Abstract Bioturbation by Sipunculus nudus in a fish-Sipunculus polyculture system was investigated using two controlled experiments. The first experiment was conducted in 20 breeding tanks (water volume 550 L) in which unfed S. nudus (mean weight 1.2 ± 0.1 g) were stocked in the bottom sandy sediment at four different densities: 0 (control), 50, 100 and 150 individuals per tank. In each tank, 3 juvenile Mugil cephalus (mean weight 24.5 ± 0.5 g) were cultured with a normal ration supply in a net cage (diameter 0.8 m, height 0.6 m) suspended in the seawater above. The experiment was conducted for 60 d. The second experiment, which was conducted for 20 d, quantified the transportation ability of S. nudus on the surface sediment. S. nudus (mean weight 1.3 ± 0.1 g) were stocked in the sandy sediment in 25 aquaria (31 cm × 20 cm × 30 cm) at five densities (0, 4, 8, 12 and 16 individuals per aquarium). The sediment was prepared in layers: the bottom layer was a 6-cm-thick layer of medium sand (grain size 0.40–1.10 mm), and the top layer was a 1-cm-thick layer of fine sand (grain size 0.10–0.28 mm). In the first experiment, 1) S. nudus exhibited excellent survival in all experimental groups, but positive weight gain was only observed in the low-stocking-density group (50 individuals per tank); the best M. cephalus growth also occurred in this group (P The above results indicate that S. nudus can mitigate sediment organic waste accumulation in an aquaculture system by transporting surface sediment to the bottom; thus, the polyculture of S. nudus with other aquaculture animals is both ecologically and economically feasible.