Biogeographic patterns of modern benthic shallow-water molluscs and the roles of temperature and palaeogeographic legacy

Unveiling the processes that lead to biogeographic regionalisation is key to understanding the links between micro- and macroevolution, community processes and macroecology. However, many studies focus on present-day conditions while neglecting geological and palaeontological history. Here, we review the relevance of contemporary climatic conditions and ocean circulation patterns and their geological legacy on the distribution of marine benthic biota, using Mollusca as a model group. Based on global gridded occurrence data, we computed hierarchical cluster analyses and non-metric multidimensional scalings using Simpson's distance index at three systematic ranks (species, genus, family). Generalised additive models were applied to assess the relationship between taxon distribution and global sea-surface temperature. In addition, we introduce a novel method to quantify the geographic coherence of clusters identified by cluster analysis to ascertain biogeographically meaningful interpretation. We show that contemporary climate and palaeogeographic changes, which have shaped ocean circulation patterns over geological time, have had a significant impact on the global distribution of benthic shallow-water marine molluscs. Our results indicate a high level of provincialism for species, slightly less so for genera, and a polar vs. circum-temperate-tropical structure for families. The biogeographic units defined by our cluster analyses match existing ocean currents for species, while the poorer regionalisation for genus- and family-level data is the result of geologically young seaways or land bridges. Our findings evidence the importance of considering historical processes for the biogeography of modern faunas.