High-resolution multibeam mosaics show that the seafloor across the southwestern Marmara Sea is host to remarkably organized near-circular bioherm mounds, which commonly are arranged into large, tightly packed clusters. Grab samples and gravity cores reveal that the bioherms are predominantly composed of very fine-grained, calcareous, silty mud with abundant bioclasts, including centimetre-scale masses of coralline red algae and intact disarticulated mollusc shells (mainly the genera Modiolus and Mytilus). Geometric analysis of multi beam images reveals that the average bioherm is 15.6 m in diameter, occupies similar to 190 m(2) of seabed, stands 113 cm above the adjacent seafloor, and its crest is 20.6 m from the crests of neighbouring bioherms. In regions of tightly packed bioherm clusters (referred to as 'bioherm colonies') the inter-mound depressions are on average 4.4 m wide and 33 cm deep. Although each bioherm mound is nearly circular, the surrounding inter-mound channels form a more rectilinear mesh of linked pentagonal and hexagonal polygons suggesting densest possible spatial packing of the mounds. Near-neighbour statistics of R = 2.11-2.14 indicate an essentially uniform spacing between the bioherms, which is the expected result for close packing on a plane and full utilization of the available space.