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?

Solution 1:

The idiomatic way of saying this is

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

To gulp: (from TFD)

to gasp, as if taking large drafts of a liquid.

Fish gulping for air:

After feeding time all the other fish are on the surface gulping or gasping. Sometimes this happens later in the day as well.

( from www.fishchannel.com)