The extreme tidal range in the Bay of Fundy (up to 17 m) exposes intertidal mud and sand flats up to 4 km wide. These flats support large populations of invertebrates, whose zonation is markedly influenced by sediment type. The fauna of the sand flats on the south shore of the Minas Basin is characterized by large numbers of deposit-feeding polychaete worms, both sedentary and errant. Two tube-building species have a marked influence on the surrounding sediment. Clymenella torquata , an upside-down deposit-feeding maldanid, reaches maximum population densities of 500/m 2 in medium sands of the lower intertidal. Sediment incorporated into the tubes is significantly coarser and lower in heavy mineral content than the substrate. Biogenic graded bedding is not produced, as feces are removed by tidal currents. Higher population densities prevent formation of ripple marks. Spiophanes wigleyi , a deposit-feeding spionid, reaches maximum population densities of 98,000/m 2 in silts and sands of the lower energy, upper intertidal zone. Up to 50% of the sediment in the upper 5 cm may be incorporated into the tubes, which are identical in mean grain size and sorting to the substrate. Heavy minerals are again avoided in tube-building. Large populations inhibit formation of ripple marks and depress the level of the anoxic zone.