Abstract
Microhabitat structure, represented by the presence or absence of sessile organisms, may affect the composition of mobile communities. A barnacle, Chthamalus challengeri, forms dense patches of different sizes on an intertidal boulder shore in Amakusa, Japan, where mobile, herbivorous gastropods and chitons are the most dominant. To study the effects of the barnacle patches on the density and size structure of several herbivorous molluscs, we carried out quantitative quadrat sampling both inside and outside barnacle patches in July, 1994. Molluscan community composition and the size structure of dominant species inside barnacle patches were apparently different from those outside. The small gastropods Littorina brevicula and Peasiella inffracostata occurred only in the barnacle patches. The mean sizes of Nipponacmea teramachii, Monodonta labio, Patelloida pygmaea, Nipponacmea nigrans, and Lunella coronata were smaller inside than outside the patches, but Acanthochiton defilippi showed the reverse trend. Densities of A. defilippi, L. coronata, and the predatory gastropod Thais clavigera were higher inside the patches than outside, while the densities of M. labio, P. pygmaea, and N. nigrans were lower inside than outside. The influences of barnacles on herbivorous gastropods on the boulder shore are discussed in the context of some hypotheses proposed in recent studies of rocky shore community ecology.
