The effect of bioturbation by the ghost shrimp Trypaea australiensis and semaphore crab Heloecious cordiformis was compared in sediment-filled tanks in the laboratory. Effect of bioturbator density was also investigated with high- and low-density treatments. It was hypothesised that the two species would influence the sediment profile in different ways owing to their contrasting burrowing and feeding habits. Both species increased porosity of surface sediments relative to control tanks. Crab activity did not alter redox potential, but low densities of shrimp created more oxidising conditions and high densities of shrimp created more reducing conditions than controls. Burial of tracer particles by crabs was restricted to the top 5 cm, whereas shrimp mixed particles to depths of 25 cm. Bioturbator density had little effect on the extent of particle mixing. The presence of both shrimp and crabs increased benthic microalgae in the sediments relative to the controls. Again, crabs had the greatest effect at the sediment surface, whereas shrimp also enhanced concentrations at 25 cm depth. High densities of shrimp had the greatest effect. Overall, shrimp bioturbation influenced deeper sediments than crab bioturbation, but there was no clear density-dependent effect.