Mussels of the family Mytilidae occur worldwide on soft-bottoms of temperate coastal waters, forming dense epibenthic aggregations. Since associated species abundances and diversity within mussel beds are usually higher than in surrounding sand flats, these aggregations are generally considered hot spots of biodiversity. In this study, we tested whether this pattern is common in soft bottom mussel beds worldwide. We investigated beds of different species of mytilid mussels and their associated species communities in comparison to adjacent areas without mussels in Germany (beds of Mytilus edulis), Chile (mixed beds of Perumytilus purpuratus and Mytilus edulis), South Korea (Musculista senhousia) and Australia (Xenostrobus inconstans). Two major types of mussel beds were identified: i. beds with most mussels lying attached to one another on the sediment surface (Germany, Chile); ii. beds consisting of bivalves and accumulated sediments forming hummocks on the bottom (South Korea, Australia). Type ii. beds are considered semi-endobenthic, because mussels sit in the accumulated sediment. In all systems investigated, species assemblages in mussel beds were significantly different from communities of the surrounding sand flats. Many species were restricted to one habitat type. In addition, species diversity of mussel aggregations may depend on mussel bed structure. Epibenthic mussel beds (Germany and Chile) showed a strongly increased species number in comparison to the surrounding sediments. In semi-endobenthic beds, by contrast, the number of associated species was lower (Korea) or similar (Australia) to adjacent sand flats. We think that the high structural complexity of epibenthic beds causes the observed pattern. We conclude that mussel beds have an important function as ecosystem engineers in coastal systems worldwide. They generally enhance diversity by increasing habitat heterogeneity. However, within mussel beds only the epibenthic type constitutes a hot spot of biodiversity.