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Macrophyte architecture can structure predator-prey interactions, but it is the chemicals within the plant that may actually be lethal. We conducted aquarium experiments to study the effects of common aquatic macrophytes (Myriophyllum spicatum, Myriophyllum sibiricum, and Chara tomentosa) and a predator (perch, Perca fluviatilis) on the survival, habitat choice, swimming, and feeding activities of Baltic littoral planktivores, mysids Neomysis integer and Praunus flexuosus, and three-spined stickleback (Gasterosteus aculeatus) larvae. Chemicals excreted by M. spicatum in a dense patch caused high mortality (73% to 89%) in both mysid species but not in sticklebacks, whereas M. sibiricum and C. tomentosa had no lethal effects. In lower stem densities stickleback larvae and N. integer avoided M. spicatum even in the presence of predator signals, and M. spicatum lowered the swimming and feeding activities of stickleback larvae. Only P. flexuosus did not avoid M. spicatum vegetation. Areas occupied by M. spicatum seem to be highly unsuitable habitats for littoral mysids and three-spined stickleback larvae. Because M. spicatum is a dominant macrophyte in the study area and eutrophication further increases its abundance, it may strongly influence the occurrence and distribution of mysids and fish larvae in the littoral ecosystems of the Baltic Sea.

The swarming behaviour of a Baltic littoral mysid shrimp, Neomysis integer, was studied both in the presence and absence of a predator (European perch, Perca fluviatilis L.). I performed two kinds of laboratory experiments. First, the swarming tendency of mysids and the effect of swarm size on swarm choice were studied. Second, the ingestion rate of mysids was measured when feeding alone versus in a swarm. The results indicate that N. integer actively join swarms. The avoidance of the perch by N. integer individuals was stronger when there was a swarm present. Larger swarms were preferred over smaller ones regardless of presence or absence of the predator. The overall feeding rate was similar when feeding alone and in swarm, but predator cues reduced feeding rate only when the mysids were feeding alone. This study demonstrates the capability of N. integer to assess predation risk and social context and alter their behaviour accordingly.