Most generalisations about marine benthic diversity (species richness) are derived from few studies, few samples and low species numbers. It is questionable whether the data on which most paradigms, especially about the deep sea, are based truly represent general patterns of marine diversity. Available information from deep-sea studies are summarised and compared with some extensive data sets from the shallow coasts of Norway and Australia. We show that species richness per unit area is as high, if not higher in shallow sedimentary habitats as was reported for the deep-sea data by Grassle & Maciolek (1992; Am Nat 139:313-341). Apparent high diversity in the deep sea may be explained, in part, by the vast area of this environment. All surveys in both the deep-sea and coastal habitats are shown to traverse a variety of microhabitats and thus sediment heterogeneity is not an explanation for the high species richness in coastal environments.