow. Speaking of growing, chimaeras don’t get pretty big (with the maximum reported size being 1.25 m/4 ft) and have a sex organ on their head that sharks lack. Yes, you read that correctly: while sharks have two “penises” known scientifically as “claspers”, male chimaeras have retractable sex organs on their heads.
And like their relatives, the elasmobranchs (which include the sharks, stingrays and skates), chimaeras have an ecological importance. “Although they feed on things like small crustaceans, if we didn’t have chimaeras there could be negative impacts on the populations of other species,” said Amber Reichert, a graduate student of the Pacific Shark Research Center.
These animals shot to momentary fame when the Monterey Bay Aquarium Research Institute (MBARI) set out to explore the depths around Hawaii and California, where a small remotely operated vehicle (ROV) captured the attention of a pointy-nosed blue chimaera (Hydrolagus trolli), that seemed to repeatedly investigate the ROV. The media went wild.
“Hydrolagus trolli can be identified from other species based on a short, blunt snout, pale blue-grey body coloration, a large, curved spine that is longer than the first dorsal fin (that fin on its back), and pointed, triangular pectoral fins (the two large fins on the side that help chimaeras swim), and broad pectoral fins (the other paired fins) Additionally, the fins of these specimens all have a pale blue margin on their edge,” said Reichert. “The surveys where Hydrolagus cf. trolli were observed were initially intended to be geological surveys of rock formations in the Monterey Canyon.”
Why the cf? “The cf is latin for ‘compare,’ which is essentially saying, we believe it is this species, of all the ghost shark species we have observed it is closest to Hydrolagus trolli. We can’t yet say with certainty that we know it is the same species until we can get genetic samples and the ability to take more distinguishing measurements.” explained Reichert.
And these ghost sharks may turn into just that – ghosts- in the future. While only a few commercial chimaera fisheries exist, these animals are caught as ‘bycatch,’ the part of a fishery’s catch that is made up of animals that they did not mean to catch. Reichert agrees, adding, ”Even if discarded, they are caught from such depths that there is a low survival rate if any. Deep-sea trawling poses a potential problem to populations if overfished.”
The inky deep holds many mysterious – including many animals, like chimaeras. “Twenty-two of the 52 valid chimaera species [have only been] discovered and named since 2002! My lab, the Pacific Shark Research Center, have been major contributors to discovering and naming new species. Eleven of the 52 valid species were discovered and named by researchers at the PSRC. Of those new species, [half] were named by members in my lab.” says Reichert. Amon agrees that the deep ocean holds many untold secrets: “The importance of the deep ocean is not to be underestimated. It harbors the majority of the world’s unknown biodiversity and it provides crucial ecosystem services that keep our planet healthy and keep up alive. And more and more, our deep sea is providing us with much-needed resources such as food, energy, minerals, pharmaceuticals and more.”
So this Halloween, think about the oddities that call the deep sea home… the glowing eyes of a chimaera, waiting to be discovered.