Paramecium Busaria find the light

While observing P. Busaria through a newly acquired--and badly adjusted--phase microscope, I discovered an interesting phenomenon. When I moved to look at a new area of the slide, over time a large number of Busaria would slowly gather together and fill the illuminated(!) field of view. I did this several times with the same result. I'm sure there is a biochemical, stimulus-response explanation for this behavior, but I find the appearance of cooperation at this micro-level between these two quite distinct kinds of microbes really amazing!