Effect of light on the feeding rates of pelagic and littoral mysid shrimps: a trade-off between feeding success and predation avoidance.

It has been shown that freshwater pelagic mysid shrimps capture zooplankton at a higher rate in light than in darkness. This has been suggested to be due to facilitation of visual predation on evasive zooplankton prey. To test this hypothesis with Baltic mysid shrimps, and to see whether pelagic (migrating) and littoral (non-migrating) mysids differ in this respect, we compared the feeding rates of Mysis mixta and Praunus flexuosus on the copepod Acartia spp. As light conditions change radically from the beginning of summer to late autumn at the Baltic latitudes, we conducted the experiments during three different times of the year to determine if there was a seasonal response to light in mysids. The feeding rates of pelagic mysids were significantly higher in total darkness than in light. In contrast, the feeding rates of littoral mysids did not differ in the dark and the light in the three different seasons. The decreased feeding of pelagic mysids under well-lit conditions may be an adaptation to avoid visual predation by pelagic fish. In contrast, littoral mysids, which live in the well-lit layer throughout the day, do not show a similar response. The fact that light did not increase feeding in either of the mysid species indicates that these mysid species do not use vision in capturing prey, but rely mainly on mechano-reception.

Effects of toxic and non-toxic cyanobacteria on grazing, zooplanktivory and survival of the mysid shrimp Mysis mixta.

We measured the effect of toxic and non-toxic cyanobacteria strains on grazing, predation rates and survival of the mysid shrimp Mysis mixta by means of laboratory experiments. Juvenile and adult M. mixta fed most actively on the non-toxic strains Aphanizomenon flos-aquae and Nodularia sphaerocarpa as on high quality food, the green flagellate Brachiomonas submarina, whereas grazing on the toxic N. spumigena was significantly lower than on the other strains. The mysids showed normal functional responses; decreasing clearance rates at increasing concentrations of cyanobacteria. In a separate predation experiment, the feeding rates on copepods declined in the presence of aggregated non-toxic cyanobacteria. However, we could not demonstrate increased mortality in a 7-week experiment where mysids were exposed to high concentrations of toxic N. spumigena