Patiriella exigua (Lamarck) fed mostly on apparently bare rock surfaces, though they were frequently found with the gut everted on macroalgae, detritus and gastropod shells. When caged alone in experimental pools, Patiriella inhibited development of algal cover. Nevertheless, the amount of algae on the rock surface in pools containing the starfish was greater than in uncaged areas. Minimal growth of Patiriella was observed in pools with the least algal cover. Above a threshold, the amount of algae on the rock pools was irrelevant to the starfish. No difference in growth rate was found in starfish kept in pools with different cover of algae when the amount of algae on the rock was > 1-1.5 mg.cm-2. When Patiriella were offered feeding surfaces with different cover of algae, their selection was not influenced by the amount of algae. The limpet Cellana tramoserica had a negative effect on the growth rate of Patiriella. The starfish gained weight more slowly when caged with Cellana than when caged alone, but no effects of increased densities of Cellana were recorded. A weak negative correlation between natural densities of Cellana and Patiriella was found in the field. Manipulations of density of grazing molluscs (elimination, addition) induced an increase in the density of Patiriella in experimental quadrats. No adequate explanation was found for this. Facilitation, i.e., provision of suitable feeding surfaces for Patiriella by grazing molluscs, has previously been proposed. This is unlikely to occur because Patiriella can successfully exploit substrata with a wide range of cover of algae. There is no evidence that competition occurs in the field. It appears that Patiriella could shift to alternative surfaces beyond the reach of potential competitors when grazing pressure was too great on apparently bare rock surfaces.
