Filling a gap:dynamics of space occupancy on a mussel-dominated subtropical rocky shore

Abstract

The mussel Sernirnytilus algosus (Gould) often dominates the space on exposed rocky shores of subtropical South America. Dynamics of space occupancy was investigated in a rocky intertidal habitat in central Peru. A series of colonization expenrnents and quantitative sampling of musselassociated faunas were conducted to clarify interspec~f~c relationships anlong resident organisms. Spatial gaps created within the S. algosus zone were colonised successively by the barnacles Jehlius cirratus (Darwin) and Notochtharnalus scabrosus (Darwin), a gallery-building polychaete, Phragmatopoma moerchi Kinberg, and a mussel, S. algosus. The rate of colonisation/recruitment was highest for barnacles, intermediate for l? moerchi and lowest for S. algosus. There was no evidence that barnacles either facilitate or inhibit colonisation by later colonists l? moerchi and S. algosus. Three filter-feeding taxa showed a clear competitive hierarchy. l? moerchi was competitively superior to barnacles and S. algosus was competitively superior to both barnacles and P rnoerchi. Therefore, there are trade-offs between colonisation ability and competitive ability among these taxa. In addition to recruitment of young larvae on free rock surfaces, S. algosus individuals demonstrate continuous lateral migration at the edge of their aggregation, constantly expanding the boundaries. Gaps can be closed in this manner, with an average rate of edge advance of 1.2 cm mo-', which is comparable to the values estimated for Mytllus californianus on North American rocky shores The S. algosus bed constitutes an important habitat to a range of organisms, many of them (particularly polychaetes) being restricted to this habitat on exposed rocky shores. Comparisons were made between faunas on free rock surfaces and in the mussel bed. Proportions of taxa restricted to the mussel bed were high among all invertebrates found there, in the range of 67 to 78 % in terms of number of taxa and 80 to 94 % in terms of number of individuals. Furthermore, juveniles of many organisms find refuge in the S. algosus bed, and barnacles which are competitively inferior to S. algosus can nevertheless exist epizoically on their shells. Thus, monopolisation of space by S. algosus generally leads to an Increase in total species richness.