While I'm on the topic of photographing fishes with their mouths open, here is an image of another common species (Ecsenius lineatus) with its mouth open.
As with the Lubricogobius exiguus goby, I had just set everything up when the blenny struck this pose for a fraction of a second.
I was there, ready, immersed in the flow of the blenny's life. The fish was active, darting out, coming back in, grabbing food, looking in every direction. It moved in and out of view. There came the moment when I pressed the shutter. There was no way I could have seen the fish open its mouth, received the message from my eye to my brain, processed it, then sent a signal from my brain to my finger and pressed the shutter. There simply wasn't sufficient time.
I do not, in fact, recall seeing the fish open its mouth.
Rather, I somehow pressed the shutter at the same time the fish decided to open its mouth.
I know. Sounds a bit woo-woo (wu-wu?), doesn't it?
I'm just describing what I experience and know to be the case.
One could argue, "You just got lucky!"
Sure, but I'm not just talking about once. Or twice. Or three times. But repeatedly over decades, for everything from tiny fishes like this to whales and even terrestrial subjects. Autopilot, being in the flow. I cannot prove it to you scientifically. But so what?
Finally, I must've seen thousands of these fish over the decades, but it's the first time I've ever noticed the orange colour of the bottom jaw.