Consequences of aggregated living in the ascidian Pyura stolonifera: Evidence for non-contact intraspecific competition

Non-contact intraspecific competition was examined as a possible consequence of the aggregated distribution of the ascidian, Pyura stolonifera near Melbourne, Australia. In a field experiment designed so that experimental individuals did not touch surrounding conspecific individuals, ascidians inside aggregations grew significantly more slowly than those outside aggregations: at the end of the 24-month experiment, the former animals had shorter body lengths, lighter bodies, lighter tunics, and lighter gonads. This is one of few conclusive demonstrations of the existence of this type of intraspecific competition in sessile marine invertebrates occupying hard substrata, and the only demonstration for ascidians.