France, Feb. 14 -- Some of the Greenland sharks swimming today were alive during the French Revolution, a few may even date back to the time of Shakespeare. Yet despite their age, scientists still know remarkably little about them. Where they reproduce, for example, remains a mystery. So does their number, and how their populations are changing.

For decades, these sharks were believed to be almost blind. Their eyes, often cloudy and covered in parasites, helped reinforce that view. Along with their slow movements and life far below the surface, this fed the idea of a sluggish scavenger drifting through the darkness.

That assumption has now been turned on its head. In research published in January,scientists examining Greenland sharks es...