When salt and other solutes enter a fish’s tissues, they force water out of the body. Since salt levels in marine fishes are lower than those of the surrounding environment, they must continually take in water and excrete salt through their gills. Salt levels in sharks are also lower than that of sea water but sharks manage osmosis differently. In order to maintain a stable amount of water in its body, the Greenland shark will retain a high concentration of urea in its blood, thus compensating for lower salt concentrations. However, because high levels of toxic urea will damage its body by destabilizing protein, the Greenland shark also retains even higher levels of trimethylamine oxide to counter the damaging effects of the urea. When the trimethylamine oxide and urea are combined with the salt in the shark's tissues, the osmotic pressure of the shark's body fluids is higher than that of the surrounding water. In other words, the shark is ‘saltier’ than seawater. Unlike bony fishes that must constantly take in water to replace water lost through osmosis, the Greenland shark does not need to expend energy to maintain life-sustaining water levels in its body.
In addition to contributing to the shark’s osmotic pressure, trimethylamine oxide and high levels of urea also serve as a natural antifreeze by stabilizing the enzymes and proteins in the Greenland shark’s tissues. When the shark encounters extremely deep and cold conditions, this prevents the formation of ice crystals that disrupt cell walls and cause the leakage of cellular contents, which results in tissue and organ damage, then death.
When Greenland shark flesh is consumed, the digestive process turns trimethylamine oxide (TMAO) into trimethylamine (TMA), a substance that smells like ammonia and rotting fish. In addition to causing intestinal discomfort, trimethylamine also has adverse neurological effects akin to consuming excessive quantities of alcohol. Death may ensue in extreme cases when too much shark flesh has been consumed. Greenland shark flesh is nonetheless considered a delicacy in Iceland. See ‘Fisheries’ section below.
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In Iceland, putrefied shark meat is processed for human consumption by compressing it in a large perforated container* to remove the toxic fluids, and then it is hung to dry outdoors for 2 to 4 months. It is then cut into bite-sized cubes, and served as an hors-d'oeuvre called hakárl or kæstur hákarl. Most of the hakárl produced in Iceland originates from the Bjarnahöfn Shark Museum, which uses by-catch sharks from Greenland.
* The meat is no longer buried in gravel for 6 to 12 weeks.
http://www.geerg.ca/gshark_1.html
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