Word for "opening and closing the mouth?"

Example:

The stranded fish were flapping desperately on the sand, their mouths closing and opening, seeking for the water that wasn't there.

Is there's a word that means opening and closing the mouth? I thought of gaping but I think it only means keeping the mouth open?

Answer

The idiomatic way of saying this is

The stranded fish were flapping desperately on the sand, gasping for breath.