Smoking Crab

We saw a strange thing today on a dive at St Abbs - what looked like a "smoking" edible crab. The first picture below is an edible crab taken this afternoon; the second picture is of another crab taken a bit later on the dive. You might notice that there is what looks like a puff of smoke coming from around its mouth. I have no idea what it was doing but it seemed to be blowing something out! Weird.