Factors maintaining topography-related mosaic of barnacle and mussel on a rocky shore

Abstract

Substratum heterogeneity on rocky shores can affect the distribution pattern of sessile epibenthic organisms. The rocky shore at Usujiri, southern Hokkaido, Japan, is composed of well-developed, columnar jointed dolerites. In this area, landscape is characterized by a mosaic of barnacle and mussels, where species composition in patches differs among typical topographic-classes, i.e. barnacles dominate on horizontal planes and vertical planes while mussels only inhabit in grooves. To determine the factors maintaining those distribution patterns, the recruitment of dominant sessile species in the typical topographic-classes with and without adults of barnacles and mussels were examined by field experiment. The results showed that the distribution pattern was determined by recruitment, and the recruitment pattern was strongly affected by rock-surface topography. Moreover, the presence of conspecific adults contributed to the maintenance of this topography-related mosaic by facilitating recruitment. These results show: (1) the presence of resource division for rocky topography among barnacles and mussels; and (2) that intraspecific positive interaction rather than interspecific competition may play important role in maintaining the topography-related mosaic of barnacle and mussels at Usujiri.