The Greenland shark belongs to the same family as dogfish, and can be found in the deepest parts of the Atlantic. Reaching lengths of 6.5m it is also one of the largest predatory shark species in the world, as well as one of the least known about. It is in fact a lot different from most sharks that people might have seen however as living at depths of around 550m has meant it has had to adapt to its environment.
It has even been alleged that the Greenland shark might be able to reach lengths greater than that of the great white, given sufficient time. Because as with many fish it grows continuously throughout its life, the sizes that it can potentially reach are gigantic. This being because the species of marine life that live in the deeper parts generally have a much longer lifespan than those who live nearer the surface.
One of the most apparent changes that the Greenland shark has had to make to be able to live at great depths is that its teeth are different from most other shark species. Its upper teeth are long and pointed; while it's lower teeth are strongly oblique, sharp and close set. This is almost certainly in order to allow the shark to gouge large chunks of flesh from dead animals and whales.
Like all other sleeper sharks, Greenland sharks have small, rounded snouts, lack an anal fin and have small, spineless dorsal fins. The reason that their fins are fairly small is thought to be because they don't need to move fast in the water, and in fact stay still for long periods. Having small fins also means that they require less energy to move and don't get so cold in freezing water.
Being one of the largest predators in the deeper waters of the north Atlantic means that the Greenland shark has few natural enemies. The only species that could potentially harm them would be sperm whales and killer whales. Killer whales generally tend to stay in shallower water, and so probably wouldn't encounter them very often, and because of the Greenland's size it would make it generally unsuitable as prey. The sperm whale could easily eat a Greenland shark however, and because of their tendency to inhabit the same waters, this is thought to happen to some degree.
Despite being generally suited to scavenging, the Greenland shark is also an adept hunter in its own right. Although it is generally a shark that doesn't like to expend much energy, it is known to kill and eat large fish such as salmon, as well as marine mammals such as seals and sea lions. To hunt these it is thought to use the bioluminescent copepods that attach themselves to its eyes. These tiny shrimp like creatures produce a bright glowing light that in the depths attracts other fish towards them, where the shark can easily catch and eat them.
Other than the lures that the Greenland shark has on its eyes, it is probably not fast or active enough to hunt other large species. Things such as seals or other sharks are generally a lot faster, and even smaller fish can often out swim it. Because of this it often eats smaller crustaceans from the sea floor, and tends to seek out the kills and left overs from other large predators.
Because there is often very little prey in the deepest oceans, compared to the shallower water, Greenland sharks have adapted to use less energy than most sharks. Because they are a species of sleeper shark, they don't have to keep moving constantly in order to breathe like most shark species. Also generally being able to stay still for much of the time allows them to expend much less energy, which is a great benefit to any deep dwelling species. This being because with less prey available generally, they are unable to feed very regularly.