Habitat properties and invertebrate composition in mussel-dominated rocky shores: A test of edge effects

Mussels form dense three-dimensional beds that serve as habitat to other species. In rocky shores, these beds are often interspersed by gaps due to patchy dislodgement/mortality caused by the action of waves, predators, and/ or extreme temperatures. Although mussel beds and gaps are known to support distinctive invertebrate com-munities, variations in invertebrate habitat function between the interior and edges of mussel beds and gaps were not yet examined. Here, we evaluated variations in habitat properties and invertebrate composition between the edge and interior of mussel (Brachidontes rodriguezii) beds and gaps at three rocky shore sites in the Southwestern Atlantic. Our results indicate that the interior and edge of mussel-covered areas differ in terms of mussel size, density and orientation relative to the rock surface (all surrogates of habitat structure), and in the quantity of sediments they accumulate. However, this does not directly translated into differences in temperature, desic-cation, and invertebrate composition within the mussel bed. As it concerns to gaps, we generally observed increased limpet (Siphonaria lesonii) densities at their edges, which suggests that they encounter favourable conditions by the perimeter of mussel beds. The general lack of edge effects on the invertebrates inhabiting mussel beds suggests that their species composition would remain largely unaffected by expected increases in gap and edge habitat formation due to ongoing increases in the frequency and magnitude of storms and heat waves. Yet, if increased availability of edge habitats leads to increased overall density of limpets in these rocky shores, then changes could be expected in algal production, composition, and dynamics.