Bioturbation processes of a dense ophiuroid assemblage were studied by in situ time-lapse sediment-profile imagery (t-SPI) and laboratory tracer-experiments. The burrows of 4 Amphiura fliformis (Ophiuroida) individuals were visible throughout the t-SPI deployment. Mean burrow excavation was 2.9 cm(2) h(-1). Mounding ranged from 0 to 7.12 m(2) h(-1). Loss of sediment from the sediment-water interface ranged from 0 to 7.0 cm(2) h(-1). Bioturbation at the station was dominated by A. filiformis, whose presence appeared to mask the activity of conspecifics. Species interactions were investigated in terms of particle reworking by laboratory luminophore-tracer experiments of monocultures and species mixtures using A. filiformis, Leptopentacta elongata (Holothuroida) and Nephtys hombergii (Polychaeta). Bioturbation by these large species fit a symmetric non-local model well. Species interactions led to underyielding with respect to predicted additive species-effects. Treatments containing A. filiformis were significantly different from treatments without A. filiformis, and not significantly different from each other. When the data was processed on a per capita basis, multiculture treatments containing N. hombergii underyielded due to the high biomass of N. hombergii and its relatively low bioturbation activity. Ordination of a distance matrix derived from tracer profiles showed that tracer profiles produced by multispecies treatments were most similar to the strongest bioturbator, due to destruction of conspecific structures by the strongest bioturbator. Species interactions are not additive in terms of particle bioturbation, and may be difficult to predict without knowing in situ densities.