Community structure of the macroinfauna in the sediments below an intertidal mussel bed (Mytilus chilensis (Hupe)) of southern Chile

the mytilid mussel mytilus chilensis (hupe) can form dense beds in sedimentary areas of the inland coast of the nord-patagonic archipelagos of the chilean coast (ca. 40-43° s). during the autumn of 2002, we collected replicated samples at five intertidal stations in panitao (golfo de reloncaví) ordered along a transect parallel to the low tide level and extended from the center of the bank (stations one and two with 100 and ca. 25 % of mussel cover, respectively) to the bare sediments of the intertidal (stations 3, 4 and 5, without mussels). the macroinfauna was numerically dominated by polychaeta, oligochaeta and crustacea peracarida. the total number of species collected was 14, being the most abundant the polychaete perinereis vallata, oligochaetes from the family tubificidae and the crustacean amphipod corophium insidiosum. the number of species, shannon-wiener diversity and total abundance of the macroinfauna did not differ significantly among stations. however, the percent contribution of polychaetes was significantly higher at the sediments sampled outside the mussel bed (stations three, four and five), while the percentual contribution of oligochaetes was significantly higher at the sediments sampled in the mussel bed (stations one and two). no significant differences were found between the percentual contribution of peracarid crustaceans between stations sampled in the mussel bed versus that sampled on the bare intertidal. the graphic results of nmmds show that the macroinfaunal assemblage of the stations located inside the mussel bed differed from that of stations located outside the bed. results of simper and anosim showed that the macroinfaunal composition of stations one and two was significantly dissimilar (61-54 %) to that of the stations located outside the mussel bed, which had similar composition. the graphic results of a nmmds based upon sedimentological characteristics show that most replicates of station one and some of station two separate from