MASS MORTALITY IN 2 COMMON SOFT-BOTTOM INVERTEBRATES, HYDROBIA-ULVAE AND COROPHIUM-VOLUTATOR - THE POSSIBLE ROLE OF TREMATODES

Two littoral macrofaunal invertebrates, Hydrobia ulvae (Prosobranchia) and Corophium volutator (Amphipoda) suffered mass mortality on an intertidal mudflat in the Danish Wadden Sea in May-June 1990. Dissection of collected H. ulvae individuals revealed a considerable increase from March to May in numbers of infected individuals by microphallid trematodes that use H. ulvae and C volutator as first and second intermediate host, respectively. The numbers of infested snails were hereafter reduced by an amount equal to the observed mortality rate of snails. At the same time, the C volutator population became extinct. Since other conceivable mortality factors could be ruled out, parasites are suspected to be the causative agent. Apart from the expected effects on potential predators by the decline in the two invertebrate populations, the benthic community changed and destabilization of the substratum occurred probably because of the die-off in C volutator. Meteorological data suggest high temperatures as a triggering factor of the mass-development of the studied trematodes.