I have just noticed a small colony of about a dozen or so sea-squirts - probably Ciona - growing on a small chalk rock. As these creatures grow from a free-swimming larvae it appears that recent fresh sea-water added to the tank had some larvae from this species which has subsequently started to grow.
Its odd though, that they all decided to grow on the same rock. This rock has been in the tank about 6 months and originally had a single small mussel on it which was prey to a dog whelk. It was found above the shoreline after a storm and had probably been in the air for about a day so its highly unlikely that a sea squirt could have stayed alive on it. According to most information available on sea squirts they dont survive being removed from the water for very long at all.
Perhaps there is some kind of mechanism that enables larvae to colonise a place together via a chemical signal? Bearing in mind that I employ strong circulation, any signal would have to be 'sticky'. However, the chosen rock is in an area of fairly calm water. Maybe thats all thats required - a suitable area will be colonised and thats it.