Experimental investigation of interactions between the perennial red alga Gelidium pusillum and barnacles on a New South Wales rocky shore

On rocky shores in New South Wales, the perennial red alga Gelidiurn pusillurn (Stackhouse) Le Jolis occurs only within a midshore barnacle zone. This alga was rarely found to recruit from propagules but instead persisted as patches within the zone. Although barnacles did not influence the growth rate of G. pusillurn they were crucial to the survival of the patches. G. pusillurn that had overgrown the barnacle Tesseropora rosea Krauss stabilized the dead tests, thus postponing the reallocation of bare space on the shore otherwise available for settlement by other organisms. Survival of juvenile herbivorous molluscs sheltering in tests was also affected by the abundance of an algal covering over the barnacle shells. Results suggest that many interactions are taking place and that some of these can have opposite effects depending upon the abundance of component species.