Experimental and field studies were carried out to quantify the hydrodynamic mobility under swells of a circalittoral epifauna species. The spatial distribution and structure of one population (NW Mediterranean) of the serpulid polychaete Ditrupa arietina was assessed before and after a moderate swell event in late winter 2006 with the two readings separated by a seven-week interval. The overall density in the bay did not change significantly. However, in certain locations, significant but heterogeneous changes occurred: at some stations, D. arietina disappeared, at others it appeared with high densities while at others, densities remained fairly constant, although in this third case, the population size structure was different. In addition, no trend was observed along a cross-shore gradient. Experimental studies were conducted to determine bed friction velocity thresholds for incipient motion, bed load transport and suspension transport for different sizes of D. arietina. The minimum friction velocity required to transport D. arietina as bed load over a smooth rigid bed ranged from 1.5 to for tube lengths ranging from 6 to 25 mm. Thus, calcified worms (more than 6 mm long) cannot be destabilized by ordinary currents. Conversely, during the moderate swell event, the maximum bed friction was large enough to mobilize D. arietina, making probably all sizes available for bed load transport, while only worms with a tube length of less than 12 mm were transported in suspension at 27 m water depth. Wave statistics in Banyuls Bay indicate that D. arietina transport under moderate swell events should not be a rare phenomenon (28 days per year at 20 m water depth). Thus, the spatial scale for population dynamics studies should account for the extent of D. arietina spatial redistribution by the hydrodynamics. It is also recommended D. arietina be discarded from biotic quality index computations as this species is sensitive to natural hydrodynamic conditions.