Tridimensional matrices of mussels Perumytilus purpuratus on intertidal platforms with varying wave forces in central Chile

Perumytilus purpuratus is the dominant competitor for primary substrate in the mid-intertidal zone of central Chile's rocky shores. This mussel is capable of forming dense matrices (beds) that support rich infaunal and epiphytic communities. Three platforms, exposed, semi-exposed, and sheltered, were sampled to determine aspects of the mussel P. purpuratus size structure and temporal dynamics. Palumbi's dynamometers were used to measure intertidal wave forces. Mussel samples from the 3 platforms were collected approximately every second month between April 1982 and June 1983. For sampling, mussels were removed sequentially in order to maintain the stratified structure (layers) of the mussel matrices. Populations of P. purpuratus show a bimodal size distribution when considering their entire size range. Although this bimodality is consistent through time and space, our results show important differences for the size distributions both spatially and temporally. Spatially, the differences could arise as a result of varying wave exposures. Adults showed a higher mean size with decreasing exposure. Temporally, the differences arise due to changes in the number of recruits entering the population. Within platforms, the observed differences when comparing upper and lower mussel layers could arise as a result of differential survival and growth rates between them. We also suggest that retention of recruits is higher in lower layers due to increased complexity of the matrix and reduced water flow. The sampling method developed to collect separate mussel layers proved to be crucial for the recognition of population dynamics processes occurring within the stratified structure of mussel matrices.