Spatial competition between colonial ascidians: the importance of stand-off

The outcome of 925 encounters between colon~es of 4 species of compound ascidians growing on perspex panels were recorded. Of encounters involving Diplosoma hsterjanurn, 61 % resulted in stand-off, and almost all the rest in overgrowth by D. listerianum. Of encounters involving any two of Trididemnurn tenerurn, Botryllus schlosseri and BoQlloides leachii, 82 % resulted in standoff, and the rest in overgrowth by either one of the adjacent pair. These 3 species were competitively equivalent, but subordinate to D. Listerianum. The very high proportion of stand-offs recorded between these ascidians contrasts sharply with the low proportions previously reported for other taxa.