Because a nurse shark would be toast. And so would a sun-bear. Unless it was in with the nurse shark, in which case I think they'd ignore each other. Oh, and the size of the tank, because if it were polar bear vs. tiger shark, if the water were too shallow or the tank too narrow, the shark would be unable to keep swimming and breathing and the bear would be having shark cutlets.

... And now I'm having the unpleasant thought of what shark cutlets might taste like. Not that I think they'd be disgusting, but because a lot of sharks are protected species, which makes thinking of them as food kind of disgusting for me.

a number of factors, I put it forth the shark would win. I, of course, am biased towards the shark, because they pop up in movies too much, and the local aquarium has some pretty damn cool sharks.

Of course, if they were using some bear other than a polar bear, I'd put my money on the bear. All the polar bears I've seen are lazy, even during the winter when it's a heckuva lot colder.

In conclusion: Shark wins. :D

I think the bear would win. As pointed out, polar bears are very large and very strong, and can swim very well. A shark's only attack is to ram into a thing and bite it; a bear's hit with its front paws, with claws, could do serious damage to any animal, even a shark, without putting its head right in its opponent's line of fire.

Besides, polar bears eat fish.

...that the polar bear will win.

I don't think the bear needs to rear up on its hind legs to attack though: I think it can just take a swipe at the shark from the all-fours position.

An adult polar bear can take on a walrus and win if it really wants to. They are ridiculously strong, very persistent, and are able to swim.

A shark is essentially a big fish with teeth.

Does the shark have a frickin' laser beam attached to its head?

I think that the shark would win. Bears normal position are on all fours, so it would have to lean against the wall to stay up on it's hind legs. Plus, whenever it lashes out at the shark, it would fall down to it's normal position, and would waste time getting back up and leaning up against the wall. During that time, the shark would charge at it. The shark would be faster, and the bear's strikes would be slower in water.

I think the real question is how did they get into the same tank to begin with?

Also: http://www.youtube.com/watch?v=Y-iPCTu61Z4