EFFECTS OF ARENICOLA-MARINA AND CERASTODERMA-EDULE ON DISTRIBUTION, ABUNDANCE AND POPULATION-STRUCTURE OF COROPHIUM-VOLUTATOR IN GULLMARSFJORDEN WESTERN SWEDEN

The amphipod Corophium volutator has a patchy distribution which has been related to different factors (both physical and biological) at different sites. Experiments in the Wadden Sea. involving manipulation of the densities of the lugworm Arenicola marina and the bivalve Cerastoderma edule showed strong negative effects of these two species on the abundance of C. volutator. This result could explain the distribution pattern of C. volutator found in the Wadden Sea. The question was if this would also be the case in other geographical regions. Therefore, these experiments were repeated under different physical conditions in two shallow and almost non-tidal soft-bottom bays in Gullmarsfjorden on the Swedish west coast. Again, a significant negative density-dependent effect on Corophium was found. This effect was even stronger when A. marina and C. edule occurred together. A. marina and C. edule affected the numbers of the two sexes of C. volutator in a similar way. However, in the treatments with high densities of A. marina and C. edule the proportion of females with eggs and the mean lengths of the females were lower resulting in a lower reproductive output. Surveys of several bays along Gullmarsfjorden showed that the bay-to-bay variability in the abundance of C. volutator could not be explained hy differences in abundances of A. marina, C. edule or Nereis diversicolor. Within the only, two bays with locally high densities of A. marina the densities of C. volutator showed a clear negative relationship with those of A. marina. In conclusion: at a small scale the abundance of C. volutator is closely related to densities of A. marina and C. edule, but at a larger scale other factors must be of more importance.