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Species density of macrofaunal invertebrates living in marine soft sediments was highest at the shelf-slope break (100–150 m) in Monterey Bay (449 m-2). There were 337 species m-2 in the mid-shelf mud zone (80 m). There were fewer species along the slope: 205 m-2 from the lower slope (950-2000 m) and 335 m-2 on the upper slope (250-750 m). Species density was highest inside the bay (328-446 m-2) compared to outside (336-339 m-2), when examining samples at selected water depths (60-1000 m). There was little difference in local species density from 1 km of shoreline compared to regional species density along 1000 km of shoreline at both shelf and slope depths. The highest species densities worldwide in the literature are recorded along the Carolina slope in the Atlantic Ocean, where peak species density (436/0.81 m2) at 800 m and values at the largest sample areas are similar to those on the Monterey Bay shelf. We speculate that the highest species densities occur where ocean water exchanges energy with shoaling topography at the continental margin, bringing more food to the benthos – areas such as the very productive waters in the upwelling system of Monterey Bay.