animal

[chappazhead]

is a shark a mammal or a fish??

[Dr. Bigalow]

It's a fish. It has gills not lungs!

[chicklin]

Yep - it's a fish.

[CrazyWolfman]

If you want to be a little bit more precise they're cartilaginous fish. Most fish are boney fish (and funnily enough have bones), whereas the cartilaginous ones only have (equally unsurprisingly) cartilage. They also don't have an operculum which means they are incabable of 'pumping' water cointinuously through their gills, thus they rely on their movement through the water to keep oxygen getting into their system. For this reason if they stop at all they die of asphyxiation, hence why sharks keep dying when caught in fishing nets. Other cartilaginous fish include rays and skates. Finally another point of interest (if you're into this kind of thing) is that cartilaginous fish do not have a swim bladder, so they have to keep swimming and use their fins to prevent them from either sinking or floating to the surface.

[heathfield]

The idea that sharks have to keep moving to breathe is a bit of a myth. It's only true of some species, not others. I've personally watched a sleeping nurse shark using muscle movement quite happily to breathe. Learn more here