Separating the effects of physical and biological aspects of mussel beds on their associated assemblages

Mussels provide habitat for diverse assemblages and have a major influence on local biodiversity. Organisms may be associated with the physical structure of the mussels or mussel beds, with biological characteristics of the shells of the mussels or with the living mussels themselves. We examined colonisation of experimentally-built patches of live mussels, dead mussel shells and resin mimics to test models regarding these cues. The sessile assemblage on the valves of the mussels and the mobile assemblage living in the spaces among the mussels were separately examined to evaluate whether they respond in the same way. In addition, the experiment incorporated 2 levels of clumping of mussels to represent sparse and clumped mussel beds after 5 and 9 mo of deployment of the patches. With few exceptions, there was no effect of the types of mussels provided; similar species and abundances were found on and among live mussels, dead shells and resin mimics after both time periods. Therefore, it appears that mussel beds act predominantly as physical habitats for these fauna. In contrast, the degree of clumping of mussels in a patch had major effects. After 5 mo, barnacles and spirorbid polychaetes had greater covers on the valves of sparsely packed mussels than on clumped mussels. The opposite pattern was shown by spirorbids and serpulids after 9 mo, although barnacles retained their original pattern. Some mobile taxa showed greater abundances in clumped than sparse patches, but these differences largely disappeared when the data were corrected for the number of mussels. Therefore, mobile taxa appeared to respond directly to the number of mussels in a patch; more mussels created more habitat. This was not the case for the sessile assemblage, where mussel shells in sparse beds appeared to provide different habitat from those in clumped beds.