Imagine yourself cruising down the road at night. There's no moon so, so you're driving slowly, peering into the blackness to find your way. As your eyes adjust you see a gigantic, ancient woman sidle into the glare of your headlamps. She's slow and battered, and moves almost as if she's in a trance. You slow down even more to avoid hitting her, but you can't bring yourself to get out of the vehicle. As she meanders to the edge of your lights' beam she turns slightly, and you catch a glance at her eyes. A tubular creature dangles from two filaments embedded in her cornea. Though she's surely been blinded by the parasite you sense that she can feel you there. As she's enfolded back into the darkness you wonder was she an ancient spirit? An aesthetic hiding away from society? A witch? or even a zombie?
She was a shark.
Courtesy: NOAA Photo Library via Flickr
In our imaginary scenario you're a researcher with the US' National Oceanic and Atmospheric Administration. You're cruising along in a submersible when a species of sleeper shark swims in front of your vehicle.
There are only around six species of sleeper sharks that we know of, and two of them fit the bill for this story; the Pacific sleeper shark (Somniosus pacificus) and the Greenland shark (Somniosus microcephalus). Although their cousin the southern sleeper (Somniosus antarcticus), which lives around Antarctica and is so badass it eats juvenile colossal squid, comes close it doesn't fit our story because it isn't affected by the dangling parasite; which we'll talk about later.
Pacific sleeper and Greenland sharks are truly incredible. They're massive. Adults of both species average around 4 meters; that's about 14ft, but they can grow even larger. That size is on par with all the largest predatory sharks we're more familiar with.
I'm gonna go ahead and guess this kid is not
the one who reeled in this shark
Courtesy: Wikimedia Commons
Unlike great whites or tiger sharks, which are fast, active predators, sleeper sharks are sluggish. It's believed that they hunt by sneaking up on live prey. They move so slowly, about the same speed as a crawling baby, and are so hydrodynamic that they barely disturb the water. So when prey are distracted or at rest they don't notice these incredible predators. Not only can sleeper sharks hunt, but they're also effective scavengers. Researchers have found meat in sleeper shark stomachs that's crawling with the organisms that usually consume dead flesh on the ocean bottom. We also know from stomach contents that sleeper sharks will consume seals, dead whales, fish, and even polar bears! It's not clear if the polar bear was an individual that drowned while crossing sea ice or if it was taken live, but either way....wow. Even crazier, a couple of guys in Newfoundland, Canada found a stranded Greenland shark that had 2 feet of moose tissue stuffed down its gullet!
Little Known Fact: Hastily gobbled moose is
the national dish of Canada
Courtesy: Christian Heilmann via Flickr
Now this slow lifestyle has benefits besides giving sleeper sharks the ability to hunt or scavenge literally everything. Sleepers are found in cold waters, either at the poles or deep in the ocean. In fact, Greenland sharks are the only shark species known to live under the arctic ice cap. In frigid waters it's easier to survive with a slow metabolism because with a fast one you're constantly compensating for lost heat. Plus a slow metabolism means you can live for a really... really... really... really long time.
A study published in Science used radiocarbon dating of the core of Greenland sharks' eyes to predict the age of a variety of individuals. This technique suggests that really old Greenland sharks may be 272-512 years old! If that's the case they're the longest lived vertebrate on earth, and there are sharks alive today older than the United States.
"Suck it Methuselah!"
Courtesy Public Domain via Encyclopedia of Life
The downside to growing to that great age is that sleeper sharks probably don't reach sexual maturity very quickly. It's been estimated from the development of ovaries and testes in dead sleepers that they can't reproduce until they're 10 or more feet long. Greenland sharks are thought to grow so slowly that they might need 150 years before they can get to baby making. If that's the case then these fish may be very sensitive to overfishing. Sleepers are commonly caught as bycatch in fisheries for other species, so it's very important to use programs like Seafood Watch to ensure your fish comes from sustainable sources.
In Iceland there is a small targeted fishery for Greenland sharks because they're the main ingredient in one of Iceland's traditional dishes. Hakarl is Greenland shark that's been fermented for several weeks. It has to be fermented because sleeper shark meat contains a lot of Trimethylamine-oxide; which is toxic. It's so poisonous that sled dogs, ravens, and a sea birds called fulmars have been described as getting "shark drunk" after eating too much raw shark. The symptoms include stumbling, respiratory depression, erratic behavior, and vomiting.
Pictured: Shark Drunk
Couretsy: Schroder + Schombs PR via Flickr
The Trimmethylamine-oxide probably helps protect sleeper sharks from the challenges of living in the deep and polar ocean. The chemical counteracts the protein dismantling effects of high pressure at depth, and works like anti-freeze to prevent the sharks body from locking up.
Okay so all of these adaptations make sleeper sharks a pretty good match for the woman in the creepy story from the introduction, but what was that bit about the parasites on her eyes? Well I'm both glad you asked and horrified that you reminded me.
Pacific sleeper and Greenland sharks frequently host the parasitic copepod (pronounced: co-puh-pod) Ommatokoita elongata. One study found that up 85% of Greenland sharks have these creepy-crawlies living on and in their eyeballs. The copepod, which is a crustacean related to shrimps and crabs, inserts a stud called a bulla into the shark's eye and just kind of dangles from it. As it swings there for its entire life the copepod feeds on the juices from the shark's eyeball. It also scratches the ever living heck out of the cornea as it moves around. It's no surprise that these copepods commonly blind their hosts. However, sleepers don't seem to be affected by blindness at all. It's believed that sleepers rely on their other senses so much that they don't actually need their eyes.
I propose the common name of the "Why-God-why-does-this-exist!? copepod"
Courtesy: Johnathan Wojcick via bogleech.com
While sleeper sharks are certainly bizarre, they're also a magnificent example of adaptation to challenging conditions. They're probably the longest-lived of all vertebrates, they know how to take their time and move deliberately, they enjoy a place of respect in the food web, and they survive adverse conditions every day. Not only that, but if you're a little loose with your Latin translation, Somniosus microcephalus means "sleepy, little face". That's pretty adorable for an animal most might not call a sleeping beauty.
Borucinska, J.D., Benz, G.W., & Whiteley H.E., "Ocular Lesions Associated with Attachment of the Parasitic Copepod Ommatokoita elongata (Grant) to Corneas of Greenland Sharks, Somniosus microcephalus (Bloch & Schneider), Journal of Fish Diseases, 1998, 21, pg 415-422
Courtney, D.L., & Foy, R, "Pacific Sleeper Shark Somniosus pacificus in the Eastern North Pacific Ocean Inferred from Nitrogen and Carbon Stable-isotope Ratios and Diet", Journal of Fish Biology, 2012, 80, pg 1508-1545
Hulbert, L.B., Sigler M.F., & Huntsford C.R., "Depth and Movement Behavior of the Pacific Sleeper Shark in the North-east Pacfic", Journal of Fish Biology, 2002, 69, pg 406-425
MacNeil, et Al., "Biology of the Greenland Shark Somniosus microcephalus", Journal of Fish Biology, 2012, 80, 991-1018
Nielsen, et al. "Eye Lens Radiocarbon Reveals Centuries of Longevity in the Greenland Shark Somniosus microcephalus" Science, 2016, 353, pg 702-704
No Author, "Moose-eating Shark Rescued in Newfoundland Harbour"(sic), CBC News, Nov 21 2013, Accessed via: http://www.cbc.ca/news/canada/newfoundland-labrador/moose-eating-shark-rescued-in-newfoundland-harbour-1.2434102