DOES THE ACTIVITY OF COCKLES, CERASTODERMA-EDULE (L) AND LUGWORMS, ARENICOLA-MARINA L, MAKE COROPHIUM-VOLUTATOR PALLAS MORE VULNERABLE TO EPIBENTHIC PREDATORS - A CASE OF INTERACTION MODIFICATION

In the field, both lugworms, Arenicola marina L., and cockles, Cerastoderma edule (L.), suppress the numbers of Corophium volutator Pallas. By aquarium-experiments it has been shown that this is not caused by direct lethal effects, but by stimulation of migration. In the field, the more serious reductions of Corophium numbers suggest an additional, lethal, factor. Both aquarium and field-experiments were performed to test the hypothesis that the induced migration enhanced vulnerability to epibenthic predators. Aquarium-experiments showed that survival of Corophium in the presence of shrimps as predators declined by an extra approximately 50% when also cockles and lugworms were present. Observations of the reaction of Corophium after disturbance showed a real flight behaviour, with swimming away and crawling on the sediment. The time spent out of the sediment was longer for adults (approximately 5 min) than for juveniles (approximately 2 min). Cage-experiments in shallow-water bays on the Swedish west coast showed positive effects of predator exclosure on Corophium survival and this effect was stronger in the presence than in the absence of cockles and lugworms. These positive effects were stronger for adult than for juvenile Corophium. In areas with low natural densities of Corophium volutator (but high numbers of lugworms and epibenthic predators), the introduction of lugworm- and predator-free exclosures resulted in Corophium densities comparable to those in (nearby shallower) areas with low natural lugworm and predator densities. The results of these experiments provide an example of interaction modification: the disturbing activities of cockles and lugworms alter the risk of Corophium to be taken by predators and restrict its abundance and distribution more than either factor (disturbance, predation) would do.