You can sort-of just barely make out their outlines by slightly faded patches moving around on the ground. When the come to a halt, they open up about a half second later.
That said, I don't know if there's any real way to handle them as melee classes. The best you can do is try to strafe while walking backwards and then quickly attack when one opens up, but they don't stick around long enough to fight back, really. Plus if they come to a halt slightly behind you, you'll wind up taking a hit anyway.
All I can really say is this boss isn't very fair, the leading cause of which being that the mouths don't stay on the surface long enough to actually fight back against.