Say hello to the Greenland shark (Somniosus microcephalus), also known as the gurry shark, grey shark, sleeper shark, or by the Inuit name Eqalussuaq. It's the second largest carnivorous shark after the great white, but luckily for us it lives in deep Arctic waters where it rarely encounters people.

The Greenland shark has a sluggish look, with a thickset, cylindrical body and a small head with a short snout and tiny eyes. They're one of the more unusual sharks out there, in appearance and behavior.

Read on to learn what sets this slow-moving giant apart from other sharks.

1) They're rarely observed and somewhat mysterious. The first underwater photos of a live Greenland shark were taken in the Arctic in 1995, and the first video images of a Greenland shark swimming freely in its natural environment were not obtained until 2003.

2) They like it cold. Greenland sharks are native to the North Atlantic waters around Greenland, Canada, and Iceland. They are the only true sub-Arctic shark and the only shark that can tolerate Arctic temperatures year round. They prefer very cold water (-1°C to 10°C). In the summer, they tend to stay in the ocean's depths where the water is coldest. In the winter, they make a vertical migration to the surface layer, which at that time is colder than the water on the sea floor.

3) These sharks can go deep. Greenland sharks have been observed at the water's surface all the way down to 2,200 meters deep. They are sometimes seen resting on continental shelves and slopes at these great depths.

4) They rival great white sharks in terms of size. Greenland sharks are one of the larger sharks out there, capable of growing to more than 20 feet long and weighing up to 2,500 lbs.

5) Eyeball parasites are common. Some populations of Greenland sharks are commonly parasitized by the copepod Ommatokoita elongata. This parasite latches on to the shark's eye and destroys the corneal tissue, rendering the shark partially blind. Luckily for the shark, light rarely penetrates the deep waters it prefers and so it relies on other sensory systems to get around and find prey. While over 90% of Arctic Greenland sharks sport this parasite, less than 10% of the Greenland sharks observed in the St. Lawrence are hosts.

In the last couple minutes of the last dive of the field season we found the largest fish we have ever encountered with the ROV, a Greenland Shark. Photo: NOAA Okeanos Explorer Program [Public domain], via Wikimedia Commons

6) They're not picky eaters. The Greenland shark is an opportunistic predator that will eat just about any meat it finds, either dead or alive. They eat a lot of fish and there is some evidence that they ambush sleeping seals, but some scientists believe they are primarily scavengers. Some strange things have been found in Greenland sharks' stomachs, including the remains of polar bears, horses, moose, and in one case an entire reindeer. A few months back, some passersby saved a Greenland shark that appeared to be choking on a chunk of moose.

7) Their teeth are designed to cut out plugs of flesh. The teeth on the upper jaw are narrow, pointed, and smooth, and anchor the food item as the lower jaw does the cutting. The teeth on the lower jaw are larger and broader and curve sideways. By swinging its head in a circular motion, the shark can cut out a round plug of whatever it's feeding on.

8) Greenland sharks go for a 'slow and steady' strategy. Also known as the sleeper shark for its sluggish pace, the Greenland shark is one of the slowest swimming sharks in the world. They average a cruising speed of 0.3 m/s (0.76 mph), but are capable of short bursts of speed.

9) They might be very long-lived. Some estimates put the life span of Greenland sharks at over 200 years. Where does this number come from? One study found the sharks grow about 0.5 to 1 cm a year. A Greenland shark that was captured and tagged off Greenland in 1936 was recaptured 16 years later. In that time, it had grown 6 cm longer. If we assume a constant growth rate, that would make a mature 7 meter shark over 200 years old. If that's correct, the Greenland shark may be one of the longest-lived vertebrates on the planet.

10) Their flesh is poisonous. Greenland sharks contain high levels of trimethylamine oxide (TMAO; it helps regulate their osmotic pressure and also acts as a natural antifreeze). During digestion, TMAO breaks down into trimethylamine (TMA). TMA causes intestinal distress and neurological effects similar to extreme drunkenness. Eating too much can even lead to convulsions and death. Early settlers of Iceland and Greenland figured out a way around this. Greenland shark flesh can be eaten with no ill effects if it is prepared correctly: the meat must be buried in the ground for 6 to 12 weeks, exposing it to several cycles of freezing and thawing. After this the meat is hung up to dry for several months, and finally cut into bite-sized cubes. The end product, Hákarl, is a delicacy.

References and Other Resources:

Borucinska, J. D., Benz G.W. and Whiteley, H.E. (1988). 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 21: 415-422. doi: 10.1046/j.1365-2761.1998.00122.x.

Eagle, Dane. "Greenland shark". Florida Museum of Natural History. Accessed 24 January 2014.

Greenland Shark and Elasmobranch Research and Education Group. Accessed 24 January 2014.

"Greenland Shark (Somniosus microcephalus)." Arkive. Accessed 24 January 2014.

MacNeil, M. A., McMeans, B. C., Hussey, N. E., Vecsei, P., Svavarsson, J., Kovacs, K. M., Lydersen, C., Treble, M. A. et al. (2012). Biology of the Greenland shark Somniosus microcephalus. Journal of Fish Biology 80(5): 991–1018. doi:10.1111/j.1095-8649.2012.03257.x.

Mills, Patrick (2006). "Somniosus microcephalus." Animal Diversity Web. University of Michigan.

Watanabe, Y. Y., Lydersen, C., Fisk, A. T., and Kovacs, K. M. (2012). The slowest fish: Swim speed and tail-beat frequency of Greenland sharks. Journal of Experimental Marine Biology and Ecology 426–427: 5-11. doi: 10.1016/j.jembe.2012.04.021.

Yano, K., Stevens, J. D., and Compagno, L. J. (2007). Distribution, reproduction and feeding of the Greenland shark Somniosus (Somniosus) microcephalus, with notes on two other sleeper sharks, Somniosus (Somniosus) paciﬁcus and Somniosus (Somniosus) antarcticus. Journal of Fish Biology 70: 374-390. doi: 10.1111/j.1095-8649.2007.01308.x.