Light-dependent predation by the invertebrate planktivore Bythotrephes longimanus

We evaluated predation by the invasive invertebrate planktivore Bythotrephes longimanus on a Lake Michigan prey assemblage as a function of light intensity. Daphnia mendotae was the only prey type significantly reduced and light intensity strongly affected this reduction. Specifically, Bythotrephes consumption of D. mendotae was not detected under low light intensity (<1 mmol·m^-2·s^-1) but increased with greater light intensity and leveled off under high light intensity (>100 mmol·m^-2·s^-1) at which point Bythotrephes ingestion rate was 2.3 Daphnia per hour. These results indicate that Bythotrephes predation is more sensitive to light than previously thought, a discrepancy that can be explained after considering the ability of D. mendotae to detect Bythotrephes' hydromechanic disturbance. The observed effect of light intensity on Bythotrephes predation is more like that of planktivorous fish than that of other previously studied invertebrate planktivores. Our findings elucidate the role that Bythotrephes plays in the food web and provide a novel explanation for its tendency to invade lakes of high water clarity. The importance of light-dependent predation found here may extend to other visually oriented predatory cladocerans.