The distribution and size composition of free-living marine nematodes were investigated in relation to the sediment condition at 55 stations on the Mae tidal flat, Hiroshima Bay. The relationship between environmental factors, frequency of human maintenance of Manila clam beds, and the horizontal and vertical distribution and size composition of nematodes were studied. The density of nematodes showed a significant correlation with viable numbers of bacteria and the frequency of clam bed maintenance. The maintenance of clam beds, which includes the installation of anti-predator nets and removal of Musculista senhousia mats, may reduce ignition loss, thereby promoting an increase in the percentage of small nematodes (63–125 μm in mesh size). The present results indicate that the size composition of nematodes and their distribution within the sediment are affected by differences in sediment organic content and redox potential.
