They are yumas, there are a lot on the rock, some are very large at probably 5 inches diameter. They are all over it, some small some large, I haven't counted but I would say there is over 20. Jarod, if you plan on this going in your nano than this might be too large unless it is the only rock/coral you want in it, they would quickly cover anything else if you put it by them. You are currently second in line for them, so if first person falls through than I will let you know.