Canadian fish scientists are opening a window into the mysterious world of the Greenland shark -- the top predator in the Canadian Arctic about which almost nothing is known.

Except this, says Steve Campana of the Bedford Institute of Oceanography: "These are very, very strange sharks."

Its meat is poison. Its mouth is far under its body. It has almost no spine. It's so lethargic that it doesn't even snap at the scientists who hook it and attach a radio to it.

And it may live 200 years.

Mr. Campana and Aaron Fisk of the University of Windsor took their team to the sea ice 300 kilometres north of Iqaluit, camping out in a frigid plywood shed in April to tag and release Greenland sharks.

Only one other big shark in the world is almost unknown -- the extremely rare deep-ocean "megamouth."

Why study the Greenland shark?

In the eastern Arctic "this is the apex (top) predator, the king of the food web, along with the polar bear. There's a sister species in the western Arctic. And as with any ecosystem, if you don't know anything about the apex predator, you're in a lot of trouble figuring out what's going on."

Everything about this fish is odd, Mr. Campana says.

"They are really the antithesis to the fast-swimming great white and mako (sharks)."

The cold water might make them slow, but even in warmer water they just cruise along the bottom, slurping up fish, and occasionally seals. The seals may be dead when the sharks eat them. No one really knows.

Researchers are hoping that samples of bone may hint at a fish's age; the team will look for radioactive elements released during atmospheric tests of nuclear weapons, to show which fish were alive in the 1960s.

The Greenland shark can grow to eight metres and has hundreds of sharp teeth.

"Just running your hand lightly along them you can slice yourself wide open," says Mr. Campana.

"When we found a dead shark we would open up its stomach. Every single one was jam-packed with food. A lot of it was large fish," but there were some baby seals.

It's possible the shark scavenged dead seals, but seals are also known to be curious, and some young ones may have wanted too close a look at the shark.

The sharks are incredibly abundant, says Mr. Campana, "and yet we don't have a clue how fast they grow, how old they get, where they give birth, how many they give birth to..."

The team is using radio tags, which don't hurt the sharks, to record information about their living conditions (water temperature and light) and location. The tags are programmed to release months later and then "pop up" to the surface and radio their findings to a satellite.

Inuit fisherman often catch them by accident, hooking a turbot that a shark then bites on the hook.

The meat is poison if cooked like normal fish, so full of urea that it takes boiling and re-boiling to make it safe.