So, I see a lot of people in the tags wondering why this shark is biting the submarine cable, so allow me to provide a brief overview of shark anatomy, behavior, kinematics, and fiber optic technology.
What’s that smell?
Sharks, as predators, have a few tools up their fins for finding prey in the vast oceans. With a lot of ground to cover, and poor visibility, many species have developed an extremely acute sense of smell; allowing them to pick up on scents drifting in the currents from miles away, such as whale oil. For pelagic sharks, this is vital.
Electroreception
Sharks also possess electro-receptive nerves sitting in pores on their noses, know as ampullae of Lorenzini, that are capable of detecting electric fields measuring in the nanovolts. Multicellular organisms give off faint electric fields, so if your prey hides in the sand or simply swims around in total darkness, being able to detect them from their electric field is an incredible advantage. The more ampullae you have, the more sensitive you become. This is one of the reasons for the development of the cephalofoil on hammerhead and bonnethead species, as well as the long nose on the infamous goblin shark.
o>o what’s this?
Sharks can be curious creatures, and often test the waters by nibbling. A young silky shark might nip a sea turtle’s flipper to see if it’s something tasty and food-like, and then decide it’s not, without seriously harming or killing the turtle, for instance. A blue shark may chomp a surf board because it looked like food, and realized it’s not, and move on. Perhaps a running camera maybe even pique a shark’s interest and try nibbling it. Just as a dog or cat might sniff something new because they’re curious, sharks bite because they’re curious.
Bite Kinematics
It was once believed in the past that sharks had bone-crushing bite-force, until a diver and citizen-scientist, Valarie Taylor, tested out a shark suit design she developed with her husband, Ron Taylor, in order to help prove to the public that sharks were not a threat of humans after the release of the 1975 film, Jaws (which she and her husband helped film, since they were pioneer shark handlers). She dove with blue sharks, and had to try really hard to goad one into biting her forearm, which was covered in chain mail. Despite the hypothesis of some biologists at the time, her bones did not break, and there was no imprint of the mail on her skin. This is because sharks like Prionance glauca rely on thrashing to tear away bites of meat, like a reciprocating saw. Gray and white tip reef sharks, bull sharks, tigers, great whites, salmon sharks, silky sharks, oceanic white-tips, and so on, do not rely on bite force to get a kill or a bite to eat. That is why chain mail shark suits work: they prevent cuts (anyone who’s worked at a deli and has worn chain gloves can attest).
Sharks like wobbegongs, goblins sharks, and sand tigers actually create vacuums to suck prey in, and their teeth keep them from escaping (for the sand tiger and goblin). Sharks like the port jackson feed on shellfish, so their teeth are adapted for breaking shells.
Fiber Optic Cables
Fiber optic cables are made of two cores of glass, one inside the other, about 62um thick depending on whether it is single-mode or multi-mode. They are tiny, but very effective. The light signal travels through the inner-core at a specific angle so that it frequently runs into the outer core–whose refractive index is different than that of the inner-core–and is refracted, or bent, back into the inner core. And repeat. The other layers provide light insulation and protection from the elements and tugging forces. Submarine cable may only carry few fibers, so why are they so thick? Well, when your destination is thousands of kilometers away, the light signal will eventually fade out. So, optical amplifiers must be placed along the way. These amplifiers require electricity to run, so copper cables are run with the fiber cables. They are bound together with more insulation and protection, and finally, kevlar. Now you have a complete submarine cable!
The Bite of ‘09
In the GIF that I created, we can see a bluntnose sixgill shark, Hexanchus griseus, drifting toward a submarine cable. (source: https://youtu.be/1ex7uTQf4bQ ) They are benthic predators and scavengers, growing up to 14 feet long, and inhabiting great depths up to 8,200 feet. Now, as I mentioned previously, sharks can detect electric fields, and submarine cables can give off quite an electric field because of those optical amplifies and their power cables. The male sixgill we can see here is curious about what’s giving off the field. And like most curious sharks, he decides to bite to see what it’s all about. The GIF does not show the aftermath, which is why I’ve linked the video and made a new gif, below (not sped up). After the sixgill determined that this cable is not food, he removes his jaws from the cable and moves on. He did not thrash. That outer layer of kevlar on the cable protected it from damage adequately, as intended, and all is well.
I hope this explanation was satisfying, and I hope you were able to learn something! If you are curious abotu sixgill sharks, here’s a documentary on them: https://www.youtube.com/watch?v=eQwThFPjdGM , and here’s a website dedicated to the Puget Sound sixgill population: https://www.sixgillsharks.org/
