Bioturbators affect other benthic animals by two processes: sediment reworking and changes in chemical properties. Crab exclusion experiments were conducted on an intertidal mud flat in the Tama estuary to examine the effects of the burrowing crab Macrophthalmus japonicus (Ocypodoidea) on the densities of sympatric macro-infauna in 2010 and 2011
and on sediment parameters in 2011. In 2010, approximately 1 month after M. japonicus was excluded, the densities of Corbicula japonica and Hediste sp. or spp. were significantly greater than those of the control, with M. japonicus present. Concerning sediment parameters: chlorophyll-a content, water content and total nitrogen content at the surface significantly increased and subsurface oxidation-reduction potential significantly decreased in the exclusion treatment compared with control conditions. These results appear to indicate that sediment reworking by M. japonicus was the main process affecting the sedimentary environment and density of C. japonica and Hediste sp. or spp. However, the magnitude of the effects of M. japonicus apparently varies from year to year because there were no significant differences in the densities of any macro-infauna species in 2011.