The physical structure of a habitat strongly affects species composition and diversity in benthic assemblages. In the shallow subtidal zone in the White Sea in northwestern Russia, barnacles Balanus crenatus are often found in clusters on empty bivalve shells or small stones, sometimes overgrown by solitary ascidians (mainly Styela spp. and Molgula spp.). These epibenthic patches are surrounded with muddy sediment. Sediment also fills the space between barnacle shells within the patches. The assemblages of mobile macrofauna associated with epibenthic patches and surrounding unstructured sediment are different in species composition and abundance. We hypothesised that epibenthic patches at least partially affect mobile benthic organisms with their architectural properties as complex cavity-loaded structures. In a field experiment we assessed the strength of this effect, comparing unmanipulated natural epibenthic patches (E), unmanipulated patches of bare sediment (S), and patches of bare sediment with artifici