Subject: Why do lumpfish vibrate?

I recently visited an aquarium with a lumpfish exhibit. I noticed one lumpfish 
in particular that was constantly swimming into or trying to push against 
another lumpfish sucking to the tank's glass. Every now and then the lumpfish 
that was swimming against the other would momentarily but violently vibrate and 
shake. Why is this? I tried searching everything I could about lumpfish and 
came up with only a story about a diver who raised one up on his palm to show 
his partner before it started vibrating.

Re: Why do lumpfish vibrate?