Taxonomic Composition
We investigated taxonomic composition at the species and family level. In order to show how taxonomic composition changes with latitude, presence-absence data were transformed into a dissimilarity matrix using a revised version of the Forbes index (Alroy, 2015, Forbes, 1907). We then used Principal Coordinates Analysis (PCoA: Gower [1966]) to visualise the differences. We implemented PCoA using the cmdscale function in the base R package. Based on accepted biogeographical schemes for molluscs in the study area (Ebach et al. , 2013, Wilson and Allen, 1971), two major provinces and a long transition zone were expected to be seen.