Aim We evaluated the applicability of Rapoport's rule (RR) to the marine molluscs of the Americas. The biogeographical pattern predicted by RR has been the subject of a large number of studies, some supporting it and some not. In this exercise, we attempted to generate results free of biases in taxonomy or geographical scale.
Location The study area encompassed the Pacific and Atlantic sides of the entire North and South American continents.
Methods Our analysis was based on secondary data. We tested the relationship of the geographical range to gradients of latitude and depth, using the methodologies of Stevens (1989) and midpoint (Rohde et al., 1993). By Spearman's correlation, we related the mean amplitude of the geographical distribution to each gradient. We compared all known molluscan species together, and performed a second analysis limited to certain taxonomically well-known groups.
Results Our results were generated from a databank encompassing 4067 species. The analyses corroborated RR on both the Pacific and Atlantic coasts. In applying the pattern to the Atlantic coast, certain methodological issues had to be considered, such as the exclusion of deep-water species and taxonomically biased groups. Regional features, such as the size of a biogeographical province, seemed to strongly affect the form of the pattern. The results also supported the association of RR with a depth gradient.