ABSTRACT: Spatial variation in the components of faunal assemblages, but spatial uniformity of their biodiversity metrics, as described in 2013 for a seagrass bed in an estuarine bay in the warm-temperate Indian Ocean, is probably widespread in marine environments, and not restricted to seagrass habitats. Here, it is shown to occur across unvegetated siliceous sandflats as well as seagrass beds on a high-diversity but low-abundance subtropical island (North Stradbroke Island, Australia) in the South Pacific, and on a low-diversity but high-abundance cool-temperate island (Scolt Head Island, UK) in the North Atlantic. At the Australian locality, this uniform biodiversity but non-uniform faunal assemblage composistion were observed to extend over distances of at least 6.5 km in both habitat types. Unit areas as small as 0.0275 m² in each system supported a statistically invariant percentage of the total species pool of that system (i.e. the system’s overall mean percentage constancy index at the spatial scale concerned). Further, assemblages in all localities showed very similar ranked species-constancy curves, even though the proportions of the total numbers comprised by the various species varied widely. It is therefore suggested that the factors structuring the macrofaunal assemblages in such soft-sediment habitats must be capable of promoting comparable systems of relative frequency of occurrence, regardless of local levels of faunal abundance and biodiversity.