Predatory processes in Baltic benthos

Sammanfattning: The Baltic soft-bottom community is uniquely simple, with only a few benthic macro-faunal species, and is therefore well suited for mechanistic studies of inter-specific interactions. Two of the dominating organisms in this benthic community are the amphipod Monoporeia affinis and the bivalve Macoma balthica. Field surveys have shown that M. balthica is generally absent or scarce when the density of M. affinis is high. The hypothesis that adult M. affinis kill the newly settled bivalves was confirmed experimentally in the laboratory, and it was also shown that adult Pontoporeia femorata amphipods have a negative impact on bivalve survival. Further experiments, showed that juvenile M. affinis, contrary to earlier beliefs, can kill and presumably eat the newly settled M. balthica. The response of M. affinis to increased bivalve densities was a type III like functional response, indicating that bivalves at low densities find a partial refuge from amphipod predation.The effect of the predatory isopod Saduria entomon on the Macoma balthica population was assessed both in the laboratory and the field. In the laboratory, the presence of the isopod did not affect the small just settled bivalves (0.3 mm), whereas slightly larger and larger bivalves (>0.8 mm) suffered from increased mortality, as did bivalves in the three month long field study. The isopods are physically capable of opening quite large bivalves, a 34 mm long isopod can break open a 17 mm long bivalve, but given a choice, smaller bivalves are selected. When S. entomon is offered the two prey species, Monoporeia affinis and M. balthica, the amphipod is preferred, leaving the bivalve relatively safe from predation.In the aquatic environment, chemical substances released by predators or their activities can convey information, to which prey can respond, for example, by a change in behaviour. In a three-trophic-level food chain, species-specific chemical substances from a predatory fish, the short-horned sculpin (Myoxocephalus scorpius), directly affected the behaviour of the isopod Saduria entomon. The isopod remained buried in the sediment longer, and fewer prey amphipods, Monoporeia affinis, were eaten. Further, exposure to chemical substances from isopods feeding on amphipods, lead the amphipod to lower their swimming activity, whereas water from non-feeding isopods did not have this effect.