The drawn-back mouth full of white teeth

Hi

Could you please explain the red part to me? Does it refer to the shark that was splitting it?

Both arms stopped flailing the water and the head went under and came up again. A cloud of blood welled up and darkened the sea. Two six-foot thin brown shadows backed out of the cloud and then dashed back into it. The body in the water jerked sideways. Half of The Big Man's left arm came out of the water. It had no hand, no wrist, no wrist watch.

But the great turnip head, the drawn-back mouth full of white teeth almost splitting it in half, was still alive. And now it was screaming, a long gurgling scream that only broke each time a barracuda hit into the dangling body.

Thanks,

Tom