THE DISTRIBUTION OF THE AMPHIPOD COROPHIUM-ARENARIUM IN THE DUTCH-WADDEN SEA - RELATIONSHIPS WITH SEDIMENT COMPOSITION AND THE PRESENCE OF COCKLES AND LUGWORMS

On the tidal flats in the Dutch Wadden Sea Corophium volutator is a dominant species of the upper intertidal zone; the closely related Corophium arenarium is usually found in the lower zone, but only in low densities (a few hundreds per m(2)). A survey in the Dutch Wadden Sea showed that this zonation pattern was only present when a muddy sediment was found in the upper zone and a sandy in the lower zone. C. arenarium was restricted to sandy sediments, C. volutator to muddy sediments. Where a sandy sediment was found in the upper intertidal zone, C. arenarium locally occurred in relatively high densities (a few thousands per m(2)). An aquarium experiment showed that C. arenarium actively avoided muddy sediments. Field experiments were carried out to study the influence of other macrozoobenthic species (known to affect the related C. volutator) on the abundance of C. arenarium. Within large defaunated areas small plots were stocked with different densities of the lugworm Arenicola marina and the cockle Cerastoderma edule. In small plots within a natural benthic community densities of these species were also augmented or (in A. marina) reduced. Strongly negative density-dependent effects of both A. marina and C. edule were found on the abundance of C. arenarium. In the natural situation, its densities showed A. marina to be the most important factor in determining the abundance of C. arenarium. In particular the removal of lugworms caused a strong increase in C. arenarium densities. These results agreed with the distribution of these species along a transect perpendicular to the shore of Schiermonnikoog, where a significant negative correlation was found between the densities of A. marina and C. arenarium. Aquarium experiments showed that the negative effect of cockles and lugworms must be due to migration rather than mortality in C. arenarium.