Exhaust Vents? (POSSIBLE SPOILER)

Well we will see what they are in the movie :) or we will know if we get a confirmation on what they are from gareth or an official source 

I actually find this more believable than gills. As most amphibious creatures dont have them. More like a nitrous purge in cars.

Have to say never would have thought of that, good theroy. That being said the vents or the gills or what ever the hell they are might also work both ways letting heat escape saftly and let water in to prevent godzilla from over heating.

Yeah, I sorta came up with this a long time ago, along with my other part of that theory, overall stating (and remember this is all MY own SPECULATION):

Those apparatuses that appear to be gills are indeed gills. However, they don't actually function as gills do, normally. Or, at least, not anymore. As the creatures grew larger, their reliance on oxygen as an essential component diminished, thus leaving the gills as an initially vestigial feature.

However, once new bodily concerns arose, the gills found new usage. The creatures' internal heat and molecular integrity due to their nuclear biology meant additional air and water needed to be brought into their systems regularly, which the gills aided in, thus eliminating the need to evolve new features for such a task.

As well, the gills served as an effective ventialtion system by which vital gases could be taken in to ionize for intense plasma discharge from the creatures' mouth, as well as a cooling system that prevents overheating of the region whilst this curious natural weapon is in effect.