Amphiura chiajei and Brissopsis lyrifera typically co-occur on some soft bottom areas of the North Sea, the Skagerrak and the Kattegat; they form the so called ‘Brissopsis-chiajei association’. Both species are deposit feeders that live partly ( A. chiajei) or completely ( B. lyrifera) burried in the sediment. In this association, each species is expected to affect the other one, notably through its feeding and burrowing activities. This study investigates the effects on body and gonads growth in A. chiajei and B. lyrifera as a result of their co-occurrence. The experiments were performed in aquaria with natural sediments (3 months observations) and have taken in account the population densities of both species and the availability of food. The results show that B. lyrifera can negatively affect the growth of body and gonads of A. chiajei, while A. chiajei seemingly has no effect on the growth of B. lyrifera. This situation probably results from the extensive bioturbation of the sediment by B. lyrifera, who also increased its surface feeding activity when food was added to the sediment surface.